Savings & Banking
Compound Interest Calculator
Compound interest pays you interest on your interest — the engine behind long-term wealth. Enter a starting balance, a monthly contribution, and a rate to see exactly what you'd have in 5, 10, or 40 years, how much of it is your money, and how much the compounding earned for you.
Compounded monthly. Long-run US stock returns have averaged ~7% after inflation; savings accounts pay less.
Final balance
$170,619
- You put in
- $70,000
- Compound interest earned
- $100,619
- Growth multiple
- 2.44
Final balance ÷ total contributions.
Growth by year
| Year | Balance | Contributed | Interest earned |
|---|---|---|---|
| 1 | $13,821 | $13,000 | $821 |
| 2 | $17,918 | $16,000 | $1,918 |
| 3 | $22,312 | $19,000 | $3,312 |
| 4 | $27,023 | $22,000 | $5,023 |
| 5 | $32,074 | $25,000 | $7,074 |
| 6 | $37,491 | $28,000 | $9,491 |
| 7 | $43,300 | $31,000 | $12,300 |
| 8 | $49,528 | $34,000 | $15,528 |
| 9 | $56,206 | $37,000 | $19,206 |
| 10 | $63,368 | $40,000 | $23,368 |
| 11 | $71,047 | $43,000 | $28,047 |
| 12 | $79,281 | $46,000 | $33,281 |
| 13 | $88,110 | $49,000 | $39,110 |
| 14 | $97,578 | $52,000 | $45,578 |
| 15 | $107,730 | $55,000 | $52,730 |
| 16 | $118,616 | $58,000 | $60,616 |
| 17 | $130,289 | $61,000 | $69,289 |
| 18 | $142,806 | $64,000 | $78,806 |
| 19 | $156,227 | $67,000 | $89,227 |
| 20 | $170,619 | $70,000 | $100,619 |
How this calculator works
The simulation credits interest monthly at rate ÷ 12 on the running balance, then adds your contribution at month's end (an ordinary annuity). This matches the closed-form future-value formula FV = PV(1+r)^n + PMT((1+r)^n − 1)/r within cent-rounding.
All amounts are tracked in exact cents, the way a real account posts interest. Results are estimates for education — actual account terms, fees, and market returns will differ.
Try an example
Frequently asked questions
What is compound interest?
Compound interest is interest calculated on both your original money and the interest it has already earned. Each period's earnings are added to the balance, so the next period earns interest on a bigger number — growth accelerates over time instead of staying linear.
How often is interest compounded in this calculator?
Monthly, which matches most savings accounts and is a close approximation for investment growth. Interest is credited each month on the current balance, then your monthly contribution is added.
What rate should I use?
Use your account's actual APY for savings (high-yield accounts have recently paid 4–5%). For long-term stock investing, planners commonly model 6–8% nominal returns. Lower your assumption to stress-test the plan — the habit of contributing matters more than the exact rate.
What is the Rule of 72?
A quick mental shortcut: divide 72 by your annual rate to estimate how many years money takes to double. At 7%, that's roughly 72 ÷ 7 ≈ 10 years. This calculator shows the exact path, including your ongoing contributions.
Does this account for taxes or inflation?
No — results are nominal and pre-tax, like your account statement. In taxable accounts, interest is taxed as income each year; retirement accounts defer or eliminate that. To think in today's purchasing power, subtract expected inflation (~2–3%) from your rate.
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Disclaimer: DayCents provides this calculator for educational purposes only. Results are estimates based on your inputs and the stated assumptions — they are not financial advice, a quote, or an offer of credit. Consult a qualified financial professional before making major money decisions.