Compound Interest Calculator
Compound interest is interest paid on both your original balance and the interest already earned. Use this calculator to estimate the future value of a starting amount plus regular contributions, using an annual rate and a compounding frequency such as daily, monthly, quarterly, semi-annually or annually.
How to project compound growth
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1
Enter the starting principal
Add the lump sum you already have available. Use zero if you only want to model future deposits.
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2
Add a contribution per compounding period
The tool treats the contribution as one payment at the end of each compounding period, so monthly compounding means a monthly contribution and quarterly compounding means a quarterly contribution.
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3
Set the annual rate and frequency
Use the nominal annual rate quoted by your account or investment assumption, then choose how often interest is credited. More frequent compounding gives a slightly higher effective annual return.
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4
Choose the time horizon
Read the ending balance, total contributions and interest earned side by side. Re-run with a lower real return if you want to allow for inflation, taxes or fees.
The formula
For a lump sum with no extra contributions: A = P * (1 + r/n)^(n*t) where P is principal, r is the nominal annual rate as a decimal, n is the number of compounding periods per year, and t is the number of years.
With an end-of-period contribution of PMT, add PMT * ((1 + r/n)^(n*t) - 1) / (r/n). If the rate is 0%, the contribution part is simply PMT * n * t.
A 30-year example
$10,000 starting balance, $300 contributed every month, 7% nominal annual rate, monthly compounding:
| Year | Balance | Contributed | Interest earned |
|---|---|---|---|
| 5 | $36,060 | $28,000 | $8,060 |
| 10 | $72,350 | $46,000 | $26,350 |
| 20 | $195,880 | $82,000 | $113,880 |
| 30 | $446,560 | $118,000 | $328,560 |
The table is not a market forecast. It shows how the maths behaves when the same average rate is applied consistently. In real portfolios, returns arrive unevenly and fees, taxes and inflation reduce spending power.
Nominal, effective and real returns
A nominal 6% rate compounded monthly produces an effective annual rate of (1 + 0.06/12)^12 - 1 = 6.168%. Daily compounding reaches about 6.183%. The effective rate is better for comparing accounts because it includes compounding; the real rate goes one step further by subtracting inflation.
Common pitfalls
- Using the wrong contribution period. In this tool, the contribution is per compounding period, not automatically per month.
- Ignoring taxes, fees and inflation. A 7% nominal return with 3% inflation is roughly a 4% real return before taxes and costs.
- Treating an average as a promise. Long-run investment assumptions are useful for planning, but actual yearly returns can be much higher or lower.
- Comparing nominal quotes only. Savings products are easier to compare by APY or effective annual rate, especially when compounding frequencies differ.
Frequently Asked Questions
Use the frequency stated by the account or product. Savings accounts often quote an APY that already reflects compounding, bonds commonly pay coupons semi-annually, and loans or mortgages are often calculated monthly. If you only have an annual expected return, monthly compounding is a reasonable modelling default.
No. The projection is before taxes, fees and inflation. Tax-advantaged accounts can make the estimate closer to what you keep, while taxable accounts should use a lower expected rate to allow for dividend, interest or capital-gains tax drag.
Use a conservative assumption that fits the asset mix and currency you are modelling. Broad equity-market history is often quoted near high single digits nominally, but a balanced portfolio or a real-return forecast should usually be lower.
This calculator treats contributions as end-of-period payments, also called an ordinary annuity. Deposits made at the start of each period would earn one extra period of interest and produce a slightly higher final balance.
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