How your balance grows over time
Green is the money you deposit; blue is the interest it earns. Updates as you change the inputs.
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Investment
Compound Interest Calculator
Project how an initial deposit, plus optional monthly contributions, grows over time as interest compounds on both your principal and your prior earnings.
Balance after time horizon
$140,411
Total deposited$58,000
Total interest earned$82,411
Starting principal$10,000
Growth multiple2.42×
How it works
This calculator combines the standard compound interest formula for your initial deposit with a future-value-of-an-annuity formula for your recurring monthly contributions, then sums the two so you get one combined ending balance.
The compound interest formula
A = P(1 + r/n)^(nt)
A = ending balance
P = initial principal
r = annual interest rate (decimal)
n = number of times interest compounds per year
t = number of years
A = ending balance
P = initial principal
r = annual interest rate (decimal)
n = number of times interest compounds per year
t = number of years
Worked example
Starting with $10,000, contributing $200 per month, at a 7% annual rate compounded monthly for 20 years:
| Item | Amount |
|---|---|
| Starting principal | $10,000 |
| Total contributions over 20 years | $48,000 |
| Total interest earned | $82,411 |
| Ending balance | $140,411 |
Why time matters more than timing
- The earliest deposits compound for the longest, money added in year one has 19 more years to grow than money added in year twenty.
- Higher compounding frequency (daily vs. annually) adds a small but real boost, especially at higher rates.
- Consistent monthly contributions smooth out the impact of when you start and often matter more than trying to time a lump sum perfectly.
Frequently asked questions
What is compound interest?
Interest calculated on both your original principal and previously earned interest, so your balance grows faster the longer it compounds.
How does compounding frequency affect growth?
More frequent compounding (daily vs. monthly vs. annually) grows your balance slightly faster, since interest starts earning its own interest sooner.
Does adding monthly contributions make a big difference?
Yes, often more than the rate itself, since each new contribution also gets years to compound.
What is the Rule of 72?
A quick estimate for doubling time: divide 72 by your annual rate. At 8%, money roughly doubles every 9 years.
Should I prioritize a lump sum or monthly contributions?
Both work, but monthly contributions are often easier to sustain and they benefit from compounding immediately, which can make them extremely powerful over long periods.