Quick answer: A TDEE calculator estimates total daily energy expenditure: the energy your body uses across a whole day. It is a starting estimate, not a measurement or an instruction to eat a particular amount. The UK Scientific Advisory Committee on Nutrition (SACN) explicitly warns that population energy reference values should not be applied to individuals because activity and expenditure vary widely. “Macros” means the protein, carbohydrate and fat in food; a universal percentage split is not a personal UK prescription.
AI-generated content. Always verify ingredients and allergen information yourself. Do not use an estimate here to self-treat a medical condition or set a restrictive diet.
TDEE, resting expenditure and food intake are different numbers#
Your body uses energy at rest and during movement and other processes. Resting energy expenditure (REE) is an estimate of one part of that demand. TDEE includes expenditure across the entire day; online tools typically estimate it by combining an REE equation with assumptions about activity. Food energy is what you eat and drink, which UK packs normally display as kilojoules and kilocalories (NHS: understanding calories).
A familiar tool can produce a neat number, but it has not observed every walk, shift, training session, period of illness or change in routine. Treat its result as uncertain. The NHS's 2,000 kcal for an average woman and 2,500 kcal for an average man are public guide figures, not proof of your own maintenance level. The NHS says needs can differ with age, size and activity; SACN makes the population-versus-person distinction explicit.
How does the Mifflin–St Jeor step work?#
The original Mifflin–St Jeor paper derived an equation to predict resting expenditure in healthy adults using weight, height, age and a male/female coefficient. In its commonly rounded form, with weight in kilograms, height in centimetres and age in years:
- Male coefficient:
10 × weight + 6.25 × height − 5 × age + 5 - Female coefficient:
10 × weight + 6.25 × height − 5 × age − 161
For an illustrative 30-year-old, 70 kg, 170 cm adult using the study's female coefficient, the arithmetic is 700 + 1,062.5 − 150 − 161 = 1,451.5 kcal/day of predicted resting expenditure. This is not TDEE and not a suggested food intake. Activity conversion adds more uncertainty, so a calculator result such as “2,187 kcal” should not be treated as if it were measured to the nearest calorie.
The equation's two original coefficients do not describe every body or circumstance. A calculator cannot decide which is clinically appropriate for a trans or non-binary person, someone pregnant, or someone with a condition affecting energy needs. A qualified clinician or dietitian can interpret estimates in the context of health history and goals. This article does not assign an activity factor, a deficit or a daily target to readers.
What does UK guidance actually say about macros?#
In its updated statement on energy and macronutrients, SACN describes UK reference values for fat and carbohydrate as population-average guidance, not exact individual splits. It explains that protein references are expressed per kilogram of body weight rather than as a percentage of calories. The British Nutrition Foundation protein guide reports the adult UK Reference Nutrient Intake of 0.75 g/kg/day; that is a general reference, not a body-building or illness prescription.
The statement also uses energy excluding ethanol when expressing population fat and carbohydrate recommendations. That distinction matters if you see a chart claiming that all UK adults “must eat 30% protein, 30% fat and 40% carbohydrate”. Such a chart is not an individual NHS or SACN rule. In practice, the NHS Eatwell Guide gives an easier food-based picture across a day or week: varied vegetables and fruit, starchy foods, protein foods, dairy or alternatives and small amounts of unsaturated oils.
How can you use labels without false precision?#
Start with the meal you are actually buying or cooking, not an invented target. For example, a jacket potato with beans and a side salad contains starchy food, a protein-containing food and vegetables. A tofu and vegetable rice dish can do the same with different ingredients. You cannot infer the exact macro grams from those descriptions; brands, recipes and portions differ.
The NHS guide to food labels explains how to compare energy and nutrients per 100 g or 100 ml, then consider the amount actually eaten. If a supermarket ready meal lists values per manufacturer portion as well, check whether your serving matches it. This applies whether you shop at Tesco, Sainsbury's, Asda, Morrisons, Aldi, Lidl, Waitrose or M&S, or order through Ocado. No current price, product range or nutritional profile is implied for any chain.
Allergens are a separate question from macros. A product's protein number cannot establish whether it is safe for you. GOV.UK's allergen-labelling guide explains what to check on current packaging and notes that vegan labelling does not guarantee freedom from a particular allergen. Follow your own allergy plan and verify the actual product.
When should you put the calculator aside?#
If you are pregnant, under 18, have diabetes, kidney disease or another medical diet, are recovering from illness, or have a history of disordered eating, a generic TDEE and macro calculation may be unsuitable. Ask a GP or registered dietitian for individual support. If tracking numbers makes food more stressful or drives increasingly restrictive choices, the estimate has stopped being a useful planning aid. A varied diet and appropriate professional advice matter more than a mathematically tidy output.
Frequently asked questions#
Is the Mifflin number my maintenance calorie target?#
No. It predicts resting energy expenditure, and even that is an estimate. Total daily expenditure adds activity and other variation. It cannot be converted into your personal target by a universal activity multiplier.
Is 2,000 or 2,500 kcal the right number for everyone in the UK?#
No. Those are NHS averages for public guidance. SACN warns against applying population energy reference values to an individual.
Do I need a fixed macro ratio to eat well?#
No UK source cited here sets a universal personal ratio. Use the NHS Eatwell Guide and food labels for general choices; seek individual advice if training or a health condition changes your requirements.
Medical note: This is general UK nutrition education, not an individual calorie, weight-loss or macro prescription.
Sources and further reading
- A new predictive equation for resting energy expenditure in healthy individuals — American Journal of Clinical Nutrition
- SACN Dietary Reference Values for Energy — Scientific Advisory Committee on Nutrition
- SACN statement on expressing energy, fat and carbohydrate intakes and recommendations — Scientific Advisory Committee on Nutrition
- Understanding calories — NHS
- The Eatwell Guide — NHS
- How to read food labels — NHS
- Protein — British Nutrition Foundation
- Allergen labelling — GOV.UK