Compound interest calculator
Move four sliders. Watch a thirty-year story unfold. This is the same model professional planners use to project a portfolio — pared down, kept honest, and made obvious.
What is a compound interest calculator?
A compound interest calculator shows how a starting balance plus ongoing contributions grow when each period's earnings get reinvested. The math is identical to what professional planners use; this version exposes the four inputs that move the answer most — principal, contribution, return rate, and time horizon — so you can build intuition fast.
The full breakdown.
| Year | Contributed | Interest | Balance | Interest share |
|---|---|---|---|---|
| Y01 | $16,000 | $919 | $16,919 | 5% |
| Y02 | $22,000 | $2,339 | $24,339 | 10% |
| Y03 | $28,000 | $4,294 | $32,294 | 13% |
| Y04 | $34,000 | $6,825 | $40,825 | 17% |
| Y05 | $40,000 | $9,973 | $49,973 | 20% |
| Y06 | $46,000 | $13,782 | $59,782 | 23% |
| Y07 | $52,000 | $18,299 | $70,299 | 26% |
| Y08 | $58,000 | $23,578 | $81,578 | 29% |
| Y09 | $64,000 | $29,671 | $93,671 | 32% |
| Y10 | $70,000 | $36,639 | $106,639 | 34% |
| Y11 | $76,000 | $44,544 | $120,544 | 37% |
| Y12 | $82,000 | $53,455 | $135,455 | 39% |
| Y13 | $88,000 | $63,443 | $151,443 | 42% |
| Y14 | $94,000 | $74,587 | $168,587 | 44% |
| Y15 | $100,000 | $86,971 | $186,971 | 47% |
| Y16 | $106,000 | $100,683 | $206,683 | 49% |
| Y17 | $112,000 | $115,820 | $227,820 | 51% |
| Y18 | $118,000 | $132,486 | $250,486 | 53% |
| Y19 | $124,000 | $150,790 | $274,790 | 55% |
| Y20 | $130,000 | $170,851 | $300,851 | 57% |
One equation. Thirty years of consequence.
Compound interest is the engine behind why time, not contribution size, is the biggest lever most people have. Each period your balance earns a return — and then that return earns its own return next period, recursively.
Stretch this over decades and the curve gets unintuitive fast: a 25-year-old saving $300/mo ends up with a larger balance than a 35-year-old saving $600/mo, even after the 35-year-old contributes twice as much in absolute dollars.
The calculator above runs the standard future-value formula for a series — with monthly or yearly compounding — so the numbers match what a financial planner's spreadsheet would produce.
- P
- Principal — the starting balance
- PMT
- Periodic deposit — your monthly or yearly contribution
- r
- Annual rate — your expected return, as a decimal
- n
- Compounding frequency — 12 for monthly, 1 for yearly
- t
- Time horizon — number of years
Six ways to actually use these numbers.
Start now, not next year.
One year of compounding at age 25 can outweigh five years of compounding at age 45. Time is the most valuable input.
Index funds beat stock picking.
For most long-horizon investors, a broad index fund earns the market return at minimal fee drag — the simplest path to the calculator's projection.
Automate the contribution.
Friction kills compounding. Set the monthly deposit to auto-draft on payday so you don't have to think about it.
Don't time the market.
Missing the 10 best days of a decade can halve your return. Time IN the market beats timing the market for nearly every long horizon.
Stress-test at 5%, not 10%.
Conservative return assumptions force higher savings rates. A plan that works at 5% returns is a plan that always works.
Recheck annually.
Salary changes, market conditions shift, life happens. Re-run the calculator each January and adjust your monthly contribution to stay on track.