Compound Int. Calculator

Interest on your interest. The one time "it's complicated" is good news.

Use this free compound interest calculator to see what a lump sum grows to over time. Enter your principal, annual rate, years, and how often interest compounds (monthly by default), and get the final balance. No contributions, no fees — just the raw power of leaving money alone.

Compound interest is the reason a boring index fund beats a brilliant plan you never start. Einstein probably did not call it the eighth wonder of the world, but whoever did was right. The formula below is the entire secret; the hard part is the waiting.

What This Compound Interest Calculator Does

Calculates the future value of a one-time deposit with compound growth
Lets you choose compounding frequency: 1 (yearly), 4 (quarterly), 12 (monthly), 365 (daily)
Shows how rate and time interact — time usually wins
Works for savings accounts, CDs, index-fund projections, and "what if I had started at 22" regret sessions

How to Use the Compound Interest Calculator

Inputs:

Principal — the starting amount
Rate — annual interest rate in % (a high-yield savings account might be 4–5; the S&P 500's long-run average is ~10 before inflation)
Time — years invested
Compound Freq — times per year interest is added (12 = monthly, the default)

Steps:

Enter principal, rate, and years
Leave compounding at 12 unless your account says otherwise
Click Calculate for the total amount
Total − Principal = interest earned. Try doubling the years instead of the rate and watch what happens.

How the Calculation Works

A = P × (1 + r/n)^(n × t)

A = final amount · P = principal · r = annual rate as a decimal (5% = 0.05) · n = compounds per year · t = years
More frequent compounding helps, but with diminishing returns: 5% yearly vs daily over 10 years is $1,629 vs $1,649 on $1,000
The Rule of 72: years to double ≈ 72 ÷ rate. At 8%, money doubles every ~9 years — see the Rule of 72 calculator
This is a lump sum only. For monthly contributions use the investment growth calculator

Worked Example: $10,000 at 7% for 30 years, monthly compounding

r/n = 0.07 ÷ 12 = 0.005833 · n × t = 12 × 30 = 360
(1.005833)^360 = 8.1165
A = 10,000 × 8.1165 = $81,165
Interest earned: $71,165 — seven times the original deposit, for doing nothing except not touching it. Not touching it is the skill.

Tips (From People Who Have Made These Mistakes)

Start date matters more than amount: $5,000 at 25 beats $10,000 at 40 at the same rate
Subtract inflation (~3%) from the rate to see growth in today's purchasing power — see the inflation calculator
Compound interest works against you at 24% APR just as beautifully. Check the credit card payoff calculator if that landed uncomfortably

Frequently Asked Questions

What is compound interest?

Interest calculated on both the original principal and the interest already earned. Each period's interest is added to the balance, so the next period earns interest on a bigger number. Simple interest only ever grows the principal.

How is compound interest calculated?

With A = P(1 + r/n)^(nt). For $1,000 at 5% compounded monthly for 10 years: 1,000 × (1 + 0.05/12)^120 = $1,647. Simple interest would give $1,500.

Does compounding frequency matter?

Somewhat. Going from yearly to monthly adds a noticeable amount; going from monthly to daily adds very little. Rate and time dominate everything else.

How long does it take to double my money?

Roughly 72 ÷ interest rate years. At 6% about 12 years; at 10% about 7. The exact answer is ln(2) ÷ ln(1 + r).

What rate should I use for stocks?

Historically the US market has returned ~10% per year nominal, ~7% after inflation, with huge swings year to year. Use 6–8% for conservative planning and remember that the average is not a promise.

Mini Roast Disclaimer

We roast the number, not you. Although "I'll start investing next year" has been said every year since interest was invented.

Reality Check

This calculator assumes a constant rate and no withdrawals, taxes, or fees — none of which exist in real life. Investment returns vary wildly year to year, savings rates change, and taxes take a bite. Use it to understand the mechanics and compare scenarios, not to predict your exact balance in 2056.