Retirement Calculator

Retirement planning is the hardest financial calculation most people never do, because the math involves compounding over forty years and the inputs (returns, inflation, your retirement-age expenses) are guesses. So most people don't run the numbers, contribute by feel, and hope.

Enter current age, retirement age, current savings, monthly contribution, expected real return, and expected retirement expenses. The calculator projects your portfolio's value at retirement, the monthly income it can sustain at a 4% safe withdrawal rate, and the gap between that income and your stated expenses. A scenario table varies contribution rate to show the relationship between what you save now and what you can spend later.

Real return — return after inflation — is the right input. Using nominal returns and not adjusting expenses for inflation produces wildly optimistic numbers. Long-term real returns on a stock-heavy portfolio are historically around 5–7%; treat anything above that as aggressive.

The core math chains compound interest with withdrawal arithmetic. Future value at retirement = current savings × (1 + r)^years + monthly contribution × [((1 + r)^(years × 12) − 1) / r], where r is the monthly real return rate. Sustainable annual income from that portfolio = future value × safe withdrawal rate. The compounding term is what does most of the work — contributions made early compound for decades and are worth several multiples of the same dollars contributed late. This is the famous "magic" of compound interest, and it's not magic; it's just exponential math.

Worked example: someone aged 30 with $40K saved, contributing $1,500/month, expecting 5% real return, retiring at 65. Future value = 40000 × (1.05)^35 + 1500 × 12 × [((1.05)^35 − 1) / 0.05]. The first term ≈ 220K, the second ≈ 1.7M, total ≈ 1.92M. At a 4% withdrawal, that's $77K/year in present-day purchasing power. Now run the same scenario with $1,500/month starting at age 40: only 25 years of contributions, future value ≈ 980K, sustainable income ≈ $39K. Same monthly contribution, ten years less compounding, half the retirement income. The advantage of starting early is the most consequential variable in the entire calculation.

The problem with all of this: every input is a guess. Real returns 35 years ahead are unknowable. Inflation rates over the period are unknowable. Your future spending patterns are unknowable. Health-care costs in retirement are notoriously underestimated. Social Security benefits 30+ years out depend on political decisions. The calculator output should be read as a planning anchor, not a prediction — sensitivity analysis (what if returns are 4% instead of 5%? what if I retire at 67 instead of 65?) is more valuable than a single point estimate.

A structural risk the basic model ignores: sequence-of-returns risk. The Trinity 4% rule assumes a stock/bond portfolio with returns drawn from historical sequences. A retiree who hits a bear market in their first five years of withdrawals is in much worse shape than one who hits the same returns in years 25-30, because the early bear depletes principal that can't recover. Modern guidance (Wade Pfau, Michael Kitces) suggests 3.5% as a more conservative anchor for current valuations and longer retirements. The calculator can swap in 3.5% as a sensitivity check.