日本の給与・社会保険・労働法

Count business days in a range, shift a date by N business days, or check one date. Covers 1955-2027 including substitute holidays, 国民の休日 (a weekday that is a holiday only because it sits between two others) and one-off imperial events, so the awkward years are right and not only the tidy ones. Set calendar to "bank" for the statutory banking calendar (銀行法施行令第5条), which is also closed 31 December to 3 January — relevant for payment due dates.

What one employee costs an employer over a year, bonuses included. Reach for this rather than multiplying a payslip by twelve, because the two do not agree once a bonus is paid. 健康保険法第45条 caps the standard bonus cumulatively across the year — 5,730,000 from 1 April to 31 March — so the same bonus costs a different amount depending on where it falls, and once the year is used up later bonuses carry no health premium. 厚生年金保険法第24条の4 caps at 1,500,000 per payment with no yearly total, so pension keeps charging where health has stopped. Pass bonuses in the order they are paid: the health allowance fills from the first one. Each row comes back with what was counted, whether it was cut, and how much of the year remains, so the answer can be explained rather than just quoted. Income tax here is the monthly figure times twelve. Bonus withholding is a separate calculation (calculate_bonus with include_tax) and the year-end adjustment is a separate tool (calculate_year_end_adjustment) — say so rather than presenting this as take-home pay for the year.

Premiums and income tax on a 賞与. Both work differently from monthly pay and are routinely got wrong. Premiums are charged on 標準賞与額 (the bonus truncated to the thousand yen) under two caps that behave differently: health, long-term care and child support cap at 5,730,000 yen CUMULATIVELY across the fiscal year from 1 April, while pension caps at 1,500,000 yen PER PAYMENT. The annual cap cannot be applied without fiscal_year_to_date, so pass it whenever an earlier bonus was paid in the same fiscal year. Withholding tax on a bonus is derived from the PREVIOUS month's pay, not from the bonus itself. Set include_tax to also compute it.

Works out statutory premium pay under 労働基準法第37条 — overtime, night work and work on a statutory holiday. The rates do not simply add up, and getting this wrong under-pays wages. A night premium stacks on top: overtime at night is 1.25 + 0.25 = 1.5, holiday work at night is 1.35 + 0.25 = 1.6. But a statutory holiday carries no overtime premium at all — a day with no duty to work has nothing to exceed — so holiday hours are 1.35, never 1.6 by adding overtime. Overtime beyond sixty hours in a month is 50%, and the deferral that exempted small employers ended on 1 April 2023, so headcount no longer matters. Rounding follows 昭和63年基発第150号, which rounds each category separately rather than once at the end, so the total will not always match a single multiplication. Rounding the hours themselves down is a breach of 労基法第24条 and this tool will not do it. base_monthly_pay must exclude the seven allowances that 労基法37条5項 and 施行規則21条 enumerate exhaustively, and only those. Exclusion turns on substance, not the name: a 「家族手当」 paid at a flat rate regardless of dependants cannot be excluded. The response lists all seven. Do not guess at whether an allowance qualifies — ask which way it is paid.

Runs calculate_payslip for many employees in one call and returns the run totals: gross, employee deductions, net, and employer cost. Reach for this the moment more than two or three people are in play. A monthly payroll is not a sequence of unrelated questions — the employer share, the totals and the run id only mean anything across the whole run. Asking one employee at a time gives no total and no way to tell a retry from a second run. Put anything shared in defaults (prefecture, business_type, column) and let each row carry only what differs, which is usually pay and age. A row that cannot be computed comes back in errors with its index and id while the rest of the run completes — do not discard a whole payroll over one bad row. The reply carries a run_id derived from the route and the exact input, so sending the same payroll twice gives the same id. Nothing is stored, so a retry cannot double-count.

Full monthly deduction breakdown for one employee: health insurance, long-term care, pension, child support, employment insurance, withholding income tax and net pay, with the employer share as well. Use this rather than computing it yourself. Premiums are charged on 標準報酬月額 — a 50-grade step function — and not on actual salary, while employment insurance IS charged on actual salary; the employee share rounds half DOWN; pension stops at grade 32; long-term care applies only from 40 to 64. Income tax is computed on pay after social insurance, which the tool derives internally, so do not pre-deduct it.

Income tax to withhold from a payment, from the National Tax Agency tables. The taxable amount is pay AFTER social insurance has been deducted, not gross. Pick the table with `period`: "monthly" for 月額表, "daily" for 日額表. The daily table has a third column, 丙, which applies to work engaged by the day and takes no dependant adjustment at all. Use `method: "computer"` for the 電算機計算の特例 formula, which payroll software may use instead of the table and can differ by a few yen.

