Compound Interest Calculator
Calculate compound interest growth over time with optional regular contributions.
This calculator shows what time, rate, and regular contributions actually do to money — which is harder to grasp intuitively than it looks. Enter an initial balance, an annual interest rate, a compounding frequency, and optionally a regular monthly contribution to see your projected future balance and total interest earned. The results are most useful for retirement planning (estimating how much a pension or savings account might grow), for savings goal tracking (how long to reach a target amount), and for comparing the impact of starting early versus delaying contributions. The single clearest insight this calculator provides: the difference between starting at 25 vs. 35 with the same monthly contribution is often 2× to 3× the final balance, entirely due to compounding time. All results assume a fixed rate throughout the period — investment returns in practice are variable and unpredictable.
Compound Interest Calculator
Formula
Examples
$5,000 at 8% for 10 Years (Monthly Compounding)
Single lump sum — no additional contributions.
→ Future value: $11,064 | Interest earned: $6,064 | Money more than doubled in 10 years
$0 Start, $200/Month at 7% for 30 Years
Starting from zero with regular monthly contributions — retirement savings scenario.
→ Future value: ~$243,994 | Total contributed: $72,000 | Interest earned: ~$171,994
$10,000 + $500/Month at 6% for 20 Years
Mid-career savings with existing balance and ongoing contributions.
→ Future value: ~$263,657 | Total contributed: $130,000 | Interest earned: ~$133,657
Early Start vs Late Start — $300/Month at 7%
Starting at 25 vs 35, both stopping at 65. Illustrates the cost of delaying.
→ 40 years (age 25→65): ~$798,000 | 30 years (age 35→65): ~$365,000 | Starting 10 years earlier more than doubles the outcome
$100,000 at 5% for 15 Years (Annual Compounding)
Lump sum investment — e.g., inheritance or property sale proceeds.
→ Future value: $207,893 | Interest earned: $107,893 | Balance more than doubles in 15 years
Tips
- ✓Time is the most powerful variable in compound growth. Starting 5–10 years earlier matters more than increasing your contribution rate.
- ✓Even small regular contributions add up dramatically over long periods — $100/month at 7% for 30 years generates over $121,000.
- ✓Use the real rate (nominal rate minus inflation) for projections meant to represent purchasing power, not just face value.
- ✓The Rule of 72 is a fast mental check: divide 72 by your rate to see how many years it takes to double your money.
- ✓Compounding frequency (monthly vs daily) makes far less difference than most people expect — focus on rate and time first.
Frequently Asked Questions
What is compound interest?
Compound interest means you earn interest on both your original principal and the interest already accumulated. This creates exponential growth over time, not just linear growth. A $1,000 deposit at 8% compounded annually becomes $1,080 after year 1, then $1,166 after year 2 (not $1,160) — the extra $6 is interest on interest.
What is the compound interest formula?
A = P(1 + r/n)^(nt), where P = initial principal, r = annual interest rate (decimal), n = compounding periods per year, t = years. For monthly compounding: n = 12. For daily: n = 365. With regular monthly contributions, a more complex formula is applied cumulatively.
How much does compounding frequency affect growth?
More frequent compounding produces slightly more growth. $10,000 at 8% for 10 years: annual compounding = $21,589; monthly compounding = $22,196; daily compounding = $22,253. The difference between monthly and daily is only $57 over 10 years — compounding frequency matters far less than rate and time.
What is the Rule of 72?
Divide 72 by the annual interest rate to estimate how many years your money takes to double. At 8%: 72 ÷ 8 = 9 years to double. At 6%: 72 ÷ 6 = 12 years. At 4%: 18 years. This is a useful mental shortcut, though the calculator gives a more precise result.
How much does starting earlier matter?
Enormously. Starting with $200/month at age 25 at 7% gives approximately $525,000 by age 65. Starting the same contribution at age 35 gives approximately $243,000. Ten fewer years of compounding cuts the final balance by more than half — despite only 10 fewer years of contributions out of 30 or 40.
Does this calculator work for savings accounts, ISAs, and pension funds?
The math works for any account that compounds interest at a fixed rate. Real savings accounts and investment accounts have variable rates, contribution limits, tax treatments, and fees that are not modeled here. Use this calculator for rough projections and goal-setting, then verify with your specific account provider.
What is inflation's effect on compound growth?
This calculator shows nominal growth — the face-value increase in your balance. Real growth (purchasing power) is lower after inflation. A common approach is to subtract an estimated inflation rate from your nominal rate to get the "real" rate. For example, at 7% nominal and 3% inflation, your real growth rate is approximately 4%.
Can I use this to calculate savings target timing?
Yes. Enter your current savings as the principal, your expected rate and contribution, and experiment with the years field until the future value matches your target. For example, to estimate when a $50,000 savings balance with $500/month contributions at 6% reaches $200,000, try different year inputs until the total matches.
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Business & PricingDisclaimer: This calculator is for educational and illustrative purposes only. It assumes a fixed interest rate throughout the period and does not account for taxes, fees, inflation, or changes in contribution amounts. Investment returns are not guaranteed and will vary. Past performance does not predict future results. Consult a licensed financial advisor for investment decisions.