How the Retirement Runway Calculator Works
Most retirement calculators stop at one big number: what you might have saved by the day you retire. That number matters less than what it can pay you each month, and for how long. This calculator follows your savings through both stages. It projects your balance to retirement, turns it into monthly income alongside Social Security, and then tracks how long the money lasts if you keep spending your target amount. It uses US rules for 401(k) and Roth accounts, required minimum distributions and Social Security, with 2026 figures from the SSA and IRS.
The Calculation, Step by Step
- Savings at retirement. Total = savings × (1 + r)n + (monthly contribution × 12) × [((1 + r)n − 1) ÷ r], where r is your expected return and n is the years until you retire. Growth is compounded once a year and each year's contributions are added at the end of the year. Your current 401(k) and Roth balances grow on their own; new contributions are split between them in the same proportion as today's balances (70/30 if both are empty). Anything else in your total, such as a brokerage account, simply grows.
- Social Security. The tool assumes your contributions are 10% of your pay, so monthly pay ≈ contribution ÷ 0.10 (capped at $15,375, the 2026 taxable maximum ÷ 12). It applies the SSA's 2026 benefit formula to that figure: 90% of the first $1,286, plus 32% up to $7,749, plus 15% above that. The result is kept between $200 and $4,152 (the 2026 maximum at full retirement age), then adjusted for your claiming age, from 70% at 62 to 124% at 70.
- Safe withdrawal. Monthly withdrawal = total × 4% ÷ 12, or 3.5% in FIRE mode.
- Tax. Your tax rate applies to the pre-tax (401(k)) share of each withdrawal; Roth and other savings come out untaxed. For Social Security, the tool works out the taxable part with the IRS single-filer rules (nothing is taxable below $25,000 of "provisional income", and at most 85% above $34,000) and taxes that part at your rate.
- Monthly gap = after-tax withdrawal + after-tax Social Security − target expenses. A positive number is a surplus.
- Runway. From your retirement age, each year the portfolio grows at your expected return and then pays whatever your after-tax Social Security doesn't cover, grossed up for tax. The runway is the age the money lasts to (shown as 100+ beyond that). It is compared with the average life expectancy at your current age from the SSA's 2023 period life table, taking the average of the figures for men and women.
- First RMD. Your 401(k) balance projected to your RMD start age, divided by the IRS Uniform Lifetime Table factor (26.5 at 73, 24.6 at 75).
Worked Example
A 45-year-old plans to retire at 67. They have $100,000 saved ($80,000 in a 401(k), $20,000 in a Roth IRA) and contribute $600 a month. They expect a 5% return after inflation, want $4,500 a month in retirement, will claim Social Security at 67, and expect a 12% tax rate. They were born after 1959, so their RMD age is 75. Type these into the calculator to follow along.
| Step | Calculation | Result |
|---|---|---|
| Savings at 67 | $100,000 × 1.0522 + $7,200 × 38.505 | $569,764 ($455,811 in the 401(k), $113,953 in the Roth) |
| Social Security at 67 | 0.9 × $1,286 + 0.32 × ($6,000 − $1,286) | $2,665.88 / month |
| 4% withdrawal | $569,764 × 4% ÷ 12 | $1,899.21 / month |
| Withdrawal after tax | $1,899.21 × (1 − 12% × 80% pre-tax) | $1,716.89 / month |
| Social Security after tax | $4,693.56 taxable a year × 12% | $2,618.94 / month |
| Monthly gap | $1,716.89 + $2,618.94 − $4,500 | −$164.17 / month |
| Runway | Needs $24,970 a year before tax; the fund earns about $28,488 in year one | Lasts to 100+ |
| First RMD at 75 | $728,443 projected 401(k) ÷ 24.6 | $29,612 a year |
The two headline results seem to disagree, and that is useful. The 4% rule is designed to leave the savings largely intact through bad markets, so on that basis this plan is $164 a month short. The runway assumes a steady 5% a year and lets the balance run down, and on that basis the money lasts past 100. The truth for most people falls somewhere between. Small changes matter: at $6,500 a month of spending the runway ends at 83, and with a 3% return instead of 5% it ends at 90. Average life expectancy at 45 is 81.
Reading Your Results
- Monthly Income Gap shows the surplus or shortfall from the 4% withdrawal plus Social Security, after tax, against your target spending.
- Retirement Runway is the bar: grey for the years until you retire, then green if the money lasts at least five years past your life expectancy, amber if it lasts up to five years past it, and red if it runs out first. The small mark is your life expectancy.
- Projection Breakdown shows your 401(k) and Roth balances at retirement, your first-year RMD, the estimated Social Security benefit before tax, and the lines that make up the gap.
- Compare lets you test a second retirement age side by side.
Assumptions You Should Know
- Returns arrive smoothly every year. Real markets don't, and a bad run early in retirement hurts far more than the average suggests.
- Spending stays flat in today's dollars, so enter a return after inflation.
- Social Security is counted from your retirement age even if you claim later. If you retire before your claiming age, the runway will look better than it should.
- The Social Security figure is a rough estimate from your contributions, not your earnings record.
- Tax uses single-filer thresholds. Married couples have higher thresholds ($32,000 and $44,000), so the tool may overstate their tax. It covers one person's benefit, and it leaves out state taxes and tax on gains in brokerage accounts.
- The RMD estimate keeps adding contributions until your RMD age and ignores any withdrawals before then, so treat it as an upper estimate.
- Life expectancy is an average for everyone your age; about half of people live longer, so plan beyond it.
Frequently Asked Questions
Why can the runway say 100+ when my gap is negative?
They answer different questions. The gap asks whether a cautious 4% withdrawal covers your spending. The runway asks how long the money lasts if you spend your full target and your returns hold up. A negative gap with a long runway means your plan depends on drawing the savings down and on returns not disappointing.
Should I enter my return before or after inflation?
After inflation. Your spending target stays the same every year, which only makes sense in today's dollars, so the return should be in today's dollars too.
How do I get a better Social Security estimate?
Sign in to your my Social Security account at ssa.gov, which estimates your benefit from your actual earnings record. You can then pick the claiming age here that matches the estimate you want to test.
Can I use this if I save outside the US?
Only partly. The savings projection and the 4% withdrawal apply anywhere, but Social Security, RMDs and the tax rules follow US law and can't be switched off, so results won't fit other countries' pension systems.
Related Tools and Sources
- APR / APY Interest Converter: compare savings rates on an equal footing before you choose where to keep cash.
- Insurance Premium Estimator: includes ACA health premiums, which matter if you retire before Medicare at 65.
Figures used: SSA benefit formula bend points, SSA maximum benefit, SSA period life table, IRS Publication 915 (taxation of benefits), IRS Publication 590-B (RMDs) and the IRS 2026 401(k) limit.
Last reviewed September 2026. For education only, not financial or tax advice. How we check our calculations.