Compound Interest Calculator
Calculate the power of compound interest over time. Enter your initial investment, annual interest rate, time period, compounding frequency, and optional monthly contributions to see how much your money grows.
Growth Breakdown
| Category | Amount | % of Final Balance |
|---|---|---|
| Initial Investment | $10,000.00 | 24.66% |
| Monthly Contributions | $12,000.00 | 29.60% |
| Interest Earned | $18,547.38 | 45.74% |
| Final Balance | $40,547.38 | 100.00% |
What Is Compound Interest?
Compound interest is interest earned on interest. When you invest money, you earn interest on the principal. In the next period, you earn interest on the principal plus the previously earned interest. Over time, this compounding effect accelerates growth dramatically.
Compound interest is the single most important concept in long-term investing. Albert Einstein reportedly called it "the eighth wonder of the world." Whether he actually said it is debated — but the math is undeniable. A 10-year investment at 8% more than doubles your money; a 30-year investment at 8% multiplies it by 10x.
Compound Interest Formula
For a lump sum (no contributions):
A = P × (1 + r/n)^(n×t)
With regular contributions:
A = P × (1 + r/n)^(n×t) + PMT × [((1 + r/n)^(n×t) − 1) ÷ (r/n)]
- P — principal (initial investment)
- r — annual interest rate (as decimal)
- n — compounding periods per year
- t — time in years
- PMT — regular contribution per period
Worked Example
$10,000 initial, 8% annual rate, 10 years, monthly compounding, $100/month contributions:
Initial investment grows to = $10,000 × (1 + 0.08/12)^120 = $22,196.40
Contributions with interest = $100 × [((1 + 0.00667)^120 − 1) ÷ 0.00667] = $18,350.98
Final balance = $22,196.40 + $18,350.98 = $40,547.38
Total contributed = $10,000 + ($100 × 120) = $22,000
Total interest = $40,547.38 − $22,000 = $18,547.38 — 45.7% of the final balance came from interest.
The Power of Time
The greatest factor in compound growth is time, not the interest rate. Here is $10,000 at 8% under different time horizons:
- 5 years — $14,898 (1.49x)
- 10 years — $22,196 (2.22x)
- 20 years — $49,268 (4.93x)
- 30 years — $109,357 (10.94x)
- 40 years — $242,734 (24.27x)
Doubling the time more than doubles the return. That is why starting to save in your 20s is so much more powerful than starting in your 30s — the early money does most of the heavy lifting.
Frequently Asked Questions
What is the difference between compound and simple interest?
Simple interest is calculated only on the original principal — it never grows. Compound interest is calculated on principal plus accumulated interest, so the base grows each period. Over 10 years at 8%, $10,000 with simple interest grows to $18,000; with compound interest, $22,196. The gap widens dramatically over longer periods.
What is a good compound interest rate?
It depends on the asset and risk. High-yield savings accounts pay 4–5%. Bonds pay 4–6%. Stock market index funds have historically returned ~10% per year (7% after inflation). Real estate returns 6–10%. For long-term retirement planning, 6–7% is a conservative assumption for a diversified portfolio; 8–10% assumes more risk.
Does compounding frequency matter?
Yes, but the effect is smaller than most people think. At 8% over 10 years, annual compounding gives $21,589; monthly gives $22,196; daily gives $22,253. The difference between annual and monthly is 2.8%; between monthly and daily it's only 0.26%. The returns from more frequent compounding diminish quickly. Focus on the rate and time instead — those have far bigger impact.
Why does the beginning vs end of month timing matter?
If you contribute at the beginning of the month, the money earns interest for the full month. If you contribute at the end, it earns no interest for that month. Over 30 years, beginning-of-month contributions can add 5–10% to the final balance compared to end-of-month. It's not huge, but it's free money for changing an automatic deposit date.
What is the Rule of 72?
A quick mental shortcut to estimate how long it takes money to double. Divide 72 by the annual interest rate. At 8%, 72 ÷ 8 = 9 years to double. At 6%, 12 years. At 12%, 6 years. It is an approximation — actual doubling at 8% takes 9.006 years — but it's remarkably accurate for rates between 4% and 15%.
Does this account for taxes and inflation?
No — this shows nominal growth before taxes and inflation. To get real returns, subtract both. If you earn 8%, inflation is 3%, and tax on gains is 15%, your real after-tax return is approximately 8% × (1 − 0.15) − 3% = 3.8%. That is why tax-advantaged accounts (401k, IRA, ISA) are so valuable for long-term compounding.