Compound Interest Calculator
Enter a starting amount, a rate, and a time horizon. Add regular contributions if you like, and see how compounding grows your money, split into principal, contributions, and interest earned.
1Starting principal
2Rate & time
% / year
years
3Additional contributionsOptional
Frequency
Max 240 (monthly × 20 years)
72,000 paid in over 20 years
Balance after 20 yearsRule of 72 · doubles every ~10.3 yrs
249,061
from 97,000 put in, 152,061 is interest · +157% total growth
Principal25,00010%
Contributions72,00029%
Interest152,06161%
Growth over time
PrincipalContributionsInterest
249k
187k
125k
62k
0
now4y8y12y16y20y
Around year 15, accumulated interest overtakes everything you have put in. From then on, your money is mostly earning rather than being added.
Year-by-year breakdown
How the balance builds, one year at a time.
Each row shows the running total at year-end: what you have contributed, the interest earned so far, and the resulting balance. Watch the interest column accelerate as it starts compounding on itself.
YearContributedInterestBalance
128,6001,86630,466
232,2004,11436,314
335,8006,77142,571
439,4009,86749,267
543,00013,43156,431
646,60017,49664,096
750,20022,09972,299
853,80027,27581,075
957,40033,06690,466
1061,00039,514100,514
1164,60046,665111,265
1268,20054,569122,769
1371,80063,279135,079
1475,40072,850148,250
1579,00083,343162,343
1682,60094,822177,422
1786,200107,357193,557
1889,800121,022210,822
1993,400135,895229,295
2097,000152,061249,061
This calculator compounds interest annually on the running balance. Contributions are made at the end of each chosen period and earn simple interest for the remainder of that year. Results assume a constant annual rate and are illustrative. Not financial advice.
How to use this calculator
From a starting balance to a 30-year projection, in four steps.
1
Set your starting principal
We prefill the amount you already have invested from your linked accounts. Not signed in, or starting fresh? Type any figure, even 0 works if you are building from contributions alone.
2
Enter rate and time
Add the annual interest rate you expect and how many years the money stays invested. Time is the single biggest lever in compounding: small changes here move the result more than you would think.
3
Add regular contributions
Optionally pay in a fixed amount each period. Choose the frequency and how many contributions you will make, at most one per period across the whole term, to model paying in for only part of the time.
4
Read your growth
See the final balance split into principal, contributions, and interest, watch it build year by year on the chart and table, and check the doubling time from the Rule of 72.
Key concepts
The mechanics behind the curve.
Compound interest
Interest earned not only on your original principal but also on the interest already added. Each period's growth becomes part of the base for the next, so the balance accelerates over time rather than rising in a straight line.
Principal
The money you start with: your initial deposit before any interest or contributions. It is the foundation the rest compounds on; a larger principal gives compounding more to work with from day one.
Compounding frequency
How often interest is calculated and added to the balance. This calculator compounds annually. More frequent compounding raises the effective return slightly, but the difference is small next to the rate and the number of years.
The Rule of 72
A quick mental shortcut: divide 72 by your annual interest rate to estimate the years it takes money to double. At 6%, that is about 12 years; at 9%, about 8. It is an approximation, but a remarkably good one for typical rates.
Future value
What a sum invested today, plus any regular contributions, will be worth at a future date once interest has compounded. It is the headline number this calculator solves for, and the basis of most long-term financial planning.
Regular contributions
Paying in a fixed amount each period, also called dollar-cost averaging when investing. Steady contributions can end up contributing more to the final balance than the starting principal, especially over long horizons.
The crossover point
The year when accumulated interest overtakes everything you have put in, principal plus contributions; the calculator flags it in the results readout. Before that point, your deposits do most of the work; after it, compounding does. At 7% with a steady monthly contribution and no starting lump sum, the crossover arrives around year 19. A higher rate moves it earlier; a lower one pushes it out past most people's patience. It is a useful milestone: past it, more than half your balance is money your money made.
