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Compound Interest Calculator

See what savings turn into once interest starts earning interest. Enter a starting amount, a rate and a timescale, add a monthly contribution if you make one, and the final balance plus a year-by-year breakdown appear straight away.

Built & reviewed by the TurboTeamTools team · Updated September 2026 · 0 uses on this device

Compound Interest Calculator tool

Final balance
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You put in
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Interest earned
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Year by year

YearContributedInterestBalance

How to use it

  1. Enter your starting amount, the annual rate and how many years.
  2. Choose how often interest compounds and add a monthly contribution if you make one.
  3. Read the final balance and check the year-by-year table.

Worked example

Input: $5,000 at 6% for 10 years, $200/month
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Result: About $41,800, of which roughly $12,800 is interest

How compound interest builds

Compound interest pays interest on the interest already earned, which is why savings curve upward instead of climbing in a straight line. The formula for a lump sum is A = P(1 + r/n)^(nt), where P is the starting amount, r the annual rate as a decimal, n the number of compounding periods per year and t the years.

Frequency matters less than people expect. At 6 percent, monthly compounding beats annual compounding by only a fraction of a percent per year. What genuinely changes the outcome is time and regular contributions: $200 a month added to a $5,000 start at 6 percent grows to roughly $41,800 over ten years, and only about $29,000 of that is money you put in.

Rates here are nominal and before tax or inflation. A 6 percent return with 3 percent inflation leaves roughly 3 percent of real growth, so treat the final figure as a gross number rather than spending power.

Common uses

  • Projecting savings and retirement growth
  • Comparing accounts with different rates
  • Seeing the effect of a monthly deposit
  • Understanding how debt compounds against you

Frequently asked questions

For a lump sum it is A = P(1 + r/n)^(nt). P is the principal, r the annual rate as a decimal, n how many times a year interest compounds, and t the number of years. Regular contributions add a further term for each deposit.
Slightly. At 6 percent, monthly compounding yields about 6.17 percent effective annual return against 6 percent compounded once. The gap widens at higher rates but stays small next to the effect of time.
Divide 72 by the interest rate for a close estimate. At 6 percent that is 12 years, at 8 percent about 9 years. This rule of 72 works well for rates between roughly 4 and 12 percent.
No. The result is a nominal, pre-tax figure. Subtract your expected inflation rate from the interest rate if you want an estimate in today's spending power.