Calorie deficit calculator
Calorie deficit calculator
Find the daily calorie target that produces steady fat loss, based on your maintenance calories (Mifflin-St Jeor + activity).
Maintenance is 2,457 kcal/day. This target is a 500 kcal/day deficit (~1 lb of fat ≈ 3,500 kcal).
Hit at least the lower number to protect muscle. This matters most on a GLP-1: rapid appetite loss makes it easy to under-eat protein, and without enough protein plus resistance training a meaningful share of the weight you lose can be lean mass rather than fat. Spread it across meals.
How the deficit is calculated
First we estimate your maintenance calories (TDEE) from the Mifflin-St Jeor BMR equation times your activity factor. Then we subtract your chosen daily deficit. One pound of body fat stores roughly 3,500 calories, so the weekly loss is approximately your daily deficit × 7 ÷ 3,500. Real-world results vary with water weight, adherence, and metabolic adaptation — track your trend over 2-3 weeks and adjust.
The muscle-loss trap (and how to avoid it)
Aggressive deficits and very low appetite — common on GLP-1 medications — can cause a meaningful share of weight loss to come from lean muscle rather than fat. Muscle is metabolically active, so losing it lowers your maintenance calories and makes regain easier. The two protective levers are protein (aim for the target the calculator shows) and resistance training 2-3 times per week. If you are on a GLP-1 and struggling to eat enough protein, prioritize protein-dense foods and shakes early in the day when appetite is highest.
Calorie deficit FAQ
What is a calorie deficit?+
A calorie deficit is eating fewer calories than you burn. Since roughly 3,500 calories equals about one pound of body fat, a deficit of ~500 calories per day produces about one pound of fat loss per week.
How big should my calorie deficit be?+
A moderate deficit of 500 calories per day (~1 lb/week) is sustainable for most people. Larger deficits lose weight faster but raise the risk of muscle loss, fatigue, and rebound. This calculator caps the target at a safe floor (about 1,200 kcal for women, 1,500 for men) below which you should have clinical supervision.
Do I still need a calorie deficit on Wegovy or Zepbound?+
Yes — GLP-1 medications cause weight loss by making a calorie deficit easier to reach, not by bypassing it. Appetite suppression naturally lowers your intake. The calculator shows the target so you can confirm you are in a healthy range and not accidentally eating far too little.
Why does the calculator emphasize protein?+
In any deficit, some of the weight you lose can be muscle. Protein at roughly 1.6 grams per kilogram of body weight, combined with resistance training, shifts that loss toward fat and preserves lean mass. This matters especially on GLP-1s, where low appetite makes it easy to under-eat protein.
Sources
Mifflin MD, St Jeor ST, et al. Am J Clin Nutr. 1990;51(2):241-247 (PMID 2305711). Protein target: Morton RW, et al. Br J Sports Med. 2018;52(6):376-384 (PMID 28698222).