Compound Interest Calculator
By Santaji GadepatilinUpdated October 27, 2026
Maturity amount vs Time (years) — other inputs held at your current values.
How to use this calculator
Choose the compounding frequency, then fill in any three of principal, rate, time and maturity amount. The fourth — and the interest earned — is calculated. One tool answers four questions:
- What will my savings grow to? → enter principal, rate, time.
- What rate do I need? → enter principal, target amount, time.
- How long will it take? → press × on Time, then enter the target amount.
- How much should I invest now? → press × on Principal, then enter the target amount.
The currency symbol follows your region automatically. The maths is the same in any currency.
The formula
A = P × (1 + r/n)n×t Interest = A − P
- P — principal (starting amount)
- r — annual interest rate as a decimal (7.5% = 0.075)
- n — compounding periods per year
- t — time in years
Worked example
10,000 invested at 7.5% compounded quarterly for 5 years:
- r/n = 0.075 ÷ 4 = 0.01875
- n × t = 4 × 5 = 20
- A = 10,000 × 1.01875²⁰ = 14,499.48
- Interest earned = 4,499.48
How compounding frequency changes the result
For 10,000 at 7.5% for 5 years:
| Compounding | Maturity amount |
|---|---|
| Yearly | 14,356.29 |
| Half-yearly | 14,450.44 |
| Quarterly | 14,499.48 |
| Monthly | 14,532.94 |
| Daily | 14,549.35 |
More frequent compounding earns slightly more because interest starts earning interest sooner. The gap is small compared with the effect of rate and time — use the chart to see how time dominates.
Rule of 72
To estimate how long money takes to double, divide 72 by the interest rate. At 8%, 72 ÷ 8 ≈ 9 years. The calculator gives the exact answer: clear the Time field and set the maturity amount to twice the principal.
Things to keep in mind
- Interest is usually taxable, so your after-tax return is lower.
- Inflation reduces real returns; compare your rate with current inflation.
- Stock and fund returns are not fixed, so this formula gives an estimate only.
Frequently asked questions
What is the formula for compound interest?
A = P(1 + r/n)^(nt), where P is the principal, r the annual rate as a decimal, n the compounding periods per year and t the time in years. Interest earned is A − P.
Which compounding frequency should I choose?
Use the frequency stated by your bank or investment. Savings accounts often compound daily or monthly, while many fixed deposits and CDs compound monthly or quarterly.
How long does it take to double my money?
Enter the same principal and set Maturity amount to double it. The calculator solves for time. The rule of 72 gives a quick estimate — 72 ÷ rate. At 8% that is about 9 years.
Can I find the interest rate I need?
Yes. Enter principal, target maturity amount and time. The rate is solved numerically.