How our calculators work

Last updated: 2026-11-11

Every result on MyWealthCheck comes from the same calculation engine, which runs in your browser. This page explains what it does, which assumptions it starts with, and where it is deliberately simple. You can change every assumption in the calculators.

Your FIRE number is worked out year by year

Many calculators multiply your yearly spending by 25 (the "4% rule"). We use a slower but more exact method instead:

1. Spending at FI. We take what you spend today and grow it with inflation until the age you want to stop working. That gives your first year's spending in future money.
2. Every year after that. Spending keeps rising with inflation each year until your plan age (for example 90). Any other income you enter in the planner, such as a pension or rent, is subtracted from that year's spending.
3. What each year costs today. Each future year's spending is discounted back to your FI date at the return of the money that pays for it (see the buckets below). Your FIRE number is the sum of these amounts: just enough to last until your plan age if returns are as expected.

Because the number depends on how long the money must last, retiring at 40 needs a bigger multiple of your spending than retiring at 65. The planner also offers a simple multiple of spending if you prefer that.

The bucket strategy after you stop working

The FIRE number is split into buckets, each invested for when it will be spent:

• Emergency fund: one year of spending in cash.
• Income bucket: the next 10 years of spending in bonds and cash, so a market fall doesn't force you to sell shares.
• Three growth buckets for the remaining years, with 50%, 70% and 90% in shares. The later the money is needed, the more of it is in shares.

Each year the simulation spends from the income bucket, then tops it back up from the growth buckets. After a year when shares fell, it doesn't sell shares; the income bucket is allowed to run down until markets recover.

How much to invest each month

• Your current savings grow at the expected return of their mix of shares and bonds.
• New monthly investments rise by the yearly increase you choose. They are invested 60% in shares, moving gradually down to 40% over the last 10 years before FI, so less is at risk just before you need it.
• Money grows month by month, with each month's investment made at the start of the month. The monthly amount we show is exactly what reaches the FIRE number under these assumptions.
• Earliest FI age is the first age (up to 75) at which your current investing reaches the FIRE number for that age.
• Coast FIRE number is the amount that, invested today with no new money, would grow into your FIRE number by your FI age.

The crash test

The calculators check whether the money lasts in two cases: returns as expected every year, and a 35% fall in shares in the first year after you stop working, with no rebound afterwards. A crash right at the start is the biggest risk for early retirees, because you are selling while prices are low. The planner adds more cases: two crashes, a decade of 2% returns, and inflation 2 percentage points higher.

These are fixed "what if" cases, not forecasts or probabilities. We don't run random (Monte Carlo) simulations yet.

Starting assumptions by country

Returns are long-run averages after tax and costs, set on the cautious side. Picking a country in the menu sets these starting values; you can change all of them.

Each calculator page lists the official sources for its country, such as national statistics offices and central banks.

What the calculators leave out

• Returns are smooth. Real markets go up and down, and the order of good and bad years matters.
• Tax is handled only through the after-tax returns you enter, not by country-specific tax rules.
• Pension rules, state pensions and benefits differ by country and change often. Add them as income in the planner if you expect them.
• Large one-off costs, such as care in old age, are only included if you add them as a goal.

These results are estimates for education, not advice. See the disclaimer.

Testing and corrections

The engine has automated tests with worked examples for several countries. If a result looks wrong, email hello@mywealthcheck.com with a link to it (use "Copy link to this result"). We fix errors and note the change date at the top of this page.