The Compound Interest Formula
For a lump sum with no additional contributions:
A = P × (1 + r/n)^(n×t)
P = principal | r = annual rate | n = compounds per year | t = years
With regular contributions (PMT per compounding period):
A = P × (1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)]
Alaska example — saving $$797/month (10% of median income) at 7% compounded monthly:
- Monthly rate: 7% ÷ 12 = 0.5833%
- After 10 years: $137,949 ( $95,640 contributed + $42,309 interest)
- After 20 years: $415,179
- After 30 years: $972,317 ( $286,920 contributed + $685,397 interest)
The $10,000 lump sum comparison: $10,000 invested at 7% (monthly compounding) for 30 years grows to $81,165 — a 712% total return on the original principal. This is the power of compound interest: returns on returns, compounding continuously over time.