How a calorie deficit works
Fat loss runs on one mechanism: eat fewer calories than you burn, and your body makes up the difference from stored fat. No moralizing about "good" and "bad" foods changes that arithmetic — a deficit is a deficit whether it comes from smaller portions, fewer snacks or more movement.
The calculator estimates the calories you burn per day (your TDEE) using the Mifflin-St Jeor equation (1990 — the equation the Academy of Nutrition and Dietetics recommends) multiplied by your activity level, then subtracts your chosen deficit:
The weekly projection uses the standard energy density of body fat: roughly 7,700 kcal ≈ 1 kg of fat (about 3,500 kcal per pound). A 500 kcal daily deficit therefore projects to 500 × 7 ÷ 7,700 ≈ 0.45 kg (1 lb) per week. Real-world loss is bumpier than the projection — more on that below — but over months the math holds up remarkably well.
If you want to see your maintenance number on its own first, run the daily calorie (TDEE) calculator.
Picking your deficit size
The coaching answer is almost never "as big as possible." Adherence beats aggression: a deficit you can hold for four months outperforms one you abandon after three weeks, every time.
| Preset | Daily deficit | Projected loss | Best for |
|---|---|---|---|
| Mild | −250 kcal | ~0.23 kg (0.5 lb)/week | Lean people cutting the last few kilos; anyone who hates feeling hungry |
| Standard | −500 kcal | ~0.45 kg (1 lb)/week | Most people, most of the time — meaningful progress, manageable hunger |
| Aggressive | −750 kcal | ~0.68 kg (1.5 lb)/week | Higher starting body fat with a firm deadline — the hardest to stick to |
Smaller bodies have less room to cut from. If the aggressive preset pushes your target below the safety floor — 1,200 kcal for women, 1,500 for men — the calculator clamps the number and tells you. That floor exists because below it, getting enough protein and micronutrients from food becomes genuinely difficult, and the diet usually breaks before the fat does.
What rate of loss to expect
The projection above is a straight line; your scale weight will not be. As a sanity check, most coaches aim for 0.5–1% of body weight per week. For an 80 kg person that's 0.4–0.8 kg — so the Standard preset sits right in the sweet spot, and Aggressive brushes the top of it. Rates above 1% per week are usually only appropriate at higher body-fat levels, ideally with professional supervision.
Why the scale stalls (and why that's normal)
Two things make daily weigh-ins misleading. First, water: sodium, carbs, training stress and (for women) cycle phase can swing scale weight by 1–2 kg overnight, in either direction, with zero change in body fat. Second, adaptation: as you get lighter you burn a little less, and dieting nudges you to move less without noticing — so the deficit that produced 0.5 kg/week in month one produces less by month three.
The fix is measurement discipline, not panic: weigh daily, compare weekly averages, and only call it a stall after two to three flat weeks. Then either trim another 100–200 kcal or add activity, and re-run this calculator with your new weight every 4–5 kg lost.
Keeping muscle while cutting
A deficit tells your body to burn stored energy; protein and lifting tell it to take that energy from fat rather than muscle. While cutting, aim for 1.8–2.2 g of protein per kg of body weight — the calculator prints your gram range with every result, and the protein calculator breaks it into per-meal targets. Keep resistance training in the plan even if sessions get shorter; it is the single strongest muscle-retention signal you control. And if the scale is moving but you're not sure what kind of weight you're losing, check in with the body fat calculator monthly.
Day to day, the whole game is logging honestly and adjusting early. That's exactly what the CoachRight client app is built for — a fast food diary with AI nutrition-label scanning, so hitting your number takes seconds instead of willpower. If you're a coach running cuts for clients, start a free 14-day trial and watch their logs live instead of waiting for the weekly "it's not working" message.