Weekly Interest a Kid Can Actually See

Published 2026-09-11 · All guides

Every adult who understands compound interest learned it twice. Once as a formula, which explained nothing, and once from watching a balance they cared about grow on its own.

The problem with teaching a child the first version is that the second one is unavailable to them. Compounding at real rates on real time scales is imperceptible to someone whose planning horizon is next weekend. A savings account paying two percent a year turns a ten dollar jar into ten dollars and twenty cents over twelve months. There is nothing there to notice.

So you have to forge it. You become the bank, you pay a rate that no bank would offer, and you pay it weekly instead of annually. Every part of that sentence sounds wrong and every part of it is the point.

Why the rate has to be absurd

The instinct is to teach an honest number. Give a child a realistic rate so they learn how the world works. This fails immediately, because a realistic rate produces no observable effect, and a lesson with no observable effect is not a lesson.

What a child needs to learn first is not the size of returns. It is the existence of the mechanism: money you leave alone turns into more money, and the more you have left alone, the faster that happens. That single fact is the load-bearing one. Everything else — inflation, risk, the difference between a savings account and an index fund — is refinement on top of it, and none of the refinement is reachable until the base fact is felt rather than recited.

One percent a week makes the mechanism visible. A five dollar weekly deposit becomes a jar of about three hundred and forty dollars over a year, and roughly eighty of those dollars appeared without anyone putting them there. A child can see eighty dollars. Try it in the kids savings calculator with the rate at zero and then at one percent; the difference between those two numbers is the entire curriculum.

Why weekly, and not monthly

A month is a long time. To a six-year-old it is an abstraction, roughly the same size as a year and about as motivating.

A week is different, because a week is a unit a child already lives in. There is a day the allowance arrives. If the interest arrives on the same day, and you say the number out loud, and it is a little bigger than the number you said last time, the causal link closes. The child does not need the formula to notice that waiting produced something.

Weekly also makes the compounding itself observable inside a reasonable window. Annual compounding needs a decade before the curve visibly bends. Weekly compounding bends inside a few months, which is inside the span a child can hold in memory.

The part that costs you

Here is what the calculator is really for. The rate you are choosing does not apply to the five dollars that arrives this week. It applies to the whole jar, and the jar grows, so your weekly payment grows with it.

At one percent a week, the first payment is five cents. Around week fifty, the same one percent is being charged against a few hundred dollars and the weekly payment has grown to a few dollars. That is not a problem — a few dollars a week to teach a child how compounding works is the cheapest tuition available — but it should be a decision rather than a surprise. Two percent a week looks only slightly bolder and costs about two and a half times as much interest over the same year, with the final week’s payment rising in step.

The last result on the calculator page is the final week’s payment on its own. Check that number, not the first one. If you are comfortable with it, the arrangement will survive the year, and an arrangement that survives is the only kind that teaches anything.

What to do when they spend it

Let them. The savings jar is not a trap and a child who cannot spend has not learned a choice, only a rule.

What the spending does teach is the other half of compounding, the half adults usually learn late: a withdrawal does not just cost you the amount withdrawn, it costs you everything that amount would have earned. The jar drops, and then it grows more slowly than it did before, from a smaller base. You do not have to explain this. The next few weeks explain it, and they explain it better than you would.

That is the argument for the three-jar split in the first place. The spend jar exists so the save jar can be left alone, and the save jar being left alone is the only condition under which any of this works.