Compound Interest Calculator
Project savings growth with regular monthly contributions.
Last updated: January 2025 · Built for the 2024–25 financial year
Inputs
Results
Final balance
$264,122
Total contributed
$130,000
Interest earned
$134,122
Growth over time
Year 1Year 20
Results are estimates only. For financial advice, consult a licensed adviser.
Overview
The eighth wonder of the world
Compound interest is interest earned on interest. Reinvested gains start to dwarf the original capital after about 15 years — the foundation of every long-term portfolio, super balance and FIRE plan.
This tool lets you model lump sum, regular contribution, frequency and time horizon. Try doubling the time and halving the rate to see how Buffett's '70% of my net worth came after age 60' really works.
Formula
FV with regular contributions
FV = P(1+r/n)^(nt) + PMT · [((1+r/n)^(nt) − 1) / (r/n)] P = principal, PMT = recurring contribution, r = annual rate, n = compounding periods/yr, t = years
Worked example
$10,000 + $500/month, 7% p.a., 30 years
- Lump sum future value = $76,123.
- Contribution future value = $610,162.
- Total future value ≈ $686,285.
- You contributed $190,000 — interest earned $496,285.
Answer: ≈ $686,000 (72% pure growth)
Pro tips
- •Time beats rate. 10 yrs at 8% < 30 yrs at 5%.
- •Reinvest dividends — that's what 'compounding' really means.
- •Index funds inside super are compounding tax-advantaged at 15%.