Savings Jar Calculator

The savings jar is the oldest budgeting trick in the world — drop spare coins in a jar, empty it out at the end of the year, and watch the total stack up. A modern version replaces the jar with an interest-bearing savings account, so the money compounds while it sits there. This calculator projects both how much accumulates and what it grows to with interest.

The habit works because it's low-friction. A daily 2 feels negligible — the price of a coffee or a pint of milk. Over a year, that's 730. Over 10 years in an account paying 4% annually, it compounds to roughly 8,835 in the entered currency. The compounding is what makes the number meaningful, not the daily amount itself. Larger daily targets in the 5-10 range push the 10-year figure into roughly the 22,000-44,000 range under the same 4% assumption.

The tool can help set a realistic daily target. A sudden jump to a 500-a-month savings habit rarely sticks. A 5-a-day habit tends to be easier to hold because it slots in between other small spending decisions rather than replacing a large one.

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 in the entered currency. 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 — Daily Deposit, Time Horizon, and Annual Interest Rate — don't pull on the result with equal weight. Daily deposit scales linearly: doubling it doubles the projection. Time horizon and interest rate both compound — small changes in either move the result non-linearly, especially over longer windows. Flipping one input at a time toward extreme values is the quickest way to see which lever matters most for a given situation.

How the math works

Daily deposit × 30 gives the monthly contribution figure used in the formula. The future value uses the standard ordinary-annuity formula FV = PMT × ((1 + r)^n − 1) / r, with the monthly rate r = annual rate ÷ 12 and n = 12 × years. The 30-day-month convention introduces a small understatement against actual calendar days (true days per month average 30.44), which is acknowledged below — the simpler 30 keeps the projection clean and the under-statement is small. The working is transparent and the formula is in the section below.

What this doesn't capture

The projection assumes the daily habit holds steadily and the interest rate stays constant. In practice, daily consistency varies, the rate on a savings account fluctuates with central-bank policy, and the figure is nominal — it isn't adjusted for inflation, fees, or tax. The output is best read as an illustration of what a sustained micro-savings habit could compound to under steady-rate assumptions, rather than a forecast of any specific account balance.