Compound Interest Calculator
See how $10,000 grows to $76,123 over 30 years at 7%. Monthly compounding, optional contributions.
Future Value
$40,387
Contributions: $10,000 · Interest earned: $30,387
Initial Principal
$10,000
Total Contributions
$10,000
Interest Earned
$30,387
Year-by-Year Growth
| Year | Balance | Contributions | Interest |
|---|---|---|---|
| 1 | $10,723 | $10,000 | $723 |
| 2 | $11,498 | $10,000 | $1,498 |
| 3 | $12,329 | $10,000 | $2,329 |
| 4 | $13,221 | $10,000 | $3,221 |
| 5 | $14,176 | $10,000 | $4,176 |
| 6 | $15,201 | $10,000 | $5,201 |
| 7 | $16,300 | $10,000 | $6,300 |
| 8 | $17,478 | $10,000 | $7,478 |
| 9 | $18,742 | $10,000 | $8,742 |
| 10 | $20,097 | $10,000 | $10,097 |
| 11 | $21,549 | $10,000 | $11,549 |
| 12 | $23,107 | $10,000 | $13,107 |
| 13 | $24,778 | $10,000 | $14,778 |
| 14 | $26,569 | $10,000 | $16,569 |
| 15 | $28,489 | $10,000 | $18,489 |
| 16 | $30,549 | $10,000 | $20,549 |
| 17 | $32,757 | $10,000 | $22,757 |
| 18 | $35,125 | $10,000 | $25,125 |
| 19 | $37,665 | $10,000 | $27,665 |
| 20 | $40,387 | $10,000 | $30,387 |
How This Is Calculated
This compound interest calculator computes the future value of an investment using the compound interest formula with optional periodic contributions.
Compound interest formula: A = P(1+r/n)^(nt) + PMT × [((1+r/n)^(nt) − 1) / (r/n)], where P is the initial principal, r is the annual interest rate, n is the number of compounding periods per year, t is the number of years, and PMT is the periodic contribution.
This calculator uses monthly compounding (n=12), which is the standard for most savings and investment accounts. Periodic contributions are assumed to be made at the end of each period (ordinary annuity).
Compound interest formula: A = P(1+r/n)^(nt) + PMT × [((1+r/n)^(nt) − 1) / (r/n)]. Monthly compounding (n=12).
See our full methodology for every formula, data source, and assumption.
Who should use this
Savers and investors who want to project growth of a lump sum or a regular contribution stream. Useful for retirement "what-ifs," college savings, and comparing investment accounts.
Key inputs
- Initial principal (P) — the starting amount you invest today
- Annual rate (r) — use 7% for stocks (historical real return), 4–5% for HYSA
- Monthly contribution (PMT) — how much you add each month
- Years (t) — time horizon; longer horizons benefit most from compounding
How to interpret results
Doubling time ≈ 72 ÷ annual rate (Rule of 72). At 7%, money doubles every ~10.3 years. A result that looks large in nominal dollars may buy much less in inflation-adjusted terms — subtract ~3% from the rate for a real (purchasing-power) estimate. The gap between "your contributions" and "final balance" is the compound growth; over 30+ years this typically exceeds the contributions themselves.
How to Read Your Results
What the numbers mean and how to use them
The headline number from this calculator — the final balance — is driven by three levers, and understanding which one matters most changes how you save. Time beats rate, and rate beats amount. A small contribution started early often outperforms a large contribution started late.
Contributions vs growth
The calculator breaks your final balance into "money you put in" versus "money your money earned." Over 30 years at 7%, a $200/month contribution stream ($72,000 total) grows to ~$228,000 — meaning $156,000 (68%) is pure compound growth, not your savings.
What to do: Do not be discouraged by small starting amounts — the compounding portion usually exceeds your contributions over 20+ year horizons.
The Rule of 72
Your money doubles roughly every 72 ÷ rate years. At 7%, that is ~10.3 years. At 4% (HYSA), ~18 years. At 10% (aggressive stock assumption), ~7.2 years. Two doublings = 4x. Three = 8x.
What to do: Use this as a sanity check: if the calculator shows your money doubling much faster than 72/rate, re-check your inputs.
Inflation reality check
A 7% nominal return becomes ~4% real (after-inflation) return at 3% inflation. Over 30 years, $100,000 at 7% nominal grows to $761,000 — but in purchasing power that is closer to $314,000 in today's dollars.
What to do: For retirement planning, subtract ~3% from the rate to see real purchasing power. Do not plan retirement around nominal dollars.
Starting early vs starting late
Someone who invests $300/month from age 25-35 (10 years, $36K total) and then stops, ends with more at age 65 than someone who invests $300/month from age 35-65 (30 years, $108K total) — assuming the same 7% return. Time in the market beats timing.
What to do: If you are in your 20s or 30s and not investing, the cost of waiting compounds against you — run the numbers here to see the gap.
When You'll Actually Use This
Retirement "what-if" modeling
You are 35 with $25,000 saved. Enter $25,000 principal, $400/month contribution, 7% rate, 30 years. The result (~$560,000) tells you if you are on track for a comfortable retirement or need to increase contributions.
Comparing a HYSA vs index fund
Run the same $10,000 + $200/month for 20 years at 4% (HYSA) and at 7% (stocks). The 3% gap produces roughly $130,000 difference over 20 years — this is why "boring" savings accounts underperform for long horizons.
College savings for a child
A newborn: enter $0 principal, $250/month, 7% rate, 18 years. The ~$108,000 result helps you gauge whether you are saving enough for in-state public tuition (~$100K projected) or need to increase the monthly amount.
Frequently Asked Questions
What is compound interest?
How often is interest compounded?
What is the Rule of 72?
How do monthly contributions affect compound interest?
What is a realistic rate of return?
Put compounding to work
Books that change how you think about money
Affiliate Disclosure:As an Amazon Associate, CentCalc earns from qualifying purchases. This means if you click an affiliate link and make a purchase on Amazon, we may receive a small commission at no additional cost to you. This helps support our free calculators. We only recommend products we believe are genuinely helpful.
The Psychology of Money (Morgan Housel)
The single best book on why most people underperform compounding. Short, practical, and shifts your mindset from "picking stocks" to "time in the market."
Check price on AmazonThe Simple Path to Wealth (JL Collins)
Explains index fund investing in plain English — the vehicle that makes the 7% return in our calculator realistic for most people.
Check price on AmazonJust Keep Buying (Nick Maggiulli)
Data-driven case for consistent investing regardless of market timing. Pairs perfectly with this calculator's "start early" insight.
Check price on AmazonRelated Calculators
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Estimates only. Actual investment returns vary. Past performance does not guarantee future results. Consult a financial advisor.