Compound Interest Calculator
Model monthly growth with regular additions, show paid-in money separately, and choose deposit timing.
Last updated: November 4, 2026
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Quick summary: Model monthly growth with regular additions, show paid-in money separately, and choose deposit timing.
Read the complete guide
Updated reference sections in this guide are in English.
A transparent monthly scenario
Enter a starting balance, an amount to add every month, an assumed nominal annual rate and the time horizon. Use whole years plus 0 to 11 extra months so the number of deposits is explicit. Select whether each addition is made before or after that month’s growth. Monthly additions are nonnegative in this version; withdrawals and changing contributions are outside the model.
The rate and deposit timing
End-of-month addition: next balance = current balance × (1 + r) + addition
Beginning-of-month addition: next balance = (current balance + addition) × (1 + r)
Paid in = starting amount + monthly addition × number of months
The calculator repeats the stated monthly step for the complete horizon. Growth is the final balance minus the amount paid in. It also reports (1 + r)12 − 1 as the effective annual rate implied by this model. That is not a promise about any financial product.
A reproducible monthly example
1,000 initially, plus 100 each month, at a nominal 6%
Over 12 months, a 0.5% monthly rate and end-of-month additions give a balance of approximately 2,295.23. The amount paid in is 2,200.00, so modeled growth is about 95.23. Beginning-of-month additions increase the balance to approximately 2,301.40, because each addition receives one extra month of growth.
A zero-rate baseline
With the same starting amount and additions at 0%, the ending balance is exactly 2,200. This baseline helps separate the effect of saving more from the effect of the assumed return.
Check this scenario in the calculator:
- Starting amount
- 1000
- Monthly addition
- 100
- Nominal annual rate (%)
- 6
- Additions are made
- At the end of each month
- Whole years
- 1
- Extra months
- 0
- Currency (display only)
- USD — US dollar
Use the schedule to separate growth from deposits
An increasing balance does not mean every increase is an investment gain. The schedule shows the amount paid in alongside balance and growth. Compare scenarios with the same contributions but different rates, or the same rate but different monthly additions. Changing one assumption at a time makes the comparison easier to interpret.
Nominal rate, uncertainty and exclusions
The annual input is a nominal rate divided by twelve. For example, a nominal 6% compounded monthly implies about 6.1678% effective annual growth before other effects. It should not be entered as an already effective APY without considering that distinction. Rates above −100% and up to 1,000% are accepted for mathematical scenarios, with a maximum horizon of 100 years plus 11 months; very large results are rejected. Actual investments can fluctuate, lose value and incur fees or tax. No volatility, fees, inflation, tax or contribution limits are modeled.
Reference and growth questions
Investor.gov’s Compound Interest Calculator is an independent educational reference for starting amounts, contributions, time and compounding. This page fixes the compounding interval to monthly and states its deposit timing explicitly.
What if the horizon is zero?
No deposits or growth periods occur. The result is the starting balance.
Can the modeled growth be negative?
Yes. A negative assumed rate can make the final balance lower than the amount paid in.
Does the tool predict a real return?
No. It computes the result of the rate you entered under the stated monthly assumptions.