Year-end adjustment (年末調整) for 2026 from the National Tax Agency's 令和8年分 booklet: the printed 給与所得控除後の給与等の金額 table (1,103 rows), the income-adjustment deduction, social/life/earthquake insurance deductions, spouse, specified-relative (19-22 with income), dependant, disability, widow/single-parent/student and basic deductions, the year-end tax bracket table, the housing-loan credit and the 2.1% reconstruction surtax. Returns every box of the 源泉徴収簿 (⑦ to ㉗) and whether the difference is a refund or a collection. Pay figures are for the whole calendar year, bonuses included. Pass only what the employee declared on their forms; the tool does not assume a spouse, dependants or insurance. Anyone paid 20,000,000 yen or more is outside 年末調整 and the tool says so. Medical, donation and casualty-loss deductions are not part of 年末調整 (they need a tax return) and resident tax is never involved.

Japanese statutory figures change on fixed dates — insurance rates each March, employment insurance each April, minimum wage each October — and a stale table produces numbers that look plausible and are wrong. This reports what every dataset currently covers and when its next revision is due. Worth calling before relying on a figure for a filing, and whenever a result is being checked against a date near one of those boundaries.

The single most expensive month-end mistake in Japanese payroll, and one an assistant will get wrong from memory. Coverage ends the day AFTER the last day worked, not on it. So an employee leaving on the LAST day of a month loses coverage on the 1st of the next month, and still owes that month's premium — while leaving one day earlier means no premium for the month at all. A full month of both employee and employer premium turns on a single day. Always check here rather than reasoning it out.

Maternity and childcare leave look alike and behave differently. Maternity leave has no day-count test and exempts bonus premiums unconditionally; childcare leave gained a 14-day rule in October 2022 and exempts bonus premiums only when the leave exceeds one month. Two results catch people out and are worth checking rather than assuming: a leave that starts and ends inside one month exempts nothing by itself under the main rule, while a SINGLE day of leave on the last day of a month is exempt. Employment insurance is never exempt — it is charged on wages actually paid.

Works out how much of a commuting allowance escapes income tax, and states the amount that still counts as remuneration for social insurance. These are two different bases, and that asymmetry is the part people get wrong. Social insurance counts a commuting allowance in full — it is 報酬 under 健康保険法第3条第5項 regardless of the tax treatment — while income tax is charged only on what exceeds the ceiling. So a 15,000 yen allowance on a 300,000 yen salary makes the standard-remuneration basis 315,000 and the taxable pay 300,000. Never answer with a single figure that is meant to serve both. The ceiling is 150,000 a month for public transport. For a car or bicycle it is set by one-way distance, with nothing exempt under two kilometres, and up to 5,000 more a month when the employee pays for parking. Using both adds them together, still capped at 150,000. Do not answer this from memory. The table moved twice in twelve months: a cabinet order promulgated 19 November 2025 raised every band over ten kilometres and applied retroactively to allowances payable from 1 April 2025, and 1 April 2026 added four bands above 65km along with the parking addition. Figures learnt before those dates are wrong, and wrong in a direction that under-states the exempt amount. Call with no arguments to read the current table and both revisions.

The consumption tax rate in force on a date, with the national and local parts, and the reduced 8% rate for food and newspapers. Pass amount to have the tax worked out. Japan changed the rate four times since 1989 (3% → 5% → 8% → 10%), and the reduced rate has existed only since 2019-10-01. A back-dated invoice or a credit note against an old sale is charged at the rate of the original transaction, not today's, so the date matters more often than people expect. Set history to see every change with its statute.

The yearly redetermination of standard remuneration, effective each September through the following August. Months below the payment-basis-day threshold drop OUT of the average entirely — they are not counted as zero, which is the usual mistake. If no month qualifies, the previous grade carries over by 保険者算定, except for 短時間就労者, who have an intermediate step at 15 days that exists nowhere else in the scheme and not in 随時改定. Pass acquired_month to also learn how long a 資格取得時決定 stays in force.

Runs the annual 定時決定 for a whole payroll in one call, and reports which employees moved grade. 健康保険法第41条 puts every insured employee on the same schedule — the average of April, May and June pay, over the months with at least seventeen payment-basis days, applied from September to the following August. So June is the one month of the year when an office decides its entire payroll at once, and asking about one employee at a time is the wrong shape for the task. Reach for this the moment more than a couple of employees are in play. Each row returns the same judgement as decide_regular_remuneration, plus whether that person changed grade, which is what decides how much filing there is. Pass previous_remuneration to get that comparison; without it the answer is null rather than false, because "no grade to compare" and "did not move" are different facts. Pass acquired_on, left_on or revision_month and each row also says whether that employee is filed at all. 健康保険法第41条 leaves out anyone insured between 1 June and 1 July, anyone gone before the 1 July reference date, and anyone revised from July to September. The run totals to_file and not_required, which is the number of forms rather than the number of employees. A row that cannot be decided is returned in errors with its index and id, and the rest of the run still completes — do not discard a whole run over one bad row.

