Compound Interest & SIP / 401(k) Growth Predictor
Project how an initial deposit plus recurring monthly contributions grow, with an inflation-adjusted view in today's money.
Used to show what the future balance is worth in today's money.
| Year | Contributions | Interest earned | Balance |
|---|---|---|---|
| 1 | $8,600.00 | $549.98 | $9,149.98 |
| 2 | $12,200.00 | $1,444.40 | $13,644.40 |
| 3 | $15,800.00 | $2,711.85 | $18,511.85 |
| 4 | $19,400.00 | $4,383.31 | $23,783.31 |
| 5 | $23,000.00 | $6,492.29 | $29,492.29 |
| 6 | $26,600.00 | $9,075.11 | $35,675.11 |
| 7 | $30,200.00 | $12,171.10 | $42,371.10 |
| 8 | $33,800.00 | $15,822.86 | $49,622.86 |
| 9 | $37,400.00 | $20,076.51 | $57,476.51 |
| 10 | $41,000.00 | $24,982.01 | $65,982.01 |
| 11 | $44,600.00 | $30,593.46 | $75,193.46 |
| 12 | $48,200.00 | $36,969.46 | $85,169.46 |
| 13 | $51,800.00 | $44,173.46 | $95,973.46 |
| 14 | $55,400.00 | $52,274.19 | $107,674.19 |
| 15 | $59,000.00 | $61,346.07 | $120,346.07 |
| 16 | $62,600.00 | $71,469.72 | $134,069.72 |
| 17 | $66,200.00 | $82,732.41 | $148,932.41 |
| 18 | $69,800.00 | $95,228.71 | $165,028.71 |
| 19 | $73,400.00 | $109,060.99 | $182,460.99 |
| 20 | $77,000.00 | $124,340.14 | $201,340.14 |
| 21 | $80,600.00 | $141,186.25 | $221,786.25 |
| 22 | $84,200.00 | $159,729.38 | $243,929.38 |
| 23 | $87,800.00 | $180,110.37 | $267,910.37 |
| 24 | $91,400.00 | $202,481.78 | $293,881.78 |
| 25 | $95,000.00 | $227,008.80 | $322,008.80 |
How the projection is built
Each month, the running balance earns interest at the monthly rate, then the contribution is added — repeated for every month in the term.
$5,000 initial + $300/month at 8% for 25 years
Starting with a $5,000 deposit and adding $300 every month at an 8% annual return compounded monthly, the balance grows to roughly $290,000 after 25 years. Of that, about $95,000 came directly from contributions (the $5,000 start plus 300 monthly payments) — the remaining $195,000 is growth. Adjusted for 3% average inflation, that balance is worth roughly $138,000 in today's purchasing power.
| Input | Value |
|---|---|
| Initial deposit | $5,000 |
| Monthly contribution | $300 |
| Annual return | 8% |
| Future value (25 yrs) | ≈ $290,000 |
The Rule of 72 — a mental-math shortcut
Before reaching for a calculator, you can estimate how long a lump sum takes to double at a given annual return by dividing 72 by the rate. It is not exact, but it is close enough for quick planning.
This shortcut assumes a lump sum with no further contributions — it does not account for the extra growth a recurring SIP or 401(k) contribution adds each month, which is what the calculator above models in full.
Common questions
Textbook compound interest projects a single lump sum forward. This calculator instead compounds the balance every month and then adds your monthly contribution, repeating for the full term — the same mechanics behind a SIP (systematic investment plan) or a 401(k) with regular payroll contributions. Small, regular contributions compounding for decades are usually what drives the bulk of the final balance, not the size of the initial deposit.