Calculate interest on your money over time.
Your values
Your result
Enter your values and select Calculate.
Estimate from stated assumptions, not personal financial advice. Rates, fees, tax and account rules may change actual interest. Calculator disclaimer.
How compound interest works
Compound interest earns interest on your savings and on earlier interest. Regular deposits add to the balance earning interest.
Formula & worked example
With deposits, each deposit earns interest from its selected start or end time. In a partial final period, a start deposit is included when that period begins; an end deposit requires a completed period.
The annual rate uses a 365-day model year: a week is 7 days, and a month is 1/12 of a year. Weekly compounding happens every 7 days; monthly compounding uses equal months. So 52 weeks is 364 days. Partial periods use the equivalent fractional growth factor. Choose Days and daily compounding for a duration measured in days.
Compound interest with monthly deposits
Enter GBP 10,000, 1% nominal annual interest, Monthly compounding, and 1 Year. Under Add extra money, enter GBP 100 and Monthly. Deposit timing changes how long each deposit earns interest.
Scroll the table to see all amounts.
| Deposit timing | Deposits added | Interest earned | Final balance |
|---|---|---|---|
| End of period | 1,200.00 | 105.97 | 11,305.97 |
| Start of period | 1,200.00 | 106.98 | 11,306.98 |
The same GBP 1,200 is added in both cases. Start-of-period deposits earn one extra month of interest in this example. Choose the timing that matches when you add money.
Daily, weekly or monthly compounding?
Choose the compounding convention in your account’s terms. This model uses 365 daily periods, seven-day weekly periods or 12 equal monthly periods per year; it applies a fractional growth factor for a partial period.
For GBP 10,000 at 5% nominal annual interest over 1 Year, with no extra deposits, the final balance is GBP 10,511.62 monthly, GBP 10,512.46 weekly or GBP 10,512.67 daily.
At the same nominal rate, more frequent compounding increases this model’s result slightly. Changing the choice changes an assumption; it does not change the interest your provider pays.
Using APY or AER
If your provider quotes APY or AER, enter that percentage and select APY/AER in Advanced options. For GBP 10,000 at 5% APY/AER over 1 Year, with no extra deposits, the model ends at GBP 10,500.00 for daily, weekly or monthly compounding. Adding compounding again would overstate the quoted yield.
Worked example: 1% annual interest compounded daily for 52 weeks earns GBP 100.22 on GBP 10,000 in this model.
Rates & assumptions
A nominal rate is divided by the compounding frequency. APY/AER is an effective annual yield that already includes compounding; the calculator converts it to the equivalent periodic rate.
The model uses equal periods and a constant, non-negative rate. It does not model fluctuating investment returns or market losses.
Fees, tax, inflation, rate changes and bank crediting rules are excluded. Currency changes formatting only, with no exchange conversion. Results are estimates, not guaranteed returns or personal financial advice. Full disclaimer.
Privacy
With JavaScript, calculations run in browser memory. Without it, the native form sends values to Cloudflare for a temporary calculation. Values submitted with Calculate stay out of URLs and are not saved in a database, analytics or advertising. Intentional setting links include their chosen options and amounts; these addresses may remain in browser history and hosting request records. This app adds no tracking for those links. Server results are private, not cached or indexed.
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Contact & references
Made by IT1ST. Report errors to compound_interest_calculator@it1st.com using example values, without private account details.
References: CFPB annual percentage yield formula and interest accrual and compounding. Check your provider’s terms for its specific convention.