Compound interest calculator
$10,000 invested at 8% for 30 years becomes $100,627 with compound interest — but only $34,000 with simple interest. The difference ($66,627) is interest earning interest. Einstein allegedly called compound interest the "eighth wonder of the world." Whether he said it or not, the math is undeniable: time is the most powerful factor in wealth building.
Understanding compound growth
Compound growth shows up everywhere money earns on itself: savings accounts, bonds, diversified portfolios, and employer retirement plans. The curve bends upward because each period applies the rate to a slightly larger balance. Small rate differences feel boring in year one but dominate outcomes over decades—which is why fees, tax drag, and APY shopping matter more than people expect.
Contributions add a second engine. Even flat markets reward steady deposits because you buy through highs and lows (dollar-cost averaging in volatile assets). In a fixed-rate model like this calculator, contributions still accelerate the ending balance linearly while compounding accelerates it exponentially; together they produce the classic retirement-savings curve.
Debt is the mirror image. Credit cards compound daily; student and auto loans usually use simple amortization on declining principal, which is less punishing than revolving debt but still sensitive to rate and term. When comparing "invest vs pay debt," match horizons: long-term market assumptions should not be compared to a 12-month 0% promo without reading the fine print on what happens when the promo ends.
For citation and planning, always record the calculator version and the exact inputs you used. Our methodology block lists assumptions (constant rate, no taxes or fees unless you model them elsewhere) so you can explain what the number represents in a memo, a classroom, or a client email.
Good to know
The Rule of 72 gives quick estimates. Divide 72 by your interest rate to find years to double. At 8%, your money doubles in roughly 9 years (72 ÷ 8 = 9). At 6%, about 12 years. At 10%, about 7.2 years. Works for any rate between 6-10% with decent accuracy.
Starting early beats investing more. Someone who invests $5,000/year from age 25-35 (10 years, $50,000 total) then stops will have more at 65 than someone who invests $5,000/year from age 35-65 (30 years, $150,000 total). The early starter's money compounds for 30 extra years.
Credit card debt works the same way — against you. A $5,000 balance at 20% APR, paying only minimums, takes 27 years to pay off and costs $8,549 in interest. Compound interest is devastating when you're on the paying side. This is why the first financial priority is escaping high-interest debt.
Trust & methodology: Editorially reviewed by the Howdeedo team. Content last reviewed March 2026. Calculation engine version 0.1.0. Open the section below for formula, assumptions, and sources.
Methodology & assumptions
Assumptions
- Interest compounds at specified frequency (daily, weekly, monthly, quarterly, annually, or continuously).
- Contributions are made at beginning or end of each period as specified.
- Rate is constant over the period; no taxes or fees.
- Inverse calculations use iterative methods when closed-form solutions don't exist.
References
Methodology, disclaimers & sources
How it works
- Future Value = P × (1 + r/n)^(n×t)
- P = principal, r = annual rate, n = compounding periods per year, t = years
- Effective annual rate = (1 + r/n)^n − 1
Details & assumptions
Assumes constant rate, regular compounding, no withdrawals. Real investments fluctuate. This is a mathematical model, not a market prediction.
For reference only. Not financial or investment advice. Returns are not guaranteed.