Compound Interest Calculator (2026)
Enter your starting balance, monthly contribution, annual return rate, and time horizon — the calculator returns your ending balance, total interest earned, and a year-by-year breakdown.
Use the Calculator
→ Open Compound Interest Calculator
What to enter:
- Starting balance (lump sum already saved, or $0 to start fresh)
- Monthly contribution (what you'll add each month)
- Annual return rate (use 3.14% APY for a HYSA; use 7–10% for long-run equity estimates)
- Time horizon in years
What you get:
- Final balance at the end of your time horizon
- Total principal contributed
- Total interest earned (what compounding added on top)
- Year-by-year growth table
Quick Reference: What $10,000 Grows To
At a 7% annual return, compounded monthly:
| Time horizon | Starting $10k (no contributions) | Starting $10k + $200/mo |
|---|---|---|
| 10 years | $20,097 | ~$54,700 |
| 20 years | $40,388 | ~$144,500 |
| 30 years | $81,136 | ~$325,100 |
| 40 years | $163,048 | ~$687,600 |
At the current HYSA rate of 3.14% APY:
| Time horizon | $10,000 lump sum | $10,000 + $200/mo |
|---|---|---|
| 5 years | $11,693 | ~$24,700 |
| 10 years | $13,675 | ~$41,800 |
| 20 years | $18,700 | ~$85,400 |
How Compound Interest Is Calculated
The formula for a lump sum with monthly contributions:
FV = P × (1 + r/n)^(n×t) + PMT × [(1 + r/n)^(n×t) − 1] / (r/n)Where:
- P = starting balance (principal)
- PMT = monthly contribution
- r = annual interest rate (as a decimal)
- n = compounding periods per year (12 for monthly)
- t = years
The calculator handles this automatically. The key insight: each monthly contribution earns interest for all remaining months. Contributions made early in a 30-year period have 25–30 years to compound; contributions made in year 29 barely compound at all.
The Cost of Waiting
At 7% annual return, $200/month for different start ages (retiring at 65):
| Start age | Years investing | Total contributed | Portfolio at 65 |
|---|---|---|---|
| 25 | 40 | $96,000 | ~$524,800 |
| 30 | 35 | $84,000 | ~$364,100 |
| 35 | 30 | $72,000 | ~$243,200 |
| 40 | 25 | $60,000 | ~$156,400 |
Waiting 5 years from age 25 to 30 costs ~$160,700 in final value — for a $12,000 saving in contributions. Every 5-year delay roughly halves the compounding benefit.
What Rate to Use
| Scenario | Rate to enter |
|---|---|
| High-yield savings account | 3.14% |
| Money market account | 3.0–3.3% |
| CD (5-year) | ~4.1% |
| Conservative portfolio | 5–6% |
| S&P 500 historical average | 10% |
For savings accounts, use the actual APY from your bank. For investments, any rate is an assumption — use multiple scenarios to stress-test your plan.
This article is for informational purposes only and does not constitute financial advice.