Nominal rate vs APY
The nominal (stated) rate ignores how often interest is added; the APY, or effective annual rate, includes it. At 6% nominal compounded monthly you actually earn about 6.17% a year, because each month's interest starts earning its own. This calculator compounds once a year, so the rate you enter is effectively the APY. When comparing savings accounts or loans, always compare effective rates: a 5.9% rate compounded daily can beat a 6% rate compounded annually.
Tips for the long game
Small habits that compound into big balances.
Time beats timing
Years invested is the most powerful input here. Starting five years earlier usually outweighs a higher contribution or a slightly better rate: let the curve run as long as you can.
Automate your contributions
A standing transfer each period removes the monthly decision and keeps the compounding engine fed. Consistency matters more than the size of any single contribution.
Reinvest, don't withdraw
Compounding only works if the interest stays in. Every withdrawal resets part of the base your future growth builds on: leave it to keep working.
Mind the fees
A 1% annual fee compounds against you exactly the way interest compounds for you. Over decades it can quietly erase a large share of the final balance: check what you are paying.
Be honest about the rate
Long-run returns are uncertain. Modelling a conservative rate and treating anything above it as upside is safer than planning around an optimistic number that may not arrive.
Raise contributions as income grows
The calculator assumes a fixed amount each period, but real plans can step up. When your income rises, re-run the numbers with a higher contribution: moving from 300 to 400 a month is a 33% boost to everything future contributions earn.
FAQ
What is compound interest?+
Compound interest is interest earned on both your original principal and the interest already accumulated. Because each period's interest is added to the balance and then earns interest itself, the total grows faster and faster over time.
How is it different from simple interest?+
Simple interest is paid only on the original principal, so the balance grows in a straight line. Compound interest is paid on the growing balance, so it curves upward. Over a year or two the gap is small; over decades it is enormous.
How does compounding frequency affect the result?+
More frequent compounding means interest is added, and starts earning its own interest, sooner, which nudges the effective return up. This calculator compounds annually. The frequency you choose for contributions has only a modest effect under annual compounding.
What is the Rule of 72?+
Divide 72 by your annual interest rate to estimate how many years it takes your money to double. At 6% that is roughly 12 years; at 8%, about 9. It is an approximation that works well for everyday rates.
Should I prioritise a higher rate or more years?+
Both matter, but over long horizons time usually wins. Because compounding is exponential, extra years let the largest balances earn the most interest. Chasing a higher rate often means taking more risk; extending your time horizon does not.
Are these returns guaranteed?+
No. This tool shows what a constant annual rate would produce. Real investment returns vary year to year and can be negative; cash-savings rates change over time. Treat the result as an illustration of how compounding behaves, not a promise of a specific outcome.
How do I factor in fees, tax and inflation?+
This calculator keeps things simple and ignores them. To model the real-world drag of fees, inflation, tax on gains, and withdrawals, and compare two scenarios side by side, use the advanced Investment Return Calculator.
How much will 10,000 be worth in 20 years?+
It depends entirely on the rate. At 5%, a 10,000 lump sum compounding annually becomes about 26,500 after 20 years; at 7%, about 38,700. Over 10 years those figures are roughly 16,300 and 19,700. That is with no contributions at all: the principal alone doing the work. Enter your own amount, rate and horizon above to see the exact curve, and remember that the last few years add the most.
How much can saving 300 a month grow to?+
At a 7% annual return, 300 a month grows to roughly 51,300 after 10 years and about 152,300 after 20, from deposits of 36,000 and 72,000 respectively. The 20-year figure is not double the 10-year one, it is nearly triple: the extra decade lets the earliest contributions compound the longest. That widening gap between money paid in and the final balance is the whole argument for starting now.
What happens if I stop contributing after a few years?+
The balance keeps compounding; only the new deposits stop. Using the number-of-contributions input: 300 a month at 7% for the first 10 years, then nothing, reaches about 101,000 by year 20. Making the same 36,000 of contributions in years 11 to 20 instead ends near 51,300. Identical money in, roughly double the outcome, because front-loaded contributions get the full 20 years to grow.
This calculator compounds interest annually on the running balance. Contributions are made at the end of each chosen period and earn simple interest for the remainder of that year, then compound annually. Results assume a constant annual rate of 7% and are illustrative. Not financial advice.
Standard future-value compounding · illustrative, not financial advice