Savings Jar Calculator

The savings jar is the oldest budgeting trick there is: drop spare coins in a jar, empty it at the end of the year, and see what the total comes to. The modern version swaps the jar for an interest-bearing account, so the money compounds while it sits there instead of gathering dust. This calculator projects both what accumulates and what it grows to once compound interest is added.

The habit works because it is low-friction. A daily 2 feels negligible, roughly the price of a coffee or a carton of milk, and over a calendar year that is 730, or 720 on the 30-day months this calculator counts in. Run it for 10 years in an account paying 4% and it compounds to about 8,835. The compounding is what makes the figure interesting, not the daily amount, which is precisely why the habit is worth a decade rather than a summer. Larger daily targets of 5 to 10 push the same 10-year figure into the 22,000 to 44,000 range at the same 4%.

The tool is also useful for setting a target you will actually hit. A sudden jump to saving 500 a month rarely survives contact with a real month. A 5-a-day habit tends to hold better because it slots in between small spending decisions rather than displacing a large one, and the calculator will show what that steadier, smaller number reaches over a long enough run.

Run it with the defaults

With a daily deposit of 2, a time horizon of 10 years and an annual interest rate of 4%, the calculation works out to 8,834.99. Of that, 7,200 is money deposited and 1,634.99 is interest, which is the split the result card shows beneath the headline. Adjust the inputs toward a specific situation and the output recalculates instantly. The defaults are a starting point, not a recommendation.

The levers in this calculation

The three inputs do not pull on the result with equal weight. Daily deposit scales linearly, so doubling it doubles the projection and nothing else changes. Time horizon and interest rate both compound, which makes their effect non-linear and increasingly lopsided over longer windows. At the default figures a 10% increase in the daily deposit lifts the result by exactly 10%, the same increase applied to the time horizon lifts it by 12.4%, and applied to the interest rate it lifts it by only 2.1%. Rate matters least at this scale, and matters more the longer the horizon runs.

How the math works

Daily deposit multiplied by 30 gives the monthly contribution. The future value uses the standard ordinary-annuity formula, FV = PMT × ((1 + r)^n − 1) / r, with the monthly rate r set to the annual rate divided by 12 and n set to 12 times the number of years. Contributions are treated as arriving at the end of each month. The 30-day month is a convention rather than a calendar fact: the true average is 30.44 days, so the projection understates by about 1.46%, which is small enough that the cleaner arithmetic is the better trade.

What this doesn't capture

The projection assumes the daily habit holds steadily and the interest rate stays where it is. In practice consistency varies, savings rates move with central-bank policy, and the figure is nominal, with no adjustment for inflation, fees or tax. Read the output as an illustration of what a sustained micro-savings habit compounds to under steady-rate assumptions rather than as a forecast of any particular account balance.