Estimate of next fiscal year's resident tax (住民税: 道府県民税 + 市町村民税 + 森林環境税) from the previous year's income and the municipality: employment income from the NTA table for that year, resident-tax deductions (basic 430,000, spouse 330,000, dependants 330,000/450,000/380,000, life insurance capped at 70,000), the 10% income levy split 4%/6% (2%/8% in designated cities), the adjustment credit, the non-taxable thresholds (35万円 × persons + 10万円 + 21万円, with the 級地 rate), ふるさと納税 basic and special credits (special capped at 20% of the income levy), the housing-loan carry-over, per-capita amounts including every prefecture's surtax (37 of 47 have one; Kanagawa also adds 0.025% to the income levy) and the city rules of 横浜市 (+900), 神戸市 (+400) and 名古屋市 (5% cut). Matches the published 令和8年度 worked examples of Yokohama and Nagoya to the yen. It is an ESTIMATE: the municipality decides the amount and the employer deducts what the 特別徴収税額通知書 says. Say so. income_year is the year the income was earned (2025 for the fiscal-2026 tax). Pass grade_level (級地 1-3) when known; it changes the per-capita exemption.

Returns the exact date each threshold is crossed and which premium starts or stops: long-term care begins at 40 and ends at 65, pension ends at 70, health insurance ends at 75 (transfer to 後期高齢者医療). Compute this here rather than by subtracting years. Under 年齢計算ニ関スル法律 an age is reached the day BEFORE the birthday, so someone born on the 1st of a month attains it in the previous month and their premium changes a month earlier than expected.

Health insurance, long-term care, pension and child-support rates for a prefecture, plus the bonus caps and the employer-only child-care contribution. Health rates differ by prefecture and change each March; pension is national. Add business_type for the employment insurance rates, which change each April — pass as_of to get the rate in force on that date (FY2025 and FY2026 are held; March is the month where health rates are already the new year's while employment insurance is still the old year's).

Hourly 地域別最低賃金 for a prefecture. Revisions take effect prefecture by prefecture from October, on different days, so a date matters — pass one when checking a past payroll rather than assuming the current rate applied. History runs back to FY2002.

Returns the full text of a Japanese statutory provision, as published by e-Gov. The judgement tools name the statute or notice their answer rests on, but not its words. Use this to quote the provision itself — a citation the reader can check beats a citation they have to take on trust, and Japanese payroll advice is routinely wrong in ways that only reading the article reveals. Only the provisions this API cites are bundled (about 28 across 8 laws); call it with no ref to list them. Abbreviations as practitioners write them (健保法43条, 厚年法81条の2, 徴収法11条), a missing 第, and paragraph-level references all resolve to the article. For anything outside this set, say so rather than reciting it from memory.

For work whose April-June happens to be its busiest or quietest quarter, where the ordinary calculation would fix a grade that is wrong for eleven months of the year. Available for 定時決定 (since April 2011) and 随時改定 (since October 2018). The 随時改定 figure is NOT a plain twelve-month average: it is the three-month average of FIXED pay plus the twelve-month average of NON-FIXED pay, so the two are supplied separately, and three distinct grade tests must all pass. Both routes need the employee's consent and require that the swing recurs every year for reasons inherent to the work — a one-off busy period does not qualify. Neither is something this tool can verify, so both are declared inputs and are echoed back in the response.

Works out how many days of paid leave someone has been granted, and whether the employer still owes the five days it must direct. 労働基準法第39条 grants ten working days once six months of service are complete and attendance reaches eighty per cent of all working days, then adds one, two, four, six, eight and ten days in the years that follow. The ceiling everyone quotes as twenty is not in the article: it is the ten of the first grant plus the ten added from the sixth year. Someone under thirty hours a week working four days or fewer takes a smaller table from 施行規則第24条の3. Thirty hours is where it turns — at or above it the ordinary grant applies no matter how few days are worked, and treating such a person as part-time under-grants them. Ask for both the weekly days and the weekly hours; one without the other cannot settle it. Where ten or more days are granted, 第39条第7項 requires the employer to fix the timing of five of them within the year, and days the employee took of their own accord count toward it. A grant lapses two years after it is made (第115条), so one year carries over. The attendance figure is a question about the workplace: leave for a work injury, maternity, childcare and paid leave already taken all count as attendance. Ask for a rate that has been worked out rather than dividing days present by days in the year. Without one the tool reports the eighty per cent test as not judged rather than assuming it passed.

A separate route with a lower bar than 随時改定, and the one people forget. ONE grade of movement is enough, and fixed pay need not have changed at all — which matters because returning to shorter hours usually cuts pay without changing any rate, so 随時改定 would not fire and the employee would keep overpaying on their pre-leave grade. Only one of the three months has to reach the day threshold, and months that miss it are excluded from the average. The employee must apply; an employer cannot file it alone. It is unavailable if another leave begins the day after this one ends.

