Cost Per Use: It's $180. If I wear it weekly for two years, is that expensive?

Four times the money and half the cost, because one pair lasts five times as long. Works out what each option really costs every time you use it, and the month the expensive one overtakes buying the cheap one again and again.

WHAT YOU GET

- Cost per use and cost per year, with running costs and resale counted in

- The break-even month, which is the number that actually decides it

- A chart where the cheap line steps up every time you replace it

- Says plainly when the expensive one never catches up

- And reminds you the whole thing rests on how often you'll really use it

HOW IT WORKS

1. List the options: The cheap one, the dear one, and keeping what you've got.

2. Say how often you'll use it: The number you'd bet money on, not the hopeful one.

3. Read the crossover: And decide whether you'll keep it that long.

One file. Double-click it and it opens in the browser you already have: Chrome, Edge, Safari or Firefox. Nothing to install, no account, no subscription.

Nothing you type ever leaves your computer. There is no server involved, which is why it still works with the wi-fi switched off.

30-day refund, no questions asked.