What Goes Into Working Out a Calorie Deficit
Working out a calorie deficit comes down to two numbers: how many calories your body needs each day (your total daily energy expenditure, or TDEE), and how many you are currently eating. The gap between them is your deficit.
Estimating your daily energy needs
Your TDEE is shaped by several factors: your basal metabolic rate (the calories your body uses at rest), your activity level, your age, sex, height, and weight. Standard estimation formulas — such as the Mifflin-St Jeor equation — multiply your resting calorie needs by an activity factor to give a daily maintenance figure. From there, a modest reduction (commonly around 300–500 kcal below maintenance) is often described in general nutrition guidance as a sustainable deficit, though the right figure for any individual will vary.
Why GLP-1 medicines change the picture
On Mounjaro or Wegovy, appetite suppression can make it easier to eat significantly below your needs without realising it. Many people on these medicines eat far less than usual, which is why tracking actual intake — not just estimating — matters. Eating too little can mean missing out on protein, which is important for preserving muscle during weight loss, and can leave you fatigued. A useful starting point is to understand how a calorie deficit works in the context of a GLP-1 journey before setting a target.
Calories in: logging what you actually eat
Accurate food logging is the other half of the equation. Portion sizes are notoriously difficult to estimate by eye, and GLP-1 appetite changes can make meals irregular — a snack here, a half-finished plate there. Consistent logging gives you a real picture of your intake rather than a guess. For a deeper walkthrough of the maths involved, see how to work out a calorie deficit step by step or explore how to work out your personal calorie deficit with your own numbers.
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Tracking the right four numbers — weight, waist, protein and dose — turns a noisy chart into a clear story. That's the loop GLP-1 users keep coming back for.
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Tracking Your Calorie Deficit Effortlessly with GLP Trackr
GLP Trackr's AI food photo scan is the fastest way to log what you eat on a GLP-1 journey — point your camera at a meal and get an instant breakdown of calories, protein, and macros without manually searching every ingredient. On days when appetite is unpredictable and meals are small or irregular, that speed matters.
Once your food is logged, GLP Trackr sets calorie and protein targets so you can see your deficit at a glance — not as an abstract calculation, but as a running daily picture updated with every meal. The app also tracks your weight over time, so you can see how your intake relates to your progress week by week through the insights and correlation features.
Everything you log builds into a clear summary you can bring to appointments — useful if your prescriber wants to understand your intake patterns alongside your dose history.
General nutrition guidance can point you toward a sensible deficit range, but what the right number looks like for you personally — especially alongside a prescribed medicine — is worth discussing with your prescriber or doctor. If you are feeling unusually fatigued, losing weight very rapidly, or concerned about your intake, contact your healthcare professional rather than adjusting targets on your own.
Ready to stop guessing and start seeing your actual numbers? Download GLP Trackr on the App Store or Google Play and try it free for 7 days — your calorie deficit, protein goals, and weight progress all in one place.
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Frequently asked questions.
Estimate how many calories your body burns each day (your TDEE, based on weight, height, age, and activity level), then aim to eat a modest amount less — commonly around 300–500 kcal below that figure. Consistent food logging helps you see whether you are hitting your target.
These medicines commonly reduce appetite, which can naturally lower your calorie intake. Many people eat less without planning to, making it worth tracking actual intake to ensure you are still meeting protein needs and not eating too little — something worth discussing with your prescriber.
General nutrition guidance often suggests a moderate deficit of around 300–500 kcal below daily needs, but the right figure is individual and depends on factors your prescriber knows. They can advise what is appropriate alongside your treatment.
Yes. GLP Trackr lets you set a calorie target and log meals via AI photo scan, barcode, or search. The app shows your running daily intake against your goal, so you can see your deficit in real time without manual calculations.
Low energy alongside very low intake can be a sign you are eating too little, particularly protein. Log what you eat so you can see the pattern clearly, and speak with your prescriber or doctor — they can advise whether your intake is appropriate for your current dose and health.