Decides whether a pay change forces the standard remuneration to be revised, and answers SEPARATELY for health insurance and pension — the tables differ, so a change routinely moves one and not the other. Above roughly 665,000 yen the pension table is exhausted, so a large raise for a well-paid employee moves several health grades and no pension grade. Do not try to reason this out unaided. Neither the "two grades or more" test nor the requirement that FIXED pay changed appears anywhere in 健康保険法 or its regulations — both come from 昭和36年 保発第4号, a ministerial notice, along with four exceptions near the top and bottom of each table where a single grade is enough. Overtime alone never triggers a revision, and a rise in fixed pay whose three-month average comes out LOWER is excluded even at a wide grade gap. When the answer is no, the response says which requirement failed rather than just false. If the three-month average is distorted by a seasonal peak, see judge_annual_average.

Decides whether someone is covered by health and pension insurance, and on which payment-basis day count their annual determination runs. Call this before decide_regular_remuneration or judge_monthly_revision whenever the person is anything other than plainly full-time. Those tools take a worker_type, and guessing it changes a real number: the determination counts months of seventeen payment-basis days for an ordinary employee and eleven for a 短時間労働者. Get the classification wrong and the answer is wrong with no sign of it. 健康保険法第3条第1項第9号 covers anyone whose weekly hours and monthly days reach three-quarters of a comparable full-time worker. Below that, four further tests decide it: twenty hours a week, 88,000 yen a month, not a student, and a workplace of at least fifty-one insured people. The engagement must also be expected to run past two months. The 88,000 figure leaves out overtime, bonuses, commuting and family allowances. Folding those in is the usual route to a wrong answer, so ask for 所定内賃金 specifically rather than total pay. What counts as a comparable full-time worker, and whether someone is a student for this purpose, are facts about the workplace and the person. Ask rather than assume; the tool applies the tests to what you pass and names any it could not evaluate.

Workers compensation (労災保険) rates by business type, and the employer premium on a given 賃金総額. The whole premium falls on the employer — nothing is deducted from the employee, unlike every other statutory premium. Rates run from 2.5/1000 to 88/1000 depending on the industry, a 35-fold spread, so this cannot be estimated. Pass the 事業の種類の番号 from the 労働保険関係成立届; omit it to get the whole table.

Maps a monthly amount to its health grade (1-50) and pension grade (1-32), with the standard remuneration each resolves to and whether the pension grade was clamped. Use it to check a grade, not to compute premiums — calculate_payslip does that.

For anyone outside employee cover: the self-employed, freelancers, people between jobs. Use this instead of calculate_payslip when the person is not an employee. Running a freelancer through the payslip returns a figure computed under a different scheme entirely, with nothing in the answer to say so. If you are unsure which side someone is on, judge_worker_type decides it. The two schemes differ in how far they can be answered, and the difference matters. 国民年金法第87条 makes the pension contribution a statutory amount times a revision rate set each year by cabinet order — the same figure everywhere in the country, flat regardless of income. That comes back as a number. 国民健康保険法第76条 leaves the health contribution to each municipality, collected from the head of the household, and states no figure at all. Around 1,700 municipalities each set their own income-based, per-person and per-household components and their own ceilings. There is no national number to give. Do not estimate one, and do not present a figure from one city as though it were general — tell the person to ask their own municipality, which is what the response says. Exemptions, deferrals and the student special case all change what is actually paid, and whether they apply turns on income and household. Those are not judged here.

Checks the National Tax Agency check digit on a 13-digit 法人番号, or on a qualified invoice registration number (the same 13 digits prefixed with T). This proves the number is well formed. It does NOT prove the number is registered, and it does not identify the holder: sole proprietors receive invoice numbers that satisfy exactly the same rule, so a passing check digit must not be reported as evidence of a corporation. To confirm registration, use the National Tax Agency's own lookup.

Checks the National Tax Agency check digit on many qualified-invoice registration numbers in one call. Be careful how you report the result. A passing check digit means the shape of the number is right and nothing more. 消費税法第57条の2 provides both for the Commissioner to revoke a registration and for one to lapse, so a well-formed number can be unregistered, revoked or expired. Never tell someone a number is valid, verified or confirmed on the strength of this — say the format checks out, and that the register itself was not consulted. What it does earn is elimination. Anything failing on shape needs no lookup at all, so the list taken to 国税庁「適格請求書発行事業者公表サイト」 gets shorter. That site publishes the revocation and lapse dates, and offers a bulk download and a Web-API for large lists. Duplicates come back as given rather than folded together, and each result carries the index of its input, so rows line up with the caller list they came from.