Compound Interest Visualiser

By Jenna Hale · Last checked August 19, 2026

Use the second number, not the first

Every version of this calculator on the internet shows a large number at the end, and the large number does not work on teenagers. It is too far away, and it reads as a trick.

The line underneath it is the one that lands: what the same weekly amount reaches if they start ten years later. That gap is not caused by saving less. It is the same money, arriving late. It is the only part of this subject where doing nothing has a visible price tag.

The split matters more than the total

Look at the two bars. After a few decades, the money anyone actually put in is the smaller share, and growth is the larger one.

That is the whole idea, and it is the bit that is genuinely hard to believe until you see it laid out. It also explains why time matters so much more than amount at this age. A teenager has almost no money and an enormous amount of time, which is the one advantageous position they will ever hold in this particular trade.

What it looks like

Nora is sixteen, working weekends, and clearing about a hundred and forty dollars a week after the deductions nobody warned her about. She agrees to put twenty of it away, which she describes as not very much.

Twenty dollars a week is a little over a thousand dollars a year. The point her parents make is not about the final figure. It is that if she starts at twenty six instead, having spent the decade in between, she does not get that decade back at any price.

She still spends the other hundred and twenty. That was always the deal.

Where the money goes is a separate question

The calculator is agnostic about the account, deliberately, because the arithmetic of compounding is the same everywhere and the account rules are not.

If a teenager has earned income from a job, a custodial Roth IRA is the account usually discussed, and there are real conditions: the contribution cannot exceed what they actually earned, and someone has to open and control it. The rules are here.

For anything they might need within a few years, this calculator is the wrong tool and a savings account is the right answer.

Be honest with them about the assumption

Seven percent is a convention, not a promise. It is used here as a nominal rate, which means inflation has not been taken out of it, so the figure at the end is in the dollars of that future year rather than in today’s dollars. Real markets deliver an average like that across decades containing years that are severely negative, and a teenager who is told to expect a smooth line will conclude they were lied to the first time they see a statement go down.

Move the slider to four percent while they are watching. The final number falls a long way and the argument does not change at all, because the argument was never about the rate. That is a more durable thing to teach than an impressive figure.

This is general information, not investment or tax advice, and the return here is an assumption rather than a forecast. Verify anything you plan to act on against the IRS pages linked above or ask a professional.