Kids Savings Calculator

Pick a rate before you promise one. The jar below compounds every week, the way a parent paying interest actually works.

Look at the last week, not the first

A parent picking a rate almost always picks it from the first week, where the number is small enough to feel free. One percent of five dollars is five cents. Nobody hesitates at five cents.

But the rate applies to the jar, and the jar grows. By week fifty the same one percent is being charged against a few hundred dollars, and the weekly payment has grown with it. That figure is the last result above, and it is the one worth checking before you promise anything, because it is what you will actually be handing over a year from now.

Why a silly rate is the right rate

One percent a week is roughly sixty-eight percent a year. No bank pays that, no index returns it, and if a stranger offered it you would walk away. It is still the correct number here, because you are not investing your child's money. You are buying a demonstration.

At a realistic rate the demonstration fails. Two percent a year on a ten dollar jar is twenty cents over twelve months, which a seven-year-old cannot perceive, let alone be motivated by. The lesson is not the yield. The lesson is that money left alone becomes more money, and a child only learns it if the effect is visible inside their attention span.

What this does not model

Spending. The jar above assumes nothing ever comes out, which is not how a real child behaves and not how the app works either. A withdrawal resets the compounding to a smaller base and the curve flattens accordingly. Treat the figure here as the ceiling of a perfectly patient year.

It also assumes you keep paying. The arrangement is only a lesson for as long as it holds, so the honest version of this calculator is the one where you check the final week and pick a rate you will still be comfortable with in twelve months.

Questions people ask

Why weekly, and not monthly?

Because a month is too long for a young child to connect a cause to an effect. A number that is bigger than last week, handed over by a parent who says where it came from, is a lesson that lands. A monthly statement is invisible.

What rate should I pick?

Look at the last row, not the first. One percent a week looks tiny and reaches roughly 68 percent over a year of steady deposits, which is generous but survivable on pocket-money amounts. Anything above two percent a week compounds into a commitment most parents would rather not have made, and the calculator will show you that before your child does.

Is this a real interest rate?

No, and it is not meant to be. No bank pays this. You are buying a demonstration of compounding with a small amount of your own money, at a speed a child can perceive. The number is a teaching tool, not a market rate.

Does the deposit earn interest in the week it arrives?

Yes. Allowance is credited first, then interest is paid on the jar including that deposit. SharkBucks settles a payday in the same order, so the figures here and in the app agree.

Will the app show exactly these numbers?

Within a coin or two. The app rounds each week to the smallest unit of your currency, because a ledger cannot hold a third of a cent; this page carries the full decimal through. Over a year the two drift by a few cents, never more.

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