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Compound Interest Calculator

Calculate compound interest with different compounding frequencies. See how your investment grows over time with detailed breakdown.

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Formula

A = P(1 + r/n)^(nt) where A=final amount, P=principal, r=rate, n=compounds per year, t=years

Example

principal:10000
rate:5
years:10
compounds:12
final Amount:16470.09
interest:6470.09

Complete Guide

Reviewed by PercentLab Editorial Team · Last updated:

A compound interest calculator shows what happens when interest earns interest: A = P(1 + r/n)^(nt), where P is the principal, r the annual rate as a decimal, n the compounds per year, and t the years. With monthly contributions there is a second half of the formula for the deposit stream — which is why honest tools project period by period instead of pretending every deposit compounds for the full term.

Three lessons every top result agrees on: starting early beats starting big (ten extra years can more than double the outcome at the same rate), rate dominates frequency (monthly vs yearly on $10,000 at 5% for 10 years differs by only $181 — see the examples), and nominal is not real (fees, taxes, and inflation all come out of the headline number). Treat this page as an educational estimate, not financial advice.

Use the worked examples for the math, the table for intuition on $200/month at 7%, and the Rule of 72 whenever you want a doubling estimate without touching a calculator.

Worked examples

Lump sum: $10,000 at 5% compounded yearly for 10 years

  1. Write the formula: A = 10000 x (1 + 0.05/1)^(1x10).
  2. Simplify: A = 10000 x 1.05^10 = 10000 x 1.628895.
  3. Result: A is about $16,288.95.

Interest earned is about $6,288.95 on top of the $10,000 principal.

Same inputs, monthly compounding

  1. n = 12: A = 10000 x (1 + 0.05/12)^120.
  2. Compute: A is about 10000 x 1.64701 = $16,470.10.
  3. Compare with yearly: $16,470.10 - $16,288.95 = $181.15.

Monthly compounding wins by $181 — real money, but tiny next to the effects of time and rate.

Rule of 72: doubling time at 7%

  1. Divide: 72 / 7 is about 10.3.
  2. Interpretation: money doubles roughly every 10.3 years at 7%.
  3. Sanity check: 1.07^10.3 is about 2.0.

A doubling estimate with no calculator required.

What $200/month becomes at 7% compounded monthly.

AfterTotal valueOf which interest
10 years$34,620$10,620
20 years$104,180$56,180
30 years$243,900$171,900

Mistakes to avoid

Comparing APY with nominal APR

A 5% nominal rate compounded monthly is about 5.12% effective (APY). Compare effective rates, not headline rates — otherwise monthly looks deceptively equal to yearly.

Forgetting fees, taxes, and inflation

A 7% market return minus 1% fees and 3% inflation is roughly 3% real. Lower your assumption instead of trusting the nominal projection.

Expecting frequency to rescue a late start

Daily vs monthly barely moves the needle; ten extra years more than doubles the outcome at the same rate. Start size and start date dominate.

Continue with

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Frequently Asked Questions

Compound interest is interest calculated on the initial principal plus all accumulated interest from previous periods. It grows faster than simple interest because you earn "interest on interest".

Monthly compounding calculates interest 12 times per year, while annual compounding calculates once per year. More frequent compounding results in higher returns. For example, $10,000 at 5% for 10 years: monthly = $16,470, annual = $16,289.

Use the formula A = P(1 + r/n)^(nt). For $10,000 at 5% annually for 10 years: A = 10,000(1 + 0.05/1)^(1×10) = $16,289. Interest earned = $16,289 - $10,000 = $6,289.

More frequent compounding yields higher returns. From best to worst: daily, monthly, quarterly, semi-annually, annually. However, the difference between daily and monthly is usually minimal.

Divide 72 by the annual rate to estimate doubling years. At 7%: 72 / 7 = about 10.3 years. At 5%: about 14.4 years. It is a quick estimate for compound growth, not an exact figure.

Slightly. $10,000 at 5% for 10 years: monthly is about $16,470 vs yearly about $16,289 — a $181 gap. Time invested and rate dominate; frequency is a distant third.

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Last updated: 2026-09-07