JP Kyokai-kenpo social-insurance simulator — prefecture-level + bonus

Health insurance and pension calculations expose salary and employment profile

The Kyokai-kenpo premium calculation requires monthly pay, bonus amount, age bracket (for nursing-care), and prefecture. Monthly pay plus bonus reconstructs annual gross salary. Age bracket reveals whether the employee is in the nursing-care premium zone (40–64). Prefecture signals which regional branch of Kyokai-kenpo the employer uses — and implicitly, that the employer is not a large corporation with its own health-insurance union.

Taken together, these inputs constitute an employment and income profile: approximate annual salary, age range, and insurance tier. Financial institutions and insurers use exactly this kind of profile for product targeting, and tools that send it to a server create an exposure that has no technical justification.

Why a payroll deduction check should not need a server

Computing standard remuneration grade → premium = grade × rate × 1/2 is table lookup followed by multiplication. All the data needed — the 50-tier health insurance table, the 32-tier pension table, and the prefecture-by-prefecture rate table — are static, published publicly by MHLW, and small enough to embed directly in a browser page.

This tool bundles every prefecture’s FY2026 Kyokai-kenpo rate alongside the care-insurance (1.62%), pension (18.3%), and child-care support (0.23%) rates into the page itself. Nothing is fetched from a server during calculation. Check the DevTools Network tab to confirm.

47-prefecture rate table, embedded in browser

Rates range from Niigata at 9.21% to Saga at 10.55% for FY2026 (the first national average cut in 34 years, from 10.0% to 9.9%). The tool maps your prefecture selection to the correct rate, applies the standard remuneration grade table to your monthly pay (rounded to the nearest grade), and separately applies the bonus premium calculation (annual health-cap ¥5.73M, pension per-bonus cap ¥1.5M). All three output columns — full / employee half / employer half — update live with no network call.

Cross-checking your payslip after the September grade revision

The most common reason for a payslip deduction to differ from this tool’s estimate is a grade mismatch: your standard remuneration grade is fixed at the September 定時決定 and may lag a pay raise or cut until the next annual reset or 随時改定. The second common cause is rounding: your employer may use floor rather than round-half-up for the employee half. Check the September and October payslips against this tool to identify the source of any discrepancy. For the post-employment switch to National Health Insurance, kokuho-hoken-jp handles that side; if you also have side income on top of salary, side-income-tax-jp covers the combined income-tax estimate — both stay browser-only.

Mid-year pay changes and the on-demand grade revision (随時改定)

When fixed monthly pay changes by enough to shift the standard remuneration grade by two or more levels, an employer must file a grade-change notice (月額変更届) with the Japan Pension Service (日本年金機構). The trigger is based on the average of three consecutive months of actual pay after the fixed-pay change — not a single month’s figure. Only changes to fixed compensation (base pay, fixed allowances) trigger this process; fluctuating components such as overtime or commissions alone do not.

Use this tool to compare your pre-raise and post-raise grade estimates side by side. If the grade shifts under these criteria, the new premium takes effect from the second month after the three-month assessment window closes. Knowing the upcoming premium change in advance is useful for household budget planning after a job change or promotion.

The child-care support levy added in April 2026 (令和8年度)

Starting with April 2026 premiums, a new child-care and child-rearing support levy (子ども・子育て支援金) at 0.23% was introduced, split equally between employee and employer. It is collected alongside Kyokai-kenpo health insurance but is a separate line item on payslips (see こども家庭庁 official guidance). For a standard remuneration of 300,000 JPY, the employee share is approximately 345 JPY per month (300,000 x 0.23% / 2).

The health-insurance rate cut from 10.0% to 9.9% in the same period partially offsets the new levy for most employees, so the net effect on take-home pay is typically small. This tool enables the child-care support calculation by default, reflecting current FY2026 rates. Toggle it off to model the pre-April 2026 premium structure for historical reference.