How it works
Compound growth of a starting balance, plus regular contributions, is:
FV = P(1 + r/n)nt + PMT × [ ((1 + r/n)nt − 1) / (r/n) ]
- P = starting principal · PMT = contribution per period
- r = annual rate · n = compounds per year · t = years
Worked example
$10,000 start + $200/mo for 20 years at 7%, compounded monthly:
- Future value ≈ $143,000
- Of which contributions ≈ $58,000 and interest ≈ $85,000
How we check this calculator
Results are verified against the closed-form compound-interest formula for every compounding frequency we offer (daily through annually): a $10,000 deposit at 5% compounded monthly must show $16,470.09 after 10 years. Regular-contribution scenarios are cross-checked against a brute-force month-by-month simulation, which must agree with the formula to the cent, and we confirm the annual/monthly compounding gap widens with rate — a known property that catches sign and exponent errors.