Savings Calculator
Two tools in one. In Growth mode, see how your monthly savings compound into a future balance — or flip it around and find the monthly savings you need to hit a goal. In Runway mode, find out how long your savings will last with regular withdrawals: essential for retirement planning. Runs entirely in your browser.
Frequently Asked Questions
How long will my savings last?
That depends on three things: how much you start with, how much you withdraw each month, and what return your remaining balance earns. Switch to Runway mode above — enter your balance, monthly withdrawal, expected return, and an inflation adjustment — and the calculator shows how many years and months your money lasts before it runs out.
How long will my money last in retirement?
Runway mode is built for exactly this. The classic rule of thumb is the 4% rule: withdrawing about 4% of your portfolio per year (adjusted for inflation) has historically lasted 30 years. Try your numbers above — for example, $500,000 with $1,667/month withdrawals (4%/year) at a 5% return — and see how the math holds up with your own inflation and return assumptions.
How much do I need to save per month to reach a goal?
Enter a savings goal in Growth mode and the calculator flips the formula around: it computes the exact monthly savings needed to hit that target in your chosen time frame at your expected return. The monthly amount is what you'd need to save starting today.
How is the growth calculated?
With monthly compounding: each month your balance grows by (annual return ÷ 12), then your savings are added. Future value = starting balance × (1 + r/12)12n + monthly savings × (((1 + r/12)12n − 1) ÷ (r/12)). The "growth" figure is the future balance minus your starting amount and everything you saved.
What return should I use?
Use a conservative real (after-inflation) return for planning: many advisors use 4–6% real for stock-heavy portfolios, 2–3% for conservative ones. Runway mode lets you add an inflation increase to withdrawals — keep the return as a nominal (pre-inflation) figure so the two stay consistent.
Why does my runway change so much with small tweaks?
Drawdown math is exponential: withdrawing $3,000/month instead of $2,500/month doesn't cut your runway by a sixth — it compounds, because every extra dollar withdrawn early also loses all its future growth. That sensitivity is why running multiple scenarios (best case, base case, worst case) matters more than one "exact" answer.