How to Calculate Compound Interest Monthly
Calculate growth when interest compounds 12 times per year.
Key idea
Calculate growth when interest compounds 12 times per year.
Formula or method
Worked example
Start with the values in your question, substitute them into the formula, and keep the units consistent. For example, if a rate is given as 15%, use 0.15 when a formula requires a decimal.
Common mistakes
- Mixing percentage points with percentage change.
- Using different units for the same calculation.
- Rounding too early when several steps are involved.
Why the result can differ
Real-world calculations can include assumptions, rounding, fees, tax rules, calendar conventions, or other details. Treat simple calculator results as estimates when the underlying situation is more complex.
Monthly compounding step by step
For a nominal annual rate compounded monthly, divide the annual rate by 12 to obtain the periodic rate and multiply the number of years by 12 to obtain the number of periods. The formula then applies the periodic growth repeatedly.
Worked example
At 6% nominal annual interest compounded monthly, the periodic rate is 0.06 ÷ 12 = 0.005. Over two years there are 24 periods, so the growth factor is (1.005)^24.
Watch the wording
An advertised annual rate and an effective annual rate are not always the same. Check how a real account defines its rate and compounding frequency before comparing products.