How to Figure Out How Much to Save Each Month

Every savings goal really boils down to one of two questions. If you already know what you can set aside each month, how long until you get there. If you already have a deadline, how much do you need to set aside each month to make it. Both questions use the same underlying math, the standard future-value formula that governs any account earning compound growth, just solved in two different directions.

Two directions, one formula

The first direction treats your monthly contribution as fixed and solves for time. You tell the calculator your goal amount, what you’ve already saved, your expected return, and how much you plan to contribute each month, and it works out how many months it’ll take to cross the finish line.

The second direction flips that around. You already know your deadline, maybe a wedding, a down payment, or a tuition bill, and you want to know the monthly contribution required to hit your goal by that specific month. Same inputs, just solved for a different unknown.

Both directions assume your current savings and your monthly contributions grow at the same expected rate of return, compounded monthly. That’s a simplification of real markets, but it’s the same simplification used in most retirement and compound interest calculators, and it’s close enough to plan around.

The formula

Solving for time works by simulating the balance month by month until it reaches the goal:

New Balance = Previous Balance x (1 + r) + Monthly Contribution

where r is your monthly rate (annual return divided by 12, divided by 100 to convert from a percentage). The calculator repeats that step, counting months, until the balance meets or exceeds your goal amount.

Solving for the required monthly contribution, when you already know how many months you have, uses the algebraic version of the same formula:

Growth Factor = (1 + r)^n
Required Monthly Contribution = ((Goal Amount - Current Savings x Growth Factor) x r) / (Growth Factor - 1)

Here n is the number of months in your target timeframe. This is just the future-value formula rearranged to isolate the payment instead of the ending balance.

Worked example: how long will it take?

Say you have $2,000 saved toward a $20,000 goal. You can put away $300 a month, and you’re assuming a modest 4% annual return, compounded monthly.

The monthly rate works out to 4% divided by 12, or about 0.33% per month. Running the simulation, month by month, that balance grows past $20,000 after 54 months, a little over four and a half years. That’s the same number the Savings Goal Calculator returns for these exact inputs, so you can check your own math against it before trusting a spreadsheet version.

Notice that the answer isn’t simply “$18,000 remaining divided by $300 a month,” which would suggest 60 months. The extra growth from compounding on both your existing balance and each new contribution shaves six months off that naive estimate. That gap only grows if your return assumption is higher or your timeline is longer.

Worked example: how much do I need to save each month?

Now flip the question. Say you have a $15,000 goal, $1,000 already saved, a 3% expected annual return, and a firm 24-month deadline.

The monthly rate here is 3% divided by 12, or 0.25% per month. Over 24 months, the growth factor (1.0025 raised to the 24th power) comes out to roughly 1.062. Plugging that into the required contribution formula gives a monthly amount of about $564.24. Save that much every month for two years at that assumed return, and your balance lands right at $15,000 on schedule.

That $564.24 figure is meaningfully lower than the naive “$14,000 remaining divided by 24 months” estimate of about $583.33, again because compounding does some of the work for you. The gap is smaller here than in the first example because the return rate and timeframe are both smaller, which is a useful intuition to carry forward: compounding matters more the longer your money has to work.

What return rate should you assume?

The return rate is the one input in this formula you can’t look up, you have to assume it, and the right assumption depends heavily on your timeframe.

For a goal within a year or two, like the 24-month example above, many people use 0% or a low rate that reflects a plain high-yield savings account. There’s simply not enough time for market swings to average out, and putting a vacation fund or a wedding fund into anything volatile risks coming up short right when you need the money.

For longer goals, five years or more, where the money could reasonably sit in a diversified investment account, a more moderate return assumption becomes reasonable. But any rate you enter is a planning assumption, not a promise. If you’re unsure, it’s usually safer to run the numbers twice, once with a conservative rate and once with a more optimistic one, so you can see the range rather than anchoring on a single number.

FAQ

Which mode should I use? Use “how long will it take” if you already know what you can contribute each month and just want a timeline. Use “how much do I need to save monthly” if you have a fixed deadline and need to know the contribution required to hit it.

Why isn’t the answer just the remaining amount divided by the number of months? Because that simple division ignores compounding. Both your existing balance and every contribution you add keep earning returns for the months remaining, so the actual required contribution (or the actual time needed) is usually a bit lower than the naive division suggests, especially over longer timeframes or higher return rates.

What happens if I enter a 0% return? The formula still works, it just simplifies to straight-line math with no growth. Solving for the required contribution becomes the goal amount minus current savings, divided evenly across the number of months, since there’s no compounding to help.

Use the Savings Calculator to run your own numbers.