Interest formulas
Simple: I = P × r × t
Compound: A = P × (1 + r/n)n·t, interest = A − P
Rate: r = n × ((A/P)1/(n·t) − 1)
Time: t = ln(A/P) ÷ (n × ln(1 + r/n))
Example
$10,000 at 5% for 10 years:
- Simple interest: $10,000 × 0.05 × 10 = $5,000
- Compound interest (monthly): $10,000 × (1 + 0.05/12)¹²⁰ − $10,000 ≈ $6,470
The extra $1,470 is "interest on interest", and the gap widens every year.
The Rule of 72
For a quick mental estimate of doubling time, divide 72 by the interest rate. At 8%, money doubles in about 9 years; at 3%, about 24 years. It is most accurate for rates between about 4% and 12%.
Adding money regularly? The compound interest calculator handles monthly deposits. For simple interest only, see the simple interest calculator.
Last reviewed October 2026.
Frequently asked questions
How do I calculate interest?
Simple interest = principal × rate × time. Compound interest adds the interest to the balance each period, so it grows faster: balance = principal × (1 + rate ÷ n)^(n × years). This calculator shows both so you can compare.
What is the difference between simple and compound interest?
Simple interest is only ever earned on the original amount. Compound interest is earned on the original amount plus all interest added so far. $10,000 at 5% for 10 years earns $5,000 simple interest but about $6,470 compounded monthly.
How do I find the interest rate?
Choose "Find the rate", enter the starting amount, the ending amount and the time. The calculator works out the annual rate that connects them, with compounding.
How long will it take to double my money?
Use the Rule of 72: divide 72 by the annual rate. At 6%, money doubles in about 12 years. The calculator gives the exact answer in "Find the time" mode.
Does this work for loans as well as savings?
Yes, for the growth of a balance. For loans with regular payments, use the loan or amortization calculator, which accounts for each payment reducing the balance.