How to Calculate a Calorie Deficit (Safely) to Lose Weight
How to calculate a calorie deficit: find maintenance with Mifflin-St Jeor, pick a safe deficit and adjust. Worked examples and why the 3,500-calorie rule misleads.
The short answer
- Estimate your resting burn with the Mifflin-St Jeor equation, multiply by an activity factor to get maintenance, then eat a bit less.
- A moderate deficit (often 300–500 calories a day) supports the steady 0.5–1 kg (1–2 lb) a week the NHS recommends.
- Treat the number as a starting point. Check your weight trend after 2–3 weeks and adjust. Real-world weight loss slows over time, so the old 3,500-calorie rule overestimates it.
A calorie deficit just means eating a little less energy than your body uses. Calculating one takes three steps: estimate what you burn, choose a sensible gap, then let your results fine-tune it.
The 4-step method
- 1
Estimate your resting burn (BMR)
Use the Mifflin-St Jeor equation with your weight, height and age.
- 2
Multiply by your activity level
That gives your maintenance calories (TDEE).
- 3
Subtract 300–500 calories
A moderate gap for about 0.5–1 kg (1–2 lb) a week.
- 4
Adjust after 2–3 weeks
Let your weekly average weight tell you if the number is right.
Step 1: Estimate your resting energy (BMR)
The Mifflin-St Jeor equation is one of the most widely used formulas for estimating how many calories you burn at rest (Mifflin et al., 1990). Use weight in kilograms, height in centimetres and age in years:
- Men: (10 × weight) + (6.25 × height) − (5 × age) + 5
- Women: (10 × weight) + (6.25 × height) − (5 × age) − 161
Step 2: Multiply by your activity level
Your resting number doesn't include walking, work or training. Multiply it by an activity factor to estimate your maintenance calories (often called TDEE). These multipliers are commonly used rules of thumb, not precise measurements:
| Activity level | Looks like | Multiply BMR by |
|---|---|---|
| Mostly sitting | Desk job, little exercise | 1.2 |
| Lightly active | Exercise 1–3 days a week or lots of walking | 1.375 |
| Moderately active | Exercise 3–5 days a week | 1.55 |
| Very active | Hard training 6–7 days or a physical job | 1.725 |
Step 3: Choose a sensible deficit
The NHS advises losing weight steadily, at about 0.5 to 1 kg (1–2 lb) a week (NHS). For most people that means starting with a deficit of roughly 300–500 calories a day. Smaller people, or anyone close to their goal, should lean toward the lower end.
Worked example: a woman
30 years old, 165 cm, 70 kg, exercises twice a week. BMR = 700 + 1,031 − 150 − 161 = about 1,420 calories. Maintenance = 1,420 × 1.375 = about 1,950. A 300–500 calorie deficit gives a starting target of roughly 1,450–1,650 calories a day.
Worked example: a 30-year-old woman
Worked example: a man
35 years old, 180 cm, 85 kg, trains four times a week. BMR = 850 + 1,125 − 175 + 5 = about 1,805 calories. Maintenance = 1,805 × 1.55 = about 2,800. A 500 calorie deficit gives a starting target of roughly 2,300 calories a day.
Why the 3,500-calorie rule is misleading
You've probably heard that cutting 3,500 calories equals one pound of fat lost. Researchers at the U.S. National Institutes of Health found this rule overestimates weight loss because it ignores how your body adapts. Their model suggests that for an average overweight adult, permanently cutting about 10 calories a day per pound you want to lose takes about a year to reach half of that loss, with nearly all of it reached by about three years (NIH model).
The practical lesson: expect your weight loss to be fastest at the start and slow down over time. That's normal, not failure.
Step 4: Adjust based on real results
- Weigh yourself a few mornings a week and watch the weekly average, not single days. Water and food can move the scale 1–2 kg day to day.
- After 2–3 weeks, compare your average to your goal rate. Losing faster than ~1% of body weight a week? Eat a bit more. Not losing at all? Check for missed oils and sauces (photo calorie apps often undercount them), then trim 100–200 calories or add more easy cardio like Zone 2 walking or running. Simply walking more helps too: see how many steps a day you actually need.
- Keep protein high (about 1.6 g/kg) and keep strength training so more of the weight you lose comes from fat, not muscle. See how much protein you need.
Once you have a calorie target, splitting it into protein, carbs and fat makes it easier to keep muscle while you lose fat. Our guide to counting macros takes it from here.
Frequently asked questions
What is a good daily calorie deficit?
For most people, 300–500 calories a day is a sustainable starting point that lines up with the NHS's suggested 0.5–1 kg of weight loss a week. Smaller or leaner people should use the lower end.
How accurate is the Mifflin-St Jeor equation?
It's one of the better formulas for estimating resting energy, but it's still an estimate. Two people with the same stats can burn different amounts, which is why you adjust after a few weeks based on your actual weight trend.
Why have I stopped losing weight?
As you lose weight, your body burns a little less, so the same intake becomes a smaller deficit. Small tracking slip-ups also add up. Recheck portion sizes, then trim 100–200 calories or add more daily movement.
Do I need to count calories forever?
No. Many people track closely for a few months to learn portions, then switch to tracking now and then or only during a goal phase.
Sources
- Mifflin M.D. et al. (1990). A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 51(2):241–247
- NHS. Obesity treatment: losing weight safely
- NIH: mathematical model predicts how body weight changes (Hall et al., The Lancet, 2011), via News-Medical
- Jäger R. et al. (2017). International Society of Sports Nutrition Position Stand: protein and exercise. JISSN 14:20
This article is general information, not medical advice. If you have a health condition, are pregnant, or take medication, talk to your doctor or a registered dietitian before changing how you eat or train.
