Short answer
$10,000.00 left at 5% annual interest for 20 years grows to $26,532.98, with $16,532.98 of that being interest. No new deposits are added in this example. Use the formula below with your own amount, rate and years, or try the Compound Interest Calculator for an instant result.
The formula
Final amount = P × (1 + r)t, where P is the starting amount, r is the annual interest rate, and t is the number of years. This version compounds once a year and adds no new deposits. If you add money every month, the result is higher, and the monthly-savings calculators cover that case.
Worked example: $10,000.00 at 5% for 20 years
Step 1: the growth factor is 1 + 0.05 = 1.05. Step 2: raise it to 20 years: 1.0520 = 2.6533. Step 3: multiply by the starting amount: $10,000.00 × 2.6533 = $26,532.98. Step 4: the interest earned is $26,532.98 − $10,000.00 = $16,532.98. That is the same result the Compound Interest Calculator gives for these numbers when Compounds per year is set to 1.
How $10,000.00 grows at different rates and times
Starting amount $10,000.00, no new deposits. Time matters more than most people expect: the last 10 years add far more than the first 10.
| Interest rate | Years | Final amount | Interest earned |
|---|---|---|---|
| 5% per year | 10 years | $16,288.95 | $6,288.95 |
| 5% per year | 20 years | $26,532.98 | $16,532.98 |
| 5% per year | 30 years | $43,219.42 | $33,219.42 |
| 3% per year | 20 years | $18,061.11 | $8,061.11 |
| 7% per year | 20 years | $38,696.84 | $28,696.84 |
What changes the result
Time matters most, because growth compounds: each year's interest earns interest in the next years. The rate comes second, and the starting amount third. At 5%, $10,000.00 reaches $16,288.95 after 10 years but $43,219.42 after 30 years. Starting earlier usually beats starting with more, because the extra years do the heavy work.
Common mistakes
The most common mistake is expecting straight-line growth, when compound growth curves upward over time. The next is mixing a monthly rate with years, or an annual rate with months: keep the rate and the time in the same unit. The third is forgetting that real returns vary, and inflation and tax reduce what the final number is worth in practice.
Try it with your own numbers
Open the Compound Interest Calculator, enter your starting amount, rate and years, and set Compounds per year to 1 for the annual formula in this guide. If you add money every month, use the Compound Interest With Contributions Calculator instead. More guides are on the BuildCalc Blog and in Savings & Investment calculators.
Results are estimates and general information only. Rates vary, and inflation and tax are not included. Always confirm final figures with a qualified professional.