Savings Goal Calculator — How Long to Save a Target
Enter what you are saving for, what you can put aside each month and the rate you expect, and see how long it takes — in months and years — and how much of the goal the interest covers.
Enter the goal and the monthly saving.
- Total contributed
- —
- Interest earned
- —
Amounts use the same currency as the savings target.
What this calculator does
It answers a single question — how long until I have enough? — from three inputs: the amount you are aiming for, what you have already and what you can add each month. It also separates the money you saved from the money the account earned, so you can see how much of a long goal is carried by interest rather than by discipline alone.
Method
The balance is projected one month at a time, growing at the monthly rate and adding the monthly saving. The number of months can also be solved directly from the annuity relation:
n = ln((T + PMT/r) / (P + PMT/r)) / ln(1 + r)
where T is the target, P the amount already saved, PMT the
monthly saving and r the monthly rate. When the rate is zero the formula reduces to
(T − P) / PMT. The reported figure is then confirmed against the month-by-month
simulation, so it is a real horizon rather than an approximation.
Example
| Target | Saved | Monthly | Rate | Time | Interest earned |
|---|---|---|---|---|---|
| 10,000 | 0 | 500 | 0% | 20 months | 0 |
| 50,000 | 5,000 | 400 | 4.5% | 91 months | 8,915 |
| 2,000 | 1,500 | 100 | 0% | 5 months | 0 |
In the middle row, seven and a half years of saving 400 a month is helped along by nearly 8,900 in interest — a reminder that the rate matters more over a long goal than a slightly larger contribution.
The three levers, measured
A savings goal has three inputs you can change, and they do not move the answer equally. Starting from the middle row above — 50,000 target, 5,000 already saved, 400 a month at 4.5% — here is what each lever buys on its own:
| Change | Months to goal | Effect |
|---|---|---|
| Nothing (baseline) | 91 | — |
| Monthly saving 400 → 500 | 76 | 15 months sooner |
| Already saved 5,000 → 10,000 | 79 | 12 months sooner |
| Rate 4.5% → 6.0% | 86 | 5 months sooner |
The ordering is the practical lesson: the contribution is the lever you control, the starting balance is history, and the rate is a market variable that moves both ways. Chasing an extra percentage point of interest is worth less than finding another 100 a month — and the interest rate is the one input that can fall after you have committed to the plan.
What the projection assumes
- A constant rate. One annual rate, applied monthly for the whole term. Real accounts change rates.
- Monthly compounding. Interest is credited monthly; daily compounding at the same nominal rate would reach the goal marginally sooner.
- End-of-month contributions. Each monthly saving is added at the end of the month, so it earns no interest in its first month.
- No withdrawals. A goal that pays out partway through needs two runs, not one.
- No tax, no fees, no inflation. Each of these is a reduction in the effective rate; enter the rate you actually keep, and raise the target if it is a future price.
Those assumptions are not reasons to distrust the number — they are the reason to read it as a planning horizon rather than a guarantee. The contribution schedule is the part you can keep; the rate is the part to stress-test by rerunning at a lower figure.
Using the answer
A result of 91 months is more useful when it is turned into a monthly target. If the deadline is fixed — a deposit needed in five years — run the calculation backwards: keep the rate and the starting balance, set the term to the deadline, and raise the monthly contribution until the projected balance reaches the goal. That number is the actionable one, because it is the only input you can adjust on a payslip-by-payslip basis.
It is also worth running the goal twice: once with the rate you expect, and once with zero. The zero-rate case is the amount you must save from your own income, and the difference between the two is what the account is contributing. If the zero-rate monthly figure is not affordable, the goal is not a savings-rate problem — it is a target or timeline problem, and no interest rate fixes that.
This is an estimate for planning, not financial advice. It assumes a constant rate and no tax or fees, and does not account for inflation reducing what the goal buys.
Frequently asked questions
How long will it take to reach the goal?
The tool projects the balance forward month by month — growing at the rate you enter and adding the monthly saving — and reports the month it first reaches the target. If the rate is zero it is simple division; with a rate the interest shortens the time, and the effect grows with the horizon.
What if I save nothing each month?
With no monthly saving you still reach the goal if the starting amount grows at a positive rate, but it takes far longer. With neither a contribution nor a rate the goal is never reached, and the tool says so rather than returning an infinite number.
Should I enter interest after or before tax?
Enter the rate you actually keep. If the account pays 4% but tax takes a quarter of the interest, the effective rate is about 3%. Using a lower rate gives a more honest timeline.
Does inflation change the answer?
It changes what the goal is worth, not how long the nominal balance takes to reach it. A common approach is to set the target at today’s prices and use a rate reduced by expected inflation, so the goal stays in today’s money.
Why is the interest earned so small at the start?
Interest is a percentage of the balance, and early on the balance is mostly your own contributions. The interest accelerates as the balance grows — which is why the last stretch of a long goal usually goes faster than the first.
What compounding frequency does the calculator use?
Monthly. The annual rate is divided by twelve and applied to the balance at the end of each month, with the monthly contribution added in the same step. An account that compounds daily, or that credits interest annually, will produce a slightly different result — the effective annual rate is what matters, and the tool shows the nominal rate you enter applied monthly.
What if the interest rate changes during the term?
The projection holds one rate for the whole period. In reality savings rates move, and a long goal will see several of them. A reasonable approach is to rerun the calculation with a lower rate as a stress case, and to treat the result as a range: the promise is the contribution, not the rate.
Should I include account fees?
Yes, by lowering the rate. A 4.5% account with a 0.5% annual platform fee behaves like a 4% account for this calculation. Fees are charged on the balance, so their effect grows with the horizon in exactly the same way the interest does — which is why a small percentage fee is not small over a ten-year goal.
What if I plan to increase my monthly saving later?
The calculator assumes a constant contribution. Raising it partway through saves more than the average suggests, because the larger contributions also earn interest for the remaining months. Model the plan in two runs — the current contribution until the raise, then the higher one — and add the months together, rather than averaging the two amounts.
Does the target need to allow for inflation?
If the goal is a price in the future — a house deposit, a car, tuition — yes. Either raise the target by expected inflation over the horizon, or lower the interest rate by the inflation rate and keep the target at today’s price. Mixing the two — a future price target with today’s nominal rate — overstates how quickly you will get there.
Go deeper
Longer explanations of the problems this tool solves:
Sources
- Time-to-target solution of the annuity relation, solved for the number of months and confirmed by a month-by-month simulation.
- U.S. Federal Reserve, consumer resources on saving and the effect of regular contributions. federalreserve.gov/consumerinfo
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