How to Calculate Compound Interest Manually
While calculators handle the math instantly, understanding the manual process builds intuition for how compounding works.
The Formula
A = P(1 + r/n)nt
Step-by-Step: Annual Compounding
$2,000 at 5% annually for 3 years:
- Year 1: $2,000 × 1.05 = $2,100
- Year 2: $2,100 × 1.05 = $2,205
- Year 3: $2,205 × 1.05 = $2,315.25
Step-by-Step: Monthly Compounding
$2,000 at 5% monthly (n=12) for 1 year:
- Monthly rate = 0.05 ÷ 12 = 0.004167
- A = 2000 × (1.004167)12
- A = 2000 × 1.05116 = $2,102.32
Common Mistakes to Avoid
- Forgetting to convert % — 5% must be entered as 0.05.
- Wrong compounding periods — Monthly means n=12, not n=1.
- Confusing A and interest earned — A is the total balance; subtract P to get just the interest.
Save time with our compound interest calculator — it handles all compounding frequencies instantly.