Girl math says it’s free. Your bank app disagrees.

Nothing here is a forecast. Every number comes off the sliders you set, and the arithmetic is deliberately boring.

• Girl math says is the claim taken at face value: the saving, the rebate, the refund, the cost per wear you quoted.
• It actually cost is the money that left your account, or the money that never arrived. A rebate is not income. A refund is not income. A discount is not income.
• Kept and invested instead takes the leak the claim was hiding, spreads it monthly, and compounds it at the annual rate you picked divided by twelve, contributed at the end of each month.
• Three of these come out true depending on your numbers, and the page says so when they do. That is on purpose. A tool that always tells you that you are wrong is not a tool, it is a lecture.

No taxes, no fees, no inflation adjustment. Those all matter in real life. Leaving them out keeps the shape of the argument visible.

Girl math started as a joke women told about themselves, and it went everywhere because everybody recognised it. The three claims filed under boy math are here for the same reason. The gender part is a punchline. The logic is universal, and it is old.

Every one of these is the same move: count the good half of a transaction and quietly drop the other half. Retail has known about it for a century. That is why prices end in 99, why the shipping threshold sits just above your cart, why the sticker shows the old price crossed out, and why everything is now billed per day.

You do not need more discipline. You need the second half of the sentence. That is all this page does.