Compound Interest Calculator With Monthly Contributions
Calculate how a starting balance and regular monthly deposits may grow over time. This page is built for people who save or invest every month, not just once.
Monthly contribution example
| Starting amount | Monthly deposit | Return | Years | Approx final value |
|---|---|---|---|---|
| $5,000 | $300 | 7% | 20 | About $174,000 |
| $0 | $100 | 7% | 10 | About $17,300 |
| $10,000 | $500 | 6% | 25 | About $384,000 |
Why monthly deposits need different math
A lump-sum formula assumes the whole principal grows for the full period. Monthly deposits arrive gradually, so each deposit compounds for a different amount of time. That is why this page uses period-by-period projection logic.
Related pages
FAQ
How do monthly contributions compound?
Each deposit begins compounding after it is added. Earlier deposits grow longer than later ones.
Should I use beginning or end of month?
Beginning of month gives deposits slightly more time to grow. End of month is more conservative.
Does this include fees?
No. Lower the return assumption manually if you want to account for fees or tax.
The formula, and why it has two halves
Monthly contributions need a different formula from a lump sum, because each contribution compounds for a different length of time. The money you put in this month has 30 years to grow; the money you put in next year has 29.
The first half is your starting balance compounding. The second half is the contribution stream. Most confusion about these calculators comes from expecting one formula to cover both.
| Term | Means | Typical value |
|---|---|---|
| P | Starting balance (can be zero) | $0 – $10,000 |
| PMT | Amount added each period | $100 – $500 a month |
| r | Annual rate as a decimal | 0.07 for 7% |
| n | Compounding periods per year | 12 for monthly |
| t | Years | 10 – 40 |
What monthly contributions actually produce
Starting from zero, at 7% annual return compounded monthly:
| Monthly amount | After 10 years | After 20 years | After 30 years | Total contributed (30y) |
|---|---|---|---|---|
| $100 | $17,308 | $52,093 | $121,997 | $36,000 |
| $200 | $34,617 | $104,185 | $243,994 | $72,000 |
| $500 | $86,542 | $260,463 | $609,985 | $180,000 |
| $1,000 | $173,085 | $520,927 | $1,219,971 | $360,000 |
Beginning or end of the month matters slightly
Contributing at the start of each period gives every payment one extra compounding period. Over 30 years at 7% that is worth roughly 0.6% of the final balance — about $3,600 on the $500-a-month row above. Real, but far less than most people expect, and not worth reorganising your finances over.
Contribution beats rate, up to a point
Raising a $200 monthly contribution to $250 does more over 20 years than improving the return from 7% to 8%: about $26,000 versus about $13,600. Contribution amount is also the variable you control, which the return is not.