Compound Interest Calculator
How much will it grow?
Compound interest is the interest your money earns on top of the interest it already earned. Set a starting amount, a monthly contribution and a rate, and see how much of the final total is pure growth.
How this is worked out
Your balance compounds monthly: each month it earns one-twelfth of the annual rate, then your monthly contribution is added. Total you put in is the starting amount plus every contribution; everything above that is compound growth.
These are nominal figures, they don't adjust for inflation, so real buying power would be lower. This is an educational tool, not financial or tax advice.
What compound interest actually is
Compound interest is the interest your money earns on top of the interest it has already earned. In the first month a balance earns a little; the next month that little is part of the balance, so it earns too. Repeat for years and the growth compounds on itself, curving the balance upward instead of climbing in a straight line. It's the quiet engine behind every long-term plan, from a single index fund to a full FIRE number.
Why monthly compounding and steady contributions matter
Two levers do most of the work here. Compounding monthly rather than once a year lets each month's interest start earning immediately, a small edge that stacks up over decades. And a regular monthly contribution matters more than people expect: every deposit buys time in the market, and money added early has the most years to compound. A steady amount left alone for twenty years often contributes more to the final balance than a larger lump sum added late. Time in the market, not timing the market.
One catch: this is a nominal number
The headline figure is nominal, it's the raw balance, not adjusted for inflation. Rising prices quietly erode what money buys, so a real, inflation-adjusted return would be lower, often by two to three percentage points a year. A million in twenty years won't buy what a million buys today. That's why serious retirement math, like the 4% rule and Runway's own projections, works in real terms. Treat this calculator as a feel for the shape of growth, then discount for inflation before you trust the headline.
Grow your real money, not a toy number
Runway projects your actual accounts forward with compounding and contributions, applies your country's tax and pension rules, and shows the inflation-adjusted view, so you see real buying power, not just a nominal headline. It launches on the App Store on 25 August 2026, and the beta is open now.
Runway projects your actual accounts forward with compounding and contributions, applies your country's tax and pension rules, and shows the inflation-adjusted view, so you see real buying power, not just a nominal headline. Your projection is free, forever.
Try the beta on TestFlight Download free on the App StoreFrequently asked
What is compound interest?
Compound interest is the interest your money earns on top of the interest it has already earned. Each period's growth is added to the balance, so the next period earns on a larger amount. Repeat that for years and the balance curves upward rather than rising in a straight line, and the longer money stays invested, the more of the final total comes from growth rather than from what you put in.
How is compound interest calculated here?
This calculator compounds monthly. Each month your balance earns one-twelfth of the annual rate, then your monthly contribution is added, and the new total earns interest the following month. The starting amount plus every monthly contribution is what you put in; everything above that is compound growth.
Does this account for inflation?
No. These are nominal figures: they show the raw balance without adjusting for rising prices. Because inflation erodes what money can buy, a real, inflation-adjusted return would be lower, often by two to three percentage points a year. Treat the final number as future money, not today's buying power.
Why do monthly contributions matter so much?
Regular contributions do two things: they add fresh money and they buy time in the market for each deposit to compound. Money added early has the most years to grow, so a steady monthly amount left alone for years often contributes more to the final balance than a larger lump sum added late. Consistency tends to beat timing.
Built by Dylan, maker of Runway
Italian, in Norway about eight years, building the cross-border FIRE planner that didn't exist for someone like me. This calculator is an educational tool, not financial or tax advice.
Related: FIRE calculator, Coast FIRE calculator, and the 4% rule.