Glossary
What is a calorie deficit?
Updated 2026-08-16By Chris RichardsAll terms
Definition
A calorie deficit is a sustained gap between the energy you eat and the energy you burn, covered by stored energy including body fat. It is an average over days and weeks — not a single low-calorie day.
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Your estimate
kcal / day target · maintenance 2,199 · ~2,310 kcal weekly deficit
Estimate only — verify with a weight log over 2–3 weeks. Not medical advice.
Definition
A calorie deficit exists when average intake stays below average expenditure across days and weeks. The gap is an average, not a daily rule: a single high day inside an otherwise lower week does not cancel it, and a single low day does not create it.
Nobody measures a deficit directly. You estimate expenditure, log intake, and let the weight trend supply the feedback.
A worked example
Say your estimated maintenance level is 2,400 calories and you average 1,900. That is a 500-calorie daily gap — 3,500 across a week.
The old rule of thumb converts that 3,500 calories to about one pound of fat and stops. Dynamic models of energy balance show the straight-line version overstates long-run loss: expenditure falls as body mass falls, so the curve flattens instead of continuing at a fixed rate.
The mistake people make
The first error is treating the calculator number as a measurement. Every expenditure estimate carries error, so hold intake steady for a few weeks and let the scale correct it.
The second is going too steep. A larger gap only helps if you can hold it, and an unlogged binge week erases an aggressive plan faster than a moderate one.
Related terms
A deficit is measured against TDEE, built from BMR plus activity. How full you feel inside it often comes down to calorie density.
FAQ
How do I calculate a calorie deficit?
Estimate maintenance calories, then eat 10–20% below that. At 2,400 maintenance, a 20% deficit is 1,920 calories a day. Percentages scale with the person better than a fixed 500-calorie cut.
How big should a calorie deficit be?
There is no single correct size, and this page does not set targets. Common practice favours a moderate gap you can hold for months over the steepest one you can survive for a fortnight. With a health condition or a history of disordered eating, work the number out with a clinician.
Can you be in a deficit and not lose weight?
On the scale, yes — for a while. Water, glycogen, and gut contents move body weight by a kilo either way, hiding a week of real change. Read the weekly average, not a single weigh-in.
Sources
- [1] Hall KD, Sacks G, et al. “Quantification of the effect of energy imbalance on bodyweight” (2011) — The Lancet 378:826–837 (PMID 21872751)
- [2] Body Weight Planner — National Institute of Diabetes and Digestive and Kidney Diseases (NIH)
General nutrition information, not medical advice. Calorie and macro targets are estimates — talk to a doctor or a registered dietitian before changing your diet, especially if you are pregnant, managing a health condition, or have a history of disordered eating.
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