Compound Interest Calculator
CalculatorsWhat a rate really earns over time, compounded yearly to daily, with monthly additions if you make them.
In any currency.
Leave at 0 for a single deposit.
After 10 years
1,647.01
1,000.00 put in, 647.01 of interest
P × (1 + r/n)^(n × t)
- Effective annual rate
- 5.116%
- 5% compounded monthly
- (1 + r/n)^n − 1
- Interest earned
- 647.01
- 64.7% of what went in
- Doubles in
- 13.9 years
- rule of 72 says 14.4
- ln 2 ÷ ln(1 + effective rate)
- Put in
- 1,000.00
- start + monthly × 12 × years
The same money, compounded more often
- Yearly1,628.89
- Half-yearly1,638.62
- Quarterly1,643.62
- Monthly1,647.01
- Daily1,648.66
Year by year
| Year | Put in | Interest | Balance |
|---|---|---|---|
| 1 | 1,000.00 | 51.16 | 1,051.16 |
| 2 | 1,000.00 | 104.94 | 1,104.94 |
| 3 | 1,000.00 | 161.47 | 1,161.47 |
| 4 | 1,000.00 | 220.90 | 1,220.90 |
| 5 | 1,000.00 | 283.36 | 1,283.36 |
| 6 | 1,000.00 | 349.02 | 1,349.02 |
| 7 | 1,000.00 | 418.04 | 1,418.04 |
| 8 | 1,000.00 | 490.59 | 1,490.59 |
| 9 | 1,000.00 | 566.85 | 1,566.85 |
| 10 | 1,000.00 | 647.01 | 1,647.01 |
No tax, fees or inflation are taken off, and the rate is assumed to stay the same for the whole period.
How this is calculated
balance = P × (1 + r/n)^(n × t) additions = C × ((1 + j)^(12t) − 1) ÷ j each month-end j = (1 + r/n)^(n/12) − 1 effective = (1 + r/n)^n − 1 P starting amount r annual rate n compoundings a year t years C added each month
Read this before you rely on the number
This is a projection, not financial advice.
This is arithmetic on a rate you chose, held constant for the whole period — not a forecast of what any account or investment will pay. Real rates change, investments can fall as well as rise, and tax, fees and inflation all reduce what you keep. Use it to understand how compounding behaves and to compare offers on the same basis; the figure that binds you is the one in the account's own terms.
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How it works
Compound interest is interest on interest. In the first year a deposit earns interest on what you put in; from then on it also earns interest on the interest already added. The growth is small at first and steepens every year, which is why the same rate does so much more over thirty years than over ten.
How often interest is added matters less than people expect. A nominal 5% added once a year earns exactly 5%; added monthly, the interest starts earning sooner and the year works out at 5.116%. Going from monthly to daily adds almost nothing more — on 1,000 over ten years, under 2. That is why the figure worth comparing between accounts is the effective annual rate, which banks publish as the AER in the UK and the APY in the US, not the headline rate.
Monthly additions are counted at the end of each month, and they earn the rate the compounding frequency implies for a month. That keeps the arithmetic honest when the two do not match: money added monthly to an account that compounds quarterly earns exactly what a quarterly rate delivers over a year, not a month's interest it would never be paid.
The rule of 72 says money doubles in about 72 ÷ rate years. It is a good estimate between roughly 4% and 12% and drifts outside that, so both figures are shown: at 7.2% a year the rule says 10 years, and the exact answer is 9.97.
Worked example
1,000 at 5% a year for 10 years, compounded five ways
- Yearly 1,000 × 1.05^10 = 1,628.89
- Half-yearly 1,000 × 1.025^20 = 1,638.62
- Quarterly 1,000 × 1.0125^40 = 1,643.62
- Monthly 1,000 × (1 + 0.05/12)^120 = 1,647.01
- Daily 1,000 × (1 + 0.05/365)^3650 = 1,648.66
- Monthly earns about 18 more than yearly.
- Daily earns under 2 more than monthly.
Compounding more often helps, and stops helping quickly — compare effective rates, not headline ones
How to use it
- Enter the starting amount, and anything you will add each month. Either can be zero, but not both.
- Enter the annual interest rate and how often it compounds. If you only know the effective rate, enter it with yearly compounding.
- Enter the number of years.
- Read the balance, the effective annual rate and the doubling time, then compare the frequencies and open the year-by-year table.
What this tool does not do
- The rate is fixed for the whole period. Variable rates and investment returns do not behave like this.
- No tax, fees or inflation are taken off. The balance is before all three.
- Additions are made at the end of each month, and a month is treated as a twelfth of a year. An account that credits additions on particular days will differ slightly.
- No withdrawals, and no change to the monthly addition over time.
- Whole years, from 1 to 100. Continuous compounding is not offered; daily comes very close — on 1,000 at 5% for ten years the two differ by 6 cents.
- For Indian bank fixed and recurring deposits, which compound quarterly with their own convention for part quarters, use the FD & RD calculator.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is paid only on what you put in: 1,000 at 5% earns 50 every year, 500 over ten years. Compound interest is added to the balance and then earns interest itself, so the same 1,000 at 5% compounded yearly grows to 1,628.89 over ten years — 128.89 more than simple interest.
Does compounding daily earn much more than monthly?
No. On 1,000 at 5% for ten years, monthly compounding gives 1,647.01 and daily gives 1,648.66, a difference of under 2. The big step is from yearly to anything more frequent; after monthly, the gains are tiny.
What are AER and APY?
Two names for the effective annual rate: AER, annual equivalent rate, in the UK, and APY, annual percentage yield, in the US. Both state what a rate actually earns in a year once its compounding is counted, which makes accounts that compound differently comparable. 5% compounded monthly is an AER or APY of 5.116%.
How accurate is the rule of 72?
Close in the middle and less so at the edges. At 7.2% it predicts doubling in 10 years against an exact 9.97; at 2% it predicts 36 years against an exact 35; at 20% it predicts 3.6 against an exact 3.8. The result shows both, so there is no need to rely on the estimate.
When are the monthly additions counted?
At the end of each month. Money added at the start of a month would earn one extra month of interest each time, so an account that takes contributions at the start will come out slightly higher.
Does this account for inflation?
No. The balance is in the money of the day it is paid. To see what it is worth in today's terms, run it again with the interest rate minus the inflation rate you expect — an approximation, but a far more honest picture over long periods.
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Last updated 13 September 2026.