Compound Interest Calculator (Monthly Contributions)
Project your investment growth with optional monthly contributions. See a year-by-year breakdown and interactive chart of your wealth trajectory.
Final Balance
$144,573
Total Contributed
$58,000
Interest Earned
$86,573
Year-by-Year Breakdown (20 years)
| Year | Balance | Total Contributed | Interest Earned |
|---|---|---|---|
| Year 1 | $13,201 | $12,400 | $801 |
| Year 2 | $16,634 | $14,800 | $1,834 |
| Year 3 | $20,315 | $17,200 | $3,115 |
| Year 4 | $24,262 | $19,600 | $4,662 |
| Year 5 | $28,495 | $22,000 | $6,495 |
| Year 6 | $33,033 | $24,400 | $8,633 |
| Year 7 | $37,900 | $26,800 | $11,100 |
| Year 8 | $43,118 | $29,200 | $13,918 |
| Year 9 | $48,714 | $31,600 | $17,114 |
| Year 10 | $54,714 | $34,000 | $20,714 |
| Year 11 | $61,147 | $36,400 | $24,747 |
| Year 12 | $68,046 | $38,800 | $29,246 |
| Year 13 | $75,444 | $41,200 | $34,244 |
| Year 14 | $83,376 | $43,600 | $39,776 |
| Year 15 | $91,882 | $46,000 | $45,882 |
| Year 16 | $101,003 | $48,400 | $52,603 |
| Year 17 | $110,783 | $50,800 | $59,983 |
| Year 18 | $121,270 | $53,200 | $68,070 |
| Year 19 | $132,515 | $55,600 | $76,915 |
| Year 20 | $144,573 | $58,000 | $86,573 |
What Rate Should You Use?
The interest rate input represents your expected annual return. For a high-yield savings account, use the current APY — typically 4 to 5 percent in 2026. For a broad stock market index fund (like a total market ETF), historical average real returns after inflation run around 6 to 7 percent annually over long periods. For retirement projections, 6 percent is a conservative and commonly used assumption. Avoid using past performance of individual stocks or recent bull market returns as your baseline.
Lump Sum vs Monthly Contributions
Set the principal to model a one-time investment — savings you already have, an inheritance, a bonus. Add a monthly contribution to model ongoing saving, like a pension contribution or automated transfer. The combination shows how both streams compound together. Even a small monthly amount added to a lump sum dramatically changes the outcome over 20 or 30 years because each contribution starts compounding immediately.
Compounding Frequency
Daily compounding produces slightly more than monthly, which produces slightly more than annual — but the difference is smaller than most people expect. At 7 percent, the difference between annual and daily compounding on £10,000 over 30 years is around £800. The rate and time horizon matter far more than frequency. Use monthly to match most real investment accounts.
Common Scenarios to Model
Retirement: Set years to your target retirement age minus your current age. Use 6 to 7 percent. Add your monthly pension or ISA contribution. The result shows your projected pot.
House deposit: Set years to 3 to 5, rate to current savings account APY, add your monthly savings target. See if the timeline is realistic.
Child education fund: Set years to 18 minus the child's current age. A modest monthly contribution started at birth compounds for 18 years into a meaningful sum.
Reading the Year-by-Year Table
Expand the table to see how the interest portion grows each year while contributions stay flat. In early years, most growth comes from your contributions. In later years, interest earned in a single year can exceed your total annual contributions. That crossover point — when compounding does more work than you do — is why starting early matters more than starting with a large amount.