The same figure over a year
$15,568
Give the calculator a number you want to reach and a date you want to reach it by. It returns the monthly saving that gets you there — and what it takes if the number has to hold its purchasing power as well.
The return is an assumption you supply, applied at monthly compounding, and it is not a forecast. Contributions are assumed to arrive at the end of each month. No fees, expense ratios or taxes are deducted, and the inflation figure is used only to restate the target, never to change the return. Amounts in the first table are dollars; a negative figure in the last two columns means the balance falls short of the target by that much.
| Monthly saving | Amount | Balance at the end | Against the target | Against the larger target |
|---|---|---|---|---|
| Save half | $649 | $311,352 | -188,648 | -467,631 |
| Save three quarters | $973 | $405,676 | -94,324 | -373,308 |
| The plan | $1,297 | $500,000 | 0 | -278,984 |
| Save a quarter more | $1,622 | $594,324 | 94,324 | -184,660 |
| Save half as much again | $1,946 | $688,648 | 188,648 | -90,336 |
| Point in the plan | Years in | Balance | Share of the target reached |
|---|---|---|---|
| A quarter of the way | 3.75 | $127,869 | 25.6% |
| Halfway | 7.50 | $225,332 | 45.1% |
| Three quarters | 11.25 | $347,319 | 69.5% |
| End of the plan | 15.00 | $500,000 | 100.0% |
The compound interest calculator takes a monthly contribution and returns a balance. This page takes the balance you want and returns the contribution, which is the same equation rearranged to make the payment the unknown.
goal — the balance you want at the end, in future dollarscurrent — what is already in the account, which is grown at the same rate for the whole period before being subtractedr — the monthly rate; if the annual return is 0 the equation reduces to the target divided by the number of monthsn — the number of monthly deposits, so a 15-year plan is 180 of themThe rearrangement has one consequence worth stating: the answer is exact only if every deposit is made and the return is constant. It is a schedule, not a guarantee. What the formula does give is the minimum contribution under those assumptions, and any departure from them — a missed month, a poor decade, a fee — pushes the required figure up rather than down, because the missing growth has to be replaced by more money.
A target of $500,000 in 15 years is a statement about a number, not about what the number buys. At 3% inflation the price level rises by about 56% over 15 years, so $500,000 then is roughly what $321,000 is today. Reaching the target and preserving the purchasing power it was meant to represent are two different plans with the same headline figure.
The calculator therefore reports both. Reaching $500,000 on the default inputs takes $1,297 a month. Holding the purchasing power means aiming at $778,984 instead, and that takes $2,257 a month — $959 more, every month, for fifteen years.
| Plan | Monthly saving | Target | Total paid in |
|---|---|---|---|
| Reach the number | $1,297 | $500,000 | $283,524 |
| Hold purchasing power | $2,257 | $778,984 | $456,032 |
$50,000 already saved, 15 years, 6% annual return, 3% inflation
The gap is not a rounding error, and it is the whole reason this page asks for an inflation rate that other goal calculators do not. A plan built to reach a nominal figure on a long horizon is a plan to be poorer than intended in real terms, and the shorter the horizon the more forgivable that omission becomes: over five years the two targets differ by less than 16%, over thirty by well over a hundred.
On the default plan $283,524 is paid in and $216,476 of growth is added, so 43.3% of the final balance is return rather than savings. That proportion is not fixed through the plan; it climbs steeply at the end because growth applies to a balance that has become large.
The milestone table shows the same thing from the other side. A quarter of the way in, the balance is around $127,869, or 25.6% of the target; halfway, $225,332, or 45.1%. The plan reaches half its target only after more than half its duration has elapsed, and the last quarter of the time delivers the remaining 30.5%. That is the arithmetic of compounding, and it explains why a shortfall discovered late is far more expensive to fix than the same shortfall discovered early.
The ladder table makes the same point in the form you can act on. Halving the monthly saving does not halve the outcome — it misses the target by a much larger margin than half, because the growth on the missing money is also missing. A quarter more a month does not simply add a quarter, it overshoots, for the same reason in reverse. The relationship between contribution and outcome is convex, and the useful reading of the table is how steeply the shortfall grows when the contribution is cut.
If the answer is out of reach, there are three levers and they are not equivalent. Extending the horizon is the cheapest because it adds compounding years at both ends; raising the return is the least reliable because it is an assumption rather than a decision; raising the contribution is the most certain and usually the most painful. The compound interest calculator shows what a different horizon does to the same monthly figure.
Take a $500,000 target, $50,000 already saved, 15 years, a 6% annual return and 3% inflation. The monthly rate is 6 ÷ 100 ÷ 12 = 0.005, and there are 180 months.
The existing $50,000 compounds to $50,000 × 1.005180 = $122,712. Subtracting that from the target leaves $377,288 to be assembled from deposits. The annuity factor is (1.005180 − 1) ÷ 0.005 = 290.82, so the deposit is $377,288 ÷ 290.82 = $1,297.36 a month.
For the inflation-adjusted target, $500,000 × 1.0315 = $778,984. The same two steps on that figure give $2,256.66 a month, and the difference between the two deposits is $959.30.
A difference of a few cents against your own working is the rounding order. A larger one means one of the two implementations is wrong, and the corrections page explains how to report it.
This page does not supply one, and the reason is that the assumed figure compounds over the whole plan. On these inputs a 6% return requires $1,297 a month and a 4% return requires $1,596, a difference of 23% in the monthly obligation for a two-point change in an assumption. The safer approach is to run the plan at a modest rate and again at a pessimistic one and treat the gap between the two required contributions as your uncertainty, rather than choosing one rate and building a budget on it.
It depends on what the money is for. A fixed obligation denominated in dollars — repaying a specific loan, meeting a price agreed today — is a nominal figure and needs no adjustment. A future standard of living is not: a retirement sum fixed today buys less every year it is deferred, so the purchasing-power column is the one that describes the plan you actually want. This page reports both precisely so the choice is visible rather than hidden in the tool's default.
Substantially, and the effect is larger than most people expect. The existing balance is grown at the assumed return for the entire period before it is credited against the target, so $50,000 over 15 years at 6% becomes $122,712 and removes nearly a quarter of the target from the deposits you have to make. Change the existing balance on this page and watch the required monthly figure move: the early money does more work per dollar than any contribution made later.
Because growth applies to a balance rather than to a duration. Early in the plan the balance is small, so the return adds little; late in the plan the balance is large, so the same percentage adds a great deal. On the default plan the balance passes 50% of the target at around year eight of fifteen. It is the same effect that makes late shortfalls expensive, since the growth you miss at that point would have been applied to the largest balances.
Related calculatorsCompound interest calculator · Inflation and purchasing power calculator · Savings and APY calculator
Cluster us/savings · Unit us-savings-goal-savings-calculator · Engine compound-growth / goal · Method: The required contribution solves the ordinary annuity for the payment: monthly = (target − current × (1 + r)^n) × r / ((1 + r)^n − 1), with r as the annual rate divided by twelve and n as years × 12. The inflation-adjusted target is the nominal target compounded at the stated inflation rate over the same period; no other adjustment is applied to the rate or the term.