Methodology
Last updated October 19, 2026
All amounts are in today’s money. Taxes and fees are not included. For a couple, money is entered as household totals and the timeline uses the first person’s age.
1. Real growth rate
Growth and inflation are combined into one real growth rate:
real rate = (1 + growth) ÷ (1 + inflation) − 1
With 7% growth and 3% inflation that is 1.07 ÷ 1.03 − 1 = 3.88%. We never use growth minus inflation (4%), because that overstates growth a little every year, and over 35 years the error adds up.
2. Pot needed at retirement
Your investments only need to cover the part of your spending that a pension does not:
target = max(0, yearly spending − yearly pension) ÷ withdrawal rate
Spending $60,000 with $20,000 of pension at a 4% withdrawal rate gives $40,000 ÷ 0.04 = $1,000,000. If the pension covers all your spending, the target is 0.
3. Coast FIRE number
The Coast FIRE number at any age is the target shrunk back to that age at the real growth rate:
Coast FIRE number(age) = target ÷ (1 + real rate) ^ (retirement age − age)
At age 30 with retirement at 65 that is $1,000,000 ÷ 1.0388^35 = $263,555.
Percent funded = amount invested ÷ Coast FIRE number today.
4. The age you can stop saving
We step through your life one year at a time, starting at your current age:
- If your investments are at least the Coast FIRE number for that age, that is the age you can stop saving.
- If not, your investments grow for a year at the real rate and your yearly saving (monthly saving × 12) is added.
If you never reach the Coast FIRE number by your retirement age, the calculator says so instead of giving an age.
Pot at retirement is your balance at the stop-saving age, grown at the real rate with nothing added until you retire. If you never reach Coast FIRE, it is your balance at retirement with saving continued every year.
5. Your next step
The next-step suggestion tries adding small amounts each month (in the US, $100, $200, $300, $400 and $500) and runs the same calculation each time. It shows the smallest amount that brings your stop-saving age at least one year sooner. If none of them does, it says so. It never shows a suggestion the calculation did not produce.
6. If markets do worse or better
The Plan section reruns your plan at 5%, 7% and 9% growth with everything else the same. This is not a forecast. It shows how much your result depends on growth.
7. Monthly check-ins
A check-in records the month, what your investments are worth, what you added, your percent funded and your stop-saving age, measured against your plan at the time. The change since your previous check-in is split into what you added and the rest, which is shown as growth.
How the calculator is tested
Before any change goes live, the calculator is run against eight fixed cases with known answers, including the example on this page ($263,555, stop saving at 40, about $1,014,884 at 65). If any answer moves, the change does not ship.
Defaults and sources
| Setting | Default | Source |
|---|---|---|
| Growth before inflation | 7% | A common long-run assumption for a mix of stock market index funds. It is not a forecast. You can change it. |
| Withdrawal rate | 4% | William P. Bengen, "Determining Withdrawal Rates Using Historical Data", Journal of Financial Planning, 1994. You can change it. |
Country defaults
| Country | Currency | Default inflation | Pension label | Next-step amounts | Source |
|---|---|---|---|---|---|
| United States | USD | 3% · Being checked | Social Security | $100 – $500 | Being checked |
| United Kingdom | GBP | 3% · Being checked | State Pension | £100 – £500 | Being checked |
| Canada | CAD | 3% · Being checked | CPP and OAS | C$100 – C$500 | Being checked |
| Australia | AUD | 3% · Being checked | Age Pension | A$100 – A$500 | Being checked |
| India | INR | 3% · Being checked | EPS or NPS pension | ₹5,000 – ₹25,000 | Being checked |
| Euro area | EUR | 3% · Being checked | State pension | €100 – €500 | Being checked |
| United Arab Emirates | AED | 3% · Being checked | GPSSA pension | AED 500 – AED 2,500 | Being checked |
| Japan | JPY | 3% · Being checked | Public pension (nenkin) | ¥10,000 – ¥50,000 | Being checked |
| Saudi Arabia | SAR | 3% · Being checked | GOSI pension | SAR 500 – SAR 2,500 | Being checked |
| Taiwan | TWD | 3% · Being checked | Labor Insurance and Labor Pension | NT$3,000 – NT$15,000 | Being checked |
| Hong Kong | HKD | 3% · Being checked | MPF | HK$1,000 – HK$5,000 | Being checked |
| Singapore | SGD | 3% · Being checked | CPF LIFE | S$100 – S$500 | Being checked |
| Malaysia | MYR | 3% · Being checked | EPF | RM 500 – RM 2,500 | Being checked |
Inflation defaults marked "Being checked" use 3% until each country's figure has been checked against its central bank or statistics office. The source link will appear here.