Compound Interest Calculator: project long-term growth
See how a starting balance grows over time with compound interest and optional monthly contributions. Pick your compounding frequency — daily, monthly, quarterly, or annual — and watch interest stack on interest.
What you'll use this for
Retirement planning
Project how a 401(k) or IRA grows over decades.
Education savings
Plan a 18-year college fund from a starting balance + monthly adds.
Down payment goals
See how long it takes to reach a target.
Investing milestones
Forecast portfolio milestones at a given rate of return.
How to use the compound interest calculator
Enter starting balance and contribution
Both are optional — leave one at 0 if it doesn’t apply.
Pick rate and compounding
Monthly is most common for consumer accounts. Daily is for high-yield savings.
Set the time horizon
Years can be 1–80; result updates instantly.
Frequently asked questions
Interest computed on both the original principal and previously earned interest. Compared to simple interest, the difference grows dramatically over long horizons.
Yes, but less than people think. Going from monthly to daily compounding at 7% adds about 0.025% to the effective annual rate. Going from annual to monthly is more meaningful.
End of each month (a "regular annuity" in finance terms). Most savings calculators use this convention.
No — results are nominal dollars at the rate you provide. For real (inflation-adjusted) growth, subtract expected inflation from the rate (e.g., 7% nominal − 2.5% inflation = 4.5% real).
About compound interest
Albert Einstein didn’t actually call compound interest "the eighth wonder of the world" — but the misattribution stuck because the math is genuinely jaw-dropping over long horizons. A 25-year-old saving $200/mo at 7% real returns ends up with more at 65 than a 35-year-old saving $400/mo at the same rate.
The formula
FV = P × (1 + r/n)ⁿᵗ + PMT × ((1+r/n)ⁿᵗ − 1) / (r/n)
P = starting balance
PMT = recurring contribution per compounding period
r = annual rate
n = compounding periods per year
t = yearsTwo levers, one habit
- Time — by far the most powerful. Starting 5 years earlier often beats doubling the contribution.
- Rate — diversified equity index funds historically average 6–7% real (after inflation).
- Habit — consistent monthly contributions matter more than perfect market timing.