The monthly compounding formula
Monthly compounding applies one-twelfth of your annual rate every month, so each month’s interest is earned on a balance that already includes last month’s interest. With n = 12 the standard formula becomes:
FV = P × (1 + r/12)12×t
Example: $10,000 at a 5% nominal annual rate for 10 years is 10,000 × (1 + 0.05/12)120 = $16,470.09. The same money compounded only once a year would reach $16,288.95 — monthly compounding adds $181.14 over the decade, because interest starts earning its own interest eleven months sooner each year.
Why monthly is the workhorse frequency
Most personal finance actually happens on a monthly rhythm: salaries arrive monthly, savings plans transfer monthly, and many banks credit interest monthly. That makes monthly compounding the natural default when you model a savings plan — deposits and interest land on the same schedule, so the yearly breakdown table maps cleanly onto what your bank statement will show. It is also the assumption this site’s calculator uses as its default preset on the home page.
With the preset above — $10,000 to start plus $100 added at the end of every month at 5% — the deposits contribute $12,000 over ten years, and compounding does the rest. Toggle the inflation adjustment to see the result in today’s money, or use the share button to send your scenario to someone else.
Reading a quoted rate correctly
Banks quote either a nominal rate (before compounding) or an APY (after). The conversion is APY = (1 + r/12)12 − 1 for monthly compounding, so a 5% nominal rate equals a 5.12% APY. If your bank quotes an APY, enter it with compounding set to annually; if it quotes a nominal rate compounded monthly, enter it here as-is. Mixing the two up double-counts compounding and overstates your projection.
Compare and extend
- Daily compound interest calculator — how much does 365-day compounding add over monthly? (Spoiler: at 5% on $10,000 over 10 years, $16.55.)
- Compound interest with monthly contributions — worked examples where the deposits, not the rate, do the heavy lifting.
- Full compound interest calculator — every frequency plus the inflation toggle and the step-by-step formula guide.
Frequently asked questions
What is the monthly compound interest formula?
FV = P × (1 + r/12)^(12×t). Divide the annual rate by 12 to get the monthly rate, then apply it once per month. $10,000 at 5% for 10 years: 10,000 × (1 + 0.05/12)^120 = $16,470.09.
How much more is monthly compounding worth than annual?
At a 5% nominal rate on $10,000 over 10 years, monthly compounding produces $16,470.09 versus $16,288.95 for annual — $181.14 more. The gap grows with higher rates, larger balances and longer horizons, but it stays modest compared to the effect of the rate itself.
Why does my bank quote both a rate and an APY?
The nominal rate is the raw annual figure before compounding; APY is what you actually earn in a year after compounding. A 5% nominal rate compounded monthly is a 5.12% APY, computed as (1 + 0.05/12)^12 − 1. APY is the number to compare across accounts because it neutralizes different compounding schedules.
Can I combine monthly compounding with monthly deposits?
Yes — that pairing is the most common real-world savings pattern and the cleanest mathematically, because deposits and compounding share the same period. Set a contribution amount with frequency "monthly" in the calculator above, or use our dedicated monthly-contributions calculator for worked examples.
Educational estimate only — not financial advice. Returns are not guaranteed.