Calorie Deficit Calculator: How to Estimate Your Deficit on Mounjaro or Wegovy

A calorie deficit calculator works in three steps: it estimates the calories your body burns at rest (BMR), multiplies that by an activity factor to get your total daily energy expenditure (TDEE), and then subtracts a deficit based on the rate of loss you'd like to aim for. The final number is your suggested daily calorie target.

Step 1 — Estimate your BMR

BMR (basal metabolic rate) is roughly the calories your body would use in 24 hours doing nothing but keeping you alive. Most calculators use the Mifflin–St Jeor equation, which takes into account your age, sex, height and weight. If you want to look at this number in isolation, see the BMR estimate explainer — BMR is the foundation every other calorie number is built on.

Step 2 — Apply an activity multiplier to get TDEE

TDEE is BMR multiplied by an activity factor — usually somewhere between 1.2 (sedentary) and 1.725 (very active). The result is roughly how many calories you burn on a typical day including movement, exercise and the energy cost of digesting food. This is your maintenance calorie level: eat that, and weight tends to stay stable.

Step 3 — Subtract a deficit

To lose weight, the calculator subtracts a deficit from TDEE. A commonly referenced rule of thumb is that around 7,700 calories equates to about 1 kg of body fat, so:

• A deficit of ~500 calories/day → roughly 0.5 kg (about 1 lb) per week
• A deficit of ~1,000 calories/day → roughly 1 kg (about 2 lb) per week

These are estimates, not promises — real-world loss varies week to week with water, sleep, hormones, training and how full your gut is. For a fuller picture of expected trajectories, the weight loss calculator overview and the weight-loss-focused calorie calculator walk through the trade-offs.

What a sensible deficit looks like

General educational guidance from sources such as the NHS healthy weight resources tends to point toward a moderate, sustainable deficit — typically a few hundred calories below maintenance — rather than aggressive cuts. Cutting too hard can backfire by eroding muscle, energy and adherence, all of which matter even more when appetite is already reduced by a GLP-1 medicine.

Why GLP-1 medication changes the picture

Mounjaro (tirzepatide) and Wegovy (semaglutide) act on appetite-regulating pathways, and many people report less hunger, less "food noise" and feeling full sooner. The calorie deficit doesn't change — your body still needs a deficit to lose fat — but the lived experience of being in one is often different. Two things matter more on a GLP-1:

• Protein. When total intake drops, protein needs to be protected to help preserve lean muscle.
• Not under-eating. A very large unintentional deficit on top of a suppressed appetite can leave you fatigued and nutrient-short. Some people on Mounjaro or Wegovy find it useful to set a sensible floor for the day rather than chase the lowest possible number — see how many calories to eat a day and how many calories you can have in a day for typical ranges.

Mounjaro calorie deficit calculator — is it different?

A "Mounjaro calorie deficit calculator" uses the same maths as any other deficit calculator — there isn't a separate equation for tirzepatide. What changes is context: the page on Mounjaro (and dose-stage pages like Mounjaro 5mg) covers how appetite tends to shift as the dose is adjusted by your prescriber, which is exactly the period when watching intake matters most. The calculator gives you the number; the medicine, prescribed by your clinician, helps make hitting it more sustainable.

Related calorie questions

If you'd like to explore the same number from different angles, you may also find these useful: daily calorie intake guidance, minimum daily calories, how many calories a day you can eat, calorie intake calculator, and understanding calories.

The hardest part of a calorie deficit isn't the maths — it's keeping an honest record day after day, especially when appetite is low and meals are smaller and more varied. GLP Trackr is built around that problem.

Effortless food logging

Point your camera at your plate and the AI food scanner reads back calories, protein and macros in seconds. For packaged foods, the barcode scanner does the same. Manual search is always there as a backup. This is the single biggest reason people fall off a deficit — friction in logging — and removing it tends to keep the data honest.

Daily calorie and protein targets

You can set your own daily calorie and protein targets — based on whatever deficit feels sustainable to you — and the app shows you in real time how close you are. Hit a protein floor, stay near your calorie target, and let the trend take care of the weekly result.

Insights that connect deficit to result

The weight chart, weekly transformation view and correlation insights show how your average intake actually maps to what's happening on the scale and the tape. A week where the number didn't move isn't a failure — it's data, and the app helps you read it instead of guess. This is the part of a deficit that matters most on a GLP-1 journey: holding steady through the weeks where progress isn't visible yet.

A note on personal targets

The calculator gives you an educational estimate, not a personalised medical prescription. If you have a health condition, are pregnant or breastfeeding, are managing diabetes or are unsure what intake is appropriate for you, talk it through with your prescriber or doctor — they know your full picture and can sense-check the number before you build a habit around it.

Bring it back to the hub

For a wider look at how calorie numbers fit together — maintenance, deficit, protein, daily intake — start at the main calorie calculator hub, which links out to every calculator and explainer in this section.

Ready to put a number on it? Set your calorie and protein targets in GLP Trackr, snap your meals as you go, and watch the weekly trend do the work. The app is free to try for 7 days on the App Store and Google Play — no commitment, just a clearer view of your own deficit.