Investment Growth Calculator
See what your money could grow to with regular contributions — or solve for the monthly contribution, return rate, starting amount or time a goal needs. In US dollars or Kenyan shillings, with an optional inflation adjustment.
Starting with 20,000.00 and adding 1,000.00 a month, at 6% a year for 10 years, gives about 198,290.40. Of that, 58,290.40 is growth — returns you did not pay in — 29% of the total.
See today's live rates on money market funds, Treasury bonds and fixed deposits in Kenya.
Estimate only. It assumes the same return every year; real investments, especially stocks, rise and fall, and actual results will differ. Taxes other than the optional 15% withholding tax are not included. Not financial advice.
| Year | Deposit | Growth | Ending balance |
|---|---|---|---|
| 1 | 32,000.00 | 1,526.53 | 33,526.53 |
| 2 | 12,000.00 | 2,338.12 | 47,864.65 |
| 3 | 12,000.00 | 3,198.41 | 63,063.06 |
| 4 | 12,000.00 | 4,110.31 | 79,173.37 |
| 5 | 12,000.00 | 5,076.93 | 96,250.30 |
| 6 | 12,000.00 | 6,101.55 | 114,351.84 |
| 7 | 12,000.00 | 7,187.64 | 133,539.48 |
| 8 | 12,000.00 | 8,338.90 | 153,878.38 |
| 9 | 12,000.00 | 9,559.23 | 175,437.61 |
| 10 | 12,000.00 | 10,852.79 | 198,290.40 |
How Investment Growth Is Calculated
The first term grows your starting amount. The second is the future value of every contribution you make, each one compounding for however long it has left to run — which is why a dollar added in year one is worth far more at the end than one added in year nine.
The subtlety is the contribution rate i. It is not simply the annual rate divided by twelve. It is the monthly rate equivalent to the effective annual rate, so that twelve months of growth compounds to exactly the annual figure you entered. Using rate ÷ 12 instead overstates the $20,000 + $1,000-a-month example by about $290 — small enough to pass review, large enough to be wrong.
The Compound setting is how often the quoted rate is applied. 6% compounded monthly is an effective 6.17% a year, so the same example reaches $200,267.28 instead of $198,290.40. If another calculator gives a slightly different answer for the same inputs, this — or whether contributions land at the start or the end of each month — is almost always why.
Worked Examples
Each row changes one thing from the first. The figures are what the calculator returns for those inputs.
| Scenario | Inputs | Result |
|---|---|---|
| Monthly contributions | $20,000 start, $1,000 a month at month-end, 6% compounded annually, 10 years | $198,290.40 ($140,000 paid in, $58,290.40 growth) |
| Contribute at the start of each month | As above, contributions at the beginning of each month | $199,081.24 |
| Compounded monthly | As above, 6% compounded monthly (6.17% effective) | $200,267.28 |
| In today's money | First row, with 3% inflation a year (hypothetical) | $147,546.68 of today's purchasing power |
| Time to a goal | $20,000 start, $1,000 a month, 6%: how long to reach $100,000? | 5 years 3 months |
| Monthly amount for a goal | $20,000 start, 7%, 20 years: how much a month to reach $500,000? | $832.66 a month |
| Return needed | $20,000 start, $500 a month, 30 years: what return reaches $1,000,000? | 8.48% a year |
The 6%, 7% and 3% figures are illustrations, not forecasts. Replace them in the calculator with your own assumptions.
Investment Growth Adjusted for Inflation
The end balance is a nominal figure: dollars or shillings in the year you reach them. Prices rise in the meantime, so the same amount buys less. Enter an inflation rate and the calculator also shows the real value — what the end balance would buy in today's money:
At a hypothetical 3% a year, $198,290.40 in ten years buys about what $147,546.68 buys today. Inflation is not constant either, so treat this as a second assumption rather than a correction. For US price history, see the Consumer Price Index from the Bureau of Labor Statistics; for Kenyan shillings, our Inflation Calculator.
Assumptions and Limits
- The return is the same every year. Stocks and stock funds do not grow in a straight line. They can fall sharply in some years, and once money is going in, the order of good and bad years changes the outcome. A constant-rate projection is a planning simplification, not a prediction, so try more than one rate. The SEC's Investor.gov compound interest calculator takes the same approach, showing results across a range of rates.
- Fees are not deducted separately. Enter the return after fees. A fund you expect to return 7% with a 0.5% annual expense ratio is roughly a 6.5% input.
- US taxes are not modelled. How growth is taxed depends on the account — a 401(k) or traditional IRA, a Roth account and a taxable brokerage account are each treated differently — so no single rate would be right. In the USD view every figure is before tax. The only tax option is Kenya's 15% withholding tax, in the KES view.
- Contributions stay level. The same amount goes in every period, with no annual increase and no withdrawals.
- Nothing is rounded until it is displayed. Schedule rows are rounded to the cent for display, so a column can differ from the total by a cent.
Solving Backwards: Rate, Time or Contribution
Most calculators only project forwards. The more useful question is usually the reverse one.
| If you want to know… | Pick this tab | What you enter |
|---|---|---|
| What my money becomes | End Amount | Starting amount, contribution, rate, years |
| How much a month to hit a target | Contribution | Target, starting amount, rate, years |
| What return I need | Return Rate | Target, starting amount, contribution, years |
| What lump sum to start with | Starting Amount | Target, contribution, rate, years |
| How long it takes | Length | Target, starting amount, contribution, rate |
The rate and length tabs are solved numerically, because neither has a closed-form answer once regular contributions are involved. Length is reported in whole months — the first month in which the balance reaches the target. If a target cannot be reached with the inputs given, the calculator says so rather than returning a confident wrong number.
Kenyan Withholding Tax (KES View)
Interest in Kenya is taxed before it reaches you. Money market funds and fixed deposits are subject to 15% withholding tax, deducted at source. Tick the box and the calculator compounds the after-tax rate instead, which is the number that actually accumulates in your account.
It matters more than it looks, because the deduction compounds too. A fund quoting 11% returns about 9.35% after tax, and over a decade that gap is not 1.65% — it is a materially smaller balance, since every year's tax is also money that never earned anything in the years after.
When you solve for the rate with tax switched on, the answer shown is the gross rate a fund would have to quote. That is deliberate: it is the figure you compare against the published yields on our money market fund table, not the post-tax number.
What Return Rate Should You Use?
The projection is only as honest as the rate you type. For US stocks or funds, no single rate is correct: run the projection at a cautious, a middle and an optimistic return, and plan around the cautious result.
For Kenyan shilling investments, rather than print a number here that would go stale, use the live tables: money market funds for short-term cash, Treasury bonds for a rate you can lock for years, and fixed deposits for a bank term. A money market yield is a current annualised rate, not a promise — it moves with the Treasury bill market, so projecting today's yield flat for twenty years is optimistic.
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