How to work out your calorie needs with the Mifflin-St Jeor formula: resting rate, training add-on, the honest margin of error and a safe lower limit.
The short answer first: your calorie need is your resting metabolic rate multiplied by an activity factor. You get the resting rate from the Mifflin-St Jeor formula — 10 × weight in kg + 6.25 × height in cm − 5 × age, then +5 for men and −161 for women [1]. Multiply the result by a factor between 1.2 and 1.9, depending on how often you train. That is your starting number.
Why this formula? Because in a systematic review of the common prediction equations it hit the measured resting rate within ±10 % more often than any other — with the narrowest error range [2].
And that is exactly where the honest catch sits: ±10 % of 2,500 kcal is roughly 250 kcal of blur — about one cereal bar plus a banana. So your number is a well-founded hypothesis, not a law of nature. What becomes of it is decided in the next two to three weeks.
Your resting rate is what your body burns lying in bed all day — heart, breathing, brain, cell work. For most people it is the biggest single chunk of daily expenditure.
Plug in your numbers: 10 × kg + 6.25 × cm − 5 × age, then +5 (male) or −161 (female) [1].
Example: 32 years old, 82 kg, 181 cm, male. 820 + 1,131 − 160 + 5 = 1,796 kcal. That is the cost of simply existing — and covering it would already take around seven servings of oats, without taking a single step.
Now your daily life goes on top — not booked session by session, but as a factor across the whole day. These tiers are practice values from nutrition counselling, not study numbers:
Four sessions a week means, in our example: 1,796 × 1.55 = roughly 2,780 kcal. That is the maintenance level.
Now you bend the number in your direction. Again: practice values, not study numbers.
The most common mistake here is impatience, not ignorance. A deficit twice as big does not halve the time — it costs you strength, sleep and eventually motivation.
Calories tell you how much. Macronutrients tell you what from. Start with protein, because that value is the best documented: 1.4 to 2.0 g per kilogram of body weight per day is enough for practically every training person according to the position stand of the International Society of Sports Nutrition [3]. A meta-analysis of 49 studies with 1,863 trainees found the point beyond which extra protein produced no further muscle gain at 1.62 g/kg per day [4].
At 82 kg that is around 133 g of protein — spread across the day, roughly one big tub of quark, two chicken breasts and a handful of nuts. The rest of your calories go to carbohydrates and fat, and there what matters most is what you tolerate well and actually enjoy.
Now comes the part almost everyone skips. Eat to your number consistently for two to three weeks and weigh yourself in the morning under identical conditions. Then compare weekly average against weekly average, not Tuesday against Tuesday — daily swings of one to two kilos are water and gut content, not fat.
Nothing moving even though you wanted to lose weight? Then the starting number was too high. Take off 150 to 200 kcal and give it another two weeks. The scale corrects the formula — never the other way round.
There is a floor you do not go below, however much of a hurry you are in. The technical term is energy availability — the energy left over after training for everything else: bones, hormones, immune system, head.
The IOC consensus statement on REDs describes a critical zone for women below 30 kcal per kilogram of fat-free mass per day; for men the data is thinner and the threshold blurrier [5]. More important than the exact figure is the message behind it: eating too little is not a shortcut, it is the fastest route to infections, stress fractures and a collapse in performance.
This guide explains and motivates, it does not replace medical or dietetic advice. With pre-existing conditions, medication, during pregnancy and breastfeeding, or if you suspect a nutrient deficiency, the planning belongs in expert hands.
Never talk yourself systematically downwards. If thoughts about food, calories or weight start to run your day, if meals cause anxiety, or if you keep training through exhaustion, that is not a discipline problem, it is a warning sign. The same goes for a missing period, frequent infections, recurring stress fractures or persistently poor sleep [5]. Talk to a doctor or a registered dietitian early in those cases.
For growing children and adolescents these calculations do not apply — that belongs in expert hands.
Weigh yourself seven days in a row and take the average as your real starting value. A single reading can be a kilo off after a salty pizza; a weekly average cannot. From then on you only ever compare weekly average against weekly average — the only comparison that tells you anything.
And write your starting number down with the date and your weight. In eight weeks that note is worth more than any app, because it shows you whether your original hypothesis held. If you would rather skip the maths: the nutrition plan in the DOMISPORTS training centre runs exactly this calculation and spreads the result across a week, shopping list included.
For most adults Mifflin-St Jeor is the best choice. In a systematic review it hit the measured resting metabolic rate within ±10 % more often than any other equation tested, and had the narrowest error range [2]. Older formulas tend to overestimate. None of them is perfect — they are starting points, not measurements.
Accurate enough to start with, too vague to trust blindly. A deviation of ±10 % means roughly 250 kcal either way at 2,500 kcal [2]. That is why the two-to-three-week test is not an optional extra but the actual core: only your weight trend tells you whether the number fits you.
No, not if you are using an activity factor — it already contains your training. Adding both is the single most common arithmetic error and explains a lot of stalled fat-loss attempts. Only someone planning from the bare resting rate and booking each session separately adds training on top. Pick one method and stick with it.
First check whether you actually hit what you planned: oil, drinks and snacks love to disappear from an estimate. If that holds up, your starting number was too high — take off 150 to 200 kcal and observe for another two to three weeks. Bigger jumps deliver nothing but hunger.
Less is not better here. Below a certain energy availability, bones, hormones and immune system suffer measurably; the IOC names a critical zone for women below 30 kcal per kilogram of fat-free mass per day [5]. Anyone chasing the fastest possible weight loss mostly loses performance. Plan moderately and get support.
How Much Protein Do You Actually Need?