The NHS says an average man needs 2,500 kcal a day and an average woman needs 2,000. Those figures are averages, and the NHS itself says your number depends on your age, weight, height and how much you move. If you train, you can get much closer with a pen and four minutes.
Step 1: work out your resting calories
Your basal metabolic rate (BMR) is what your body burns if you do nothing all day. The Mifflin-St Jeor equation estimates it from four things you already know:
- Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
Take a 30-year-old man who weighs 80 kg and is 178 cm tall:
(10 × 80) + (6.25 × 178) − (5 × 30) + 5 = 1,768 kcal
Why this equation
A 2005 systematic review compared the four equations most used in clinical practice, and Mifflin-St Jeor was the most likely to land within 10% of measured resting metabolism, in people with and without obesity. It came out of a 1990 study of healthy adults.
It is still an estimate. The review's authors warned that errors for an individual can be large, which is why step 4 matters.
Step 2: add your daily activity
Multiply your BMR by how active your life is outside the gym:
| Day to day | Multiply by |
|---|---|
| Desk job | 1.2 |
| On your feet some of the day | 1.375 |
| Active job | 1.55 |
| Physical job | 1.725 |
Then add your training separately. Liftprep counts 250 kcal a session, spread across the week, because gym sessions are something you can actually count. That figure is our working estimate, and a heavy session can burn more or less.
Our 80 kg man has a desk job and trains four times a week:
| Step | Sum | Result |
|---|---|---|
| Daily activity | 1,768 × 1.2 | 2,122 kcal |
| Training | 4 × 250 ÷ 7 | 143 kcal |
| Maintenance | 2,265 kcal |
That is 235 kcal a day below the NHS average for men. Over a week, eating the average would put him about 1,650 kcal over without him noticing.
Step 3: set a target for your goal
Maintenance is where your weight stays roughly level. Your goal moves you away from it. The example column is our 80 kg man:
| Goal | Change | Example |
|---|---|---|
| Lose fat, steadily | −20% | 1,812 kcal |
| Lose fat, gently | −12% | 1,993 kcal |
| Maintain | none | 2,265 kcal |
| Build muscle, lean | +10% | 2,492 kcal |
| Build muscle, standard | +15% | 2,605 kcal |
A kilogram of body fat holds roughly 7,700 kcal, so a steady cut of 453 kcal a day comes to about 0.4 kg a week. The NHS suggests aiming for 0.5 to 1 kg a week. We sit slightly under that on purpose, because slower loss is kinder to muscle. There is more on that in how to cut without losing muscle.
Set your protein next. How much protein you need to build muscle has the numbers.
Step 4: let the scales correct you
Every equation here is a starting point. Weigh yourself once a week, at the same time of day, and compare across two weeks. If your weight is not moving at the pace you planned, change your calories by a small amount and try again. Your own results beat any formula.
When to stop and ask someone
Calorie targets have a lower limit. Liftprep will not go below 1,500 kcal a day for men, 1,200 kcal for women, or 65% of maintenance, whichever is higher, and it will not build plans for anyone with a BMI under 18.5. Those limits are our own safety rails.
These sums are a guide for healthy adults. If you are pregnant, have a medical condition or have a history of disordered eating, talk to your GP or a registered dietitian first.
The short version
- Work out your BMR with Mifflin-St Jeor.
- Multiply by your daily activity, then add your training.
- Take away 12 to 20% to lose fat, or add 10 to 15% to build muscle.
- Check against the scales after two weeks and adjust.
Rather skip the maths? The free Liftprep calculator does all of this and shows its working.
Sources
- NHS. Understanding calories.
- NHS. Tips to help you lose weight.
- Mifflin MD et al. (1990). A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition 51: 241–247.
- Frankenfield D et al. (2005). Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association 105: 775–789.