What Is a Calorie Deficit, and How Big Should It Be?

A calorie deficit is simply eating below your TDEE, your real, personal maintenance number. It’s the actual mechanism behind weight loss, and the real question that matters isn’t whether to have one, it’s how big it should be.

How big should it be

The NHS’s own guidance is direct: “aim to reduce your intake by around 600 calories a day. This is a safe and sustainable way to lose weight.” That maps to a real, expected rate, the NHS separately states a safe pace of weight loss is “1 to 2lbs, or 0.5 to 1kg, a week.”

Why a bigger deficit isn’t automatically better

A much larger deficit, especially one that skips meals or drops well below a few hundred calories a day, is genuinely harder to sustain, and a real risk is losing muscle along with fat rather than just fat. The NHS’s own 600kcal figure isn’t a conservative fallback, it’s the actual recommended starting point for most healthy adults, not a minimum to beat.

A real worked example

Take the worked TDEE example from our own TDEE calculation guide, a 30-year-old woman with a TDEE of 2,124kcal. A real 600kcal deficit puts her daily target at 1,524kcal, still a real, adequate amount of food, not a crash diet.

Where you will see it used on this site

Work out your own TDEE first using our TDEE calculation guide or the calculator’s “Daily Calorie Need” tab, then subtract your real deficit from it. Our Meal Builder lets you build real meals against that new target and see exactly where you land. For keeping that same deficit intact on a night out, see Eating Out Without Blowing Your Calorie Budget.

*This article is general information, not medical or dietary advice. If you’re making dietary changes for a specific health reason, speak to a doctor or registered dietitian.