How a Calorie Deficit Works

A calorie deficit is the gap between the energy you burn and the energy you eat. Eat consistently less than you burn and your body covers the difference from stored energy; eat more and the surplus is stored. Every diet that works, works through this gap — the math just tells you how big to make it.

Step 1: BMR — the energy you burn doing nothing

Even at complete rest your body spends energy keeping the heart, brain, and organs running. That baseline is the basal metabolic rate (BMR), and it is estimated — not measured — from the things that correlate with it: weight, height, age, and sex. The most common estimate, and the default in the TDEE calculator, is the Mifflin-St Jeor formula:

men:   BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161

Worked example: a 35-year-old woman weighing 70 kg at 165 cm gets 10 × 70 + 6.25 × 165 − 5 × 35 − 161 = 700 + 1031 − 175 − 161 ≈ 1,395 kcal/day. Notice how much weight dominates the formula — it is the largest single input, which is a clue about what happens as you diet.

Step 2: from BMR to TDEE

Nobody lies perfectly still all day, so the estimate scales BMR up by an activity factor:

• Sedentary (little or no exercise) — × 1.2
• Lightly active (1–3 days/week) — × 1.375
• Moderately active (3–5 days/week) — × 1.55
• Active (6–7 days/week) — × 1.725
• Very active (hard exercise or a physical job) — × 1.9

Our example woman, moderately active: 1,395 × 1.55 ≈ 2,162 kcal/day. That is her maintenance — the intake at which weight stays flat. Worth knowing: these multipliers are population averages, not measurements of you, and an average can miss an individual by a wide margin (the same reason a median sometimes describes a typical person better than a mean — see Mean vs Median vs Mode). Treat the number as a starting estimate, then check it against a few weeks of real-world scale data.

Step 3: setting the deficit

Eat below maintenance and the body fills the gap from stored energy. A widely used rule of thumb holds that about 3,500 kcal ≈ one pound (0.45 kg) of body fat, so a 500 kcal daily deficit adds up to 3,500 kcal per week — roughly a pound a week. Our example, eating 500 less than her 2,162 maintenance, lands at about 1,662 kcal/day.

This is why calculator tools expose preset targets: a gentle cut of −250 kcal for slow, sustainable loss, or −500 kcal for the standard pace (and +250 / +500 on the gain side, by symmetry). The bigger the deficit, the faster the scale moves and the harder the diet gets — speed and adherence trade off directly.

Why weight loss slows down: the plateau

TDEE is not a constant. As you lose weight, your BMR falls right along with it — the formula multiplies your weight by 10, so every kilogram lost removes roughly 10 kcal/day from BMR directly, before accounting for the smaller amount of energy a lighter body spends moving around. Recompute our example at 65 kg instead of 70: BMR drops to about 1,345 and maintenance to about 2,085. The 50 kcal/day drop in BMR gets amplified by the 1.55 activity multiplier into roughly 77 kcal/day of lost maintenance.

On top of that, the body adapts: spontaneous movement — fidgeting, walking around — tends to decrease in a deficit, a response known as adaptive thermogenesis. The plateau is not the math failing; it's the inputs changing. The fix is to recompute TDEE at your new weight and set the deficit from there, rather than concluding the diet stopped working.

How big is too big?

General guidance most practitioners converge on: aim to lose roughly 0.5–1% of body weight per week, which for a 70 kg person is about 0.35–0.7 kg weekly. Deficits much larger than that tend to cost muscle along with fat, dent training performance, and make the diet hard enough to abandon. Two things make a moderate deficit work better: enough protein to protect muscle, and some resistance training to tell the body the muscle is still needed. The usual disclaimer applies with extra force here — these are population-level rules of thumb, not medical advice. If you have a health condition, take medication, or have a history of disordered eating, set targets with a doctor or dietitian.

Try it yourself

The TDEE Calculator runs the full pipeline above — BMR by Mifflin-St Jeor, Harris-Benedict, or Katch-McArdle, your activity multiplier, then loss and gain presets in one readout. The Calorie Calculator gives the same maintenance-and-target view from slightly different inputs. To see the exercise side of the ledger, the Calories Burned Calculator estimates what specific activities add to your daily burn. And once you have a target, the Macro Calculator splits it into protein, carb, and fat grams — including the protein intake that helps protect muscle in a deficit.