FI Tools — FIRE Calculator for Couples & Cross-Border Planners

How the PlanTogetherFI FIRE calculator works

This free FIRE calculator estimates when you can reach financial independence (FI) — the point where your investments can cover your living costs without needing a salary. Enter your ages, savings, monthly contributions, expected returns, and target retirement spending, and it projects your net worth year by year until it crosses your FI number. It is built for the situations generic calculators ignore: couples planning together, staggered retirement dates, multiple currencies, and cross-border lives in Japan and beyond.

Everything runs in your browser — nothing you enter is sent to a server or stored.

What is your FI number?

Your FI number is the size of the investment portfolio that can fund your retirement spending indefinitely. The calculator derives it from two inputs: your target retirement spending and your safe withdrawal rate:

FI number ≈ annual retirement spending ÷ withdrawal rate

At a 4% withdrawal rate that is roughly 25× your yearly spending; at a more conservative 3.5% it is closer to 29×. Because the tool applies inflation, the spending figure it uses is the future cost of today's lifestyle — not just today's number.

What is a safe withdrawal rate (the 4% rule)?

The 4% rule comes from research suggesting a retiree can withdraw about 4% of their starting portfolio in year one, adjust that amount for inflation each year after, and have a high chance of not running out over a roughly 30-year retirement. It is a planning heuristic, not a guarantee — longer retirements, weaker returns, or an early market crash all change the safe number. That is why the calculator lets you set your own withdrawal rate and stress-test more conservative assumptions. Early retirees, whose money must last far longer than 30 years, often plan around 3–3.5%.

How do couples and staggered retirement change the math?

Most calculators assume one person retiring once. Real couples rarely work that way. This tool models both partners — different ages, savings, incomes, and retirement dates — and the bridge period when one partner has stopped working while the other still earns. That bridge is where many couple plans quietly break: household spending continues while income drops, so the portfolio has to carry more of the load earlier than a single-person model predicts. Modeling both timelines together gives a far more honest FI date than adding two separate calculations.

How does cross-border and multi-currency planning change your plan?

For foreigners and international couples, money rarely sits in one currency. You might earn yen, hold investments in dollars, and plan to spend part of retirement in another country. The calculator lets you set a base currency and enter savings, income, and spending in whichever currency each actually lives in, then converts consistently so your FI number and date reflect your real, mixed situation. You can also model one-time life events (an inheritance, a property sale, a big move) and scheduled changes to contributions, so the projection bends where your real life bends.

What do the NISA and iDeCo inputs do?

If you invest in Japan, the calculator has fields for NISA and iDeCo monthly contributions (and bonus allocations), because these tax-advantaged accounts are the backbone of most FI plans here. They feed into your total invested cash flow and compound across the projection. The tool stays neutral on products — it models the contributions, not a recommendation to use any particular broker or fund.

Frequently asked questions

How accurate is the FIRE calculator? It is a planning estimate, not a prediction. Real returns, inflation, taxes, and life events vary, so treat the FI date as a well-reasoned projection you revisit as your numbers change — not a fixed promise.

Do I need an account to use it? No. There is no sign-up, and every calculation happens locally in your browser. Nothing you enter is transmitted or stored on a server.

Can I use it as a single person? Yes. Fill in one partner and leave the other blank — the tool works the same for solo planners as for couples.

What retirement spending number should I enter? Use the annual cost of the lifestyle you want in retirement, in today's money. The calculator applies inflation to project what that lifestyle will actually cost by the time you retire.

Is this financial advice? No. PlanTogetherFI is an educational planning tool, not financial, tax, investment, or legal advice. Confirm important decisions with a qualified professional and the relevant official sources.