JP National Health Insurance simulator — household + reductions

Household NHI data reveals income structure across all family members

National Health Insurance is calculated per household. Entering every member’s income, age group, and employment type means disclosing the financial structure of an entire family at once: who earns what, how old they are, and whether income is salary or self-employment. That is more information than a single tax return, and it covers a household rather than an individual.

NHI covers the self-employed, freelancers, and retirees — groups whose income is more variable and who are prime targets for financial products such as whole-life insurance, investment trusts, and income-protection plans. Tools that collect household income data in this segment have clear commercial uses.

Why household income belongs in your browser, not on a server

The 7/5/2 reduction threshold uses household total income (before the basic deduction). That aggregate figure is a direct proxy for household wealth level and tax status. Insurance comparison sites, FP portals, and household-budget apps that offer NHI calculators have an incentive to associate that income level with a user account for product targeting.

The NHI premium formula — income levy (old 旧ただし書き method: total income minus ¥430k) × rate, plus per-member flat — is static arithmetic on publicly available municipal rates. It does not require a server.

FY2026 four-category calculation with reduction logic in browser

The tool computes all four categories — medical (¥670k cap), late-elder support (¥260k), nursing care for ages 40–64 (¥170k), and child-care support (¥30k, under-18 flat waived per FY2026) — as income levy plus per-member flat for each household member. The 7/5/2 reduction is applied to the flat portion based on household total income and salary-earner count. Pre-school children get an additional 50% flat reduction. All steps run in JavaScript; the Network tab stays empty.

The Tokyo 23-ward preset embeds the FY2026 unified rates. The custom panel lets you enter your municipality’s actual rates if they differ.

Reconciling this estimate against the annual premium notice

Common sources of discrepancy between this tool and the actual notice: (1) the 65+ pension ¥150k deduction in the reduction threshold (this tool skips it — subtract ¥150k manually from that member’s income); (2) a household-flat (平等割) component if your city uses the three-pronged formula (add the flat amount to the per-member field as a workaround); (3) municipal-specific rate adjustments for tiers above the standard. Enter the correct custom rates from your July premium notice to narrow the gap. To see the NHI premium in context with full annual income and resident tax, freelance-tax-jp covers the headline figure; for the monthly due-date calendar, freelance-tax-calendar-jp handles that view — both stay browser-only.

Old 旧ただし書き method, the four-component structure, and 7/5/2 reduction thresholds

NHI premiums are assessed under Local Tax Act Article 703 and NHI Act Article 76 as a combination of up to four components: (1) income levy (each member’s total income minus the JPY 430k basic deduction × rate), (2) per-member flat (定額 per insured), (3) household flat (平等割, not used by all municipalities), (4) asset levy (property-tax base × rate, increasingly dropped). Tokyo’s 23 wards use a two-component model (income levy + per-member) with no household flat or asset levy. Many designated cities (Osaka, Nagoya, Fukuoka) use a three-component model adding the household flat.

The 7/5/2 reduction (Local Tax Act Article 703 paragraph 7) cuts the per-member and household-flat portions when household total income falls below tiered thresholds. The FY2026 thresholds are: 70% reduction at household total income ≤ JPY 430k; 50% reduction at JPY 430k + 305k × insured count; 20% reduction at JPY 430k + 560k × insured count (source: MHLW NHI premium overview). The thresholds scale with the count of salary-earner / pension-earner household members, which this tool implements. Note: the reduction test uses the pre-deduction income figure, not the post-blue-return or post-deduction taxable income — entering the wrong basis here misreports the reduction tier.

The four-category caps (medical / late-elder support / care / child-care) and FY2026 changes

NHI premiums split into four categories — medical (JPY 670k cap), late-elder support (JPY 260k cap), nursing care for ages 40–64 only (JPY 170k cap), and child-care support (JPY 30k cap, added FY2024). Total ceiling is JPY 1.13M including the child-care line. High-income households hit these caps and pay the same regardless of further income increases, so this tool’s estimate converges with actual notices once household taxable income exceeds roughly JPY 12–15M.

Notable FY2026 points: (1) the child-care support levy of 0.23% (from Children and Families Agency policy) applies to all households regardless of whether they have children, billed separately from the medical levy; (2) the 50% additional reduction for pre-school children’s per-member levy continues (since FY2022); (3) age 70–74 ‘pre-elder’ members pay only a single-tier medical levy (vs the standard two-tier) and are exempt from the late-elder support and care levies. All three rules are implemented here. At age 75, members move out of NHI to the Late-Elder Medical System (後期高齢者医療制度) and pay a separate premium under that scheme (not covered by this tool). For a member who reaches age 75 mid-year, the NHI premium covers only the months before the transition; the late-elder system covers the remainder of the year on its own rate sheet.