Calorie calculator

How this works it out

It uses the Mifflin-St Jeor equation, which is the one most dietitians reach for and the most accurate of the common formulas for people who aren't athletes. It estimates what you burn at complete rest, multiplies that by how active you are, then takes an amount off depending on what you're trying to do.

The four numbers it gives you

• At rest — what you'd burn lying in bed all day. Mostly driven by your size.
• You burn — that figure multiplied out for your activity. This is maintenance: eat this and your weight holds.
• Eat around this — your target. Maintenance minus a sensible amount.
• Never below — a floor. Going under it is how people end up bingeing.

Be honest about the activity line

This is the setting that moves the answer more than anything else, and most people are a notch lower than they think. "Moderate" means three to five real training sessions a week, not an intention to start. If you're unsure, pick the lower one — you'll lose weight either way, and you won't be left wondering why the number isn't working.

What the number is and isn't

It's an estimate. Real needs vary by around ten per cent either way, because metabolism, NEAT and how accurately anyone logs food all differ. So treat it as a starting point rather than a fact: eat to it for a fortnight, see what the scale actually does across that time, then adjust by 100 or 200 rather than starting again.

Then comes the harder bit

Knowing your number is the easy part. The weeks that go wrong aren't the ordinary ones — they're the ones with a wedding, a holiday or somebody's birthday in them, where the plan assumed every day looks the same and it doesn't.

That's what the rest of Leeway does: you tell it what's coming up and it spreads the cost across the days around it, so the night out is accounted for before you get there.