Compound Interest Calculator
Estimate compound growth from a starting amount, monthly contributions, assumed rate, time period, and compounding frequency.
Inputs
Result
Enter values on the left and select Calculate to see the result here.
What the Compound Interest Calculator does
- Estimates how a starting principal may grow when interest or an assumed return compounds over time.
- Includes user-entered monthly contributions and separates total contributions from estimated interest or growth.
- Uses the rate and compounding assumptions you enter; it does not predict or guarantee account rates or investment returns.
How to use the Compound Interest Calculator
- Select the currency used only for displaying the result.
- Enter the principal or starting amount, plus any monthly contribution you want included.
- Enter an annual interest or return rate assumption, the time period, and the compounding frequency.
- Review the estimated final balance as a mathematical projection based on your inputs, then compare alternative assumptions.
Formula / methodology used
- The starting principal is compounded using A = P × (1 + r ÷ m)^(m × t), where r is the user-entered annual rate, m is compounds per year, and t is years.
- Monthly contributions are modeled as end-of-month additions using a monthly rate derived from the same annual rate.
- Total contributions equal initial principal plus all monthly contributions over the projection period.
- Estimated interest/growth equals estimated final balance minus total contributions, with currency values rounded for display.
Example calculation
Start with £5,000, add £250 per month, assume 5% annual growth for 12 years, compounded monthly.
- Initial principal = £5,000
- Monthly contributions = £250 × 144 months = £36,000
- Total contributions = £41,000
- Apply 5% ÷ 12 monthly compounding to the starting principal and monthly contribution stream
Result: The estimated final balance is about £58,290, including about £17,290 of estimated interest/growth above the £41,000 contributed.
Common mistakes and limitations
- This is a mathematical projection, not a guaranteed savings account rate or investment return.
- Actual outcomes may differ because rates can change and investments can rise or fall.
- Fees, tax, inflation, withdrawals, provider rules, bonus-rate conditions, and exact compounding schedules are not fully modeled.
- Contribution timing can affect real results; this tool assumes end-of-month contributions.
Important note
This calculator provides an estimate based on your inputs and is not financial, investment, tax, pension, or savings advice.
Compound Interest Calculator FAQs
Is the compound interest result guaranteed?
No. It is a projection using the rate and compounding settings you enter. Real savings rates, provider terms, fees, taxes, inflation, withdrawals, and investment performance can change the outcome.
What does compounding frequency mean?
It is how often interest is applied to the starting principal in the model. More frequent compounding can slightly increase the estimate when all other inputs are unchanged.
Why are monthly contributions handled separately?
The calculator compounds the initial principal using the selected frequency and models regular contributions with a monthly growth assumption, which is suitable for a recurring monthly contribution estimate.
Can I use this for investments?
You can enter an assumed return, but investments can fall as well as rise. Treat the result as a scenario estimate, not advice or a forecast.