TDEE Calculator Explained: Mifflin-St Jeor, Activity Multipliers and How to Diet From It

What TDEE actually measures

TDEE — total daily energy expenditure — is the number of calories your body burns in an average 24-hour day, from the moment you wake up to the moment you fall asleep and every heartbeat in between. It is the single most important number in weight management, because everything else — cutting, bulking, maintaining, macro splits — is defined relative to it. Eat under TDEE and you lose weight. Eat at TDEE and you maintain. Eat over TDEE and you gain. The TDEE calculator gives you that number in about ten seconds using an equation that has been the nutrition-science standard since 1990.

TDEE has two moving parts. The first is basal metabolic rate (BMR) — the calories your body burns just to keep you alive at complete rest. Breathing, circulating blood, maintaining body temperature, running your brain, digesting food and repairing tissue all cost energy even when you are lying perfectly still. The second is activity burn: everything you spend on top through walking, fidgeting, working out, standing at a kitchen counter, climbing stairs. BMR is fixed by your size, sex and age. Activity burn is entirely under your control, and it is where the big lever for weight change lives.

The Mifflin-St Jeor equation

The equation the calculator uses to estimate BMR was published by M.D. Mifflin and S.T. St Jeor in the American Journal of Clinical Nutrition in 1990. The Academy of Nutrition and Dietetics evaluated the four major predictive BMR equations in 2005 and concluded that Mifflin-St Jeor was the most accurate for healthy non-obese and obese adults. It has been the default in clinical nutrition and the fitness industry ever since.

BMR (men) = 10·kg + 6.25·cm − 5·age + 5
BMR (women) = 10·kg + 6.25·cm − 5·age − 161

Weight is in kilograms, height in centimetres, age in years. The output is calories per day at complete rest. The two equations differ only in the constant — men add 5, women subtract 161 — a 166-kcal offset that reflects differences in average lean-body-mass ratio between the sexes. If you compare a 30-year-old man and woman of identical size, his BMR is exactly 166 kcal higher than hers.

Older BMR equations you may still see quoted — Harris- Benedict (1919, revised 1984) and Katch-McArdle (uses lean body mass) — are still in the literature. Harris-Benedict overestimates modern populations by about 5 %; Katch-McArdle is more accurate for lean subjects but needs a body-fat measurement most people do not have. Mifflin-St Jeor works with numbers you already know and is accurate to within ±10 % for most adults.

Activity multipliers, in plain English

Once BMR is set, TDEE is BMR multiplied by an activity factor. The five bands used by the calculator are the standard Harris-Benedict tiers, in use since the 1980s:

• Sedentary (× 1.2) — desk job, no structured exercise, drives everywhere. Roughly 3,000 steps a day.
• Lightly active (× 1.375) — desk job with light exercise 1-3 days a week, or a job that involves standing and walking with no structured workouts. Roughly 5,000-7,500 steps.
• Moderately active (× 1.55) — structured exercise 3-5 days a week (running, weights, sport), or a physically active job. Roughly 8,000-11,000 steps.
• Very active (× 1.725) — hard exercise 6-7 days a week, or a physically demanding job like construction or nursing. 12,000+ steps.
• Extra active (× 1.9) — professional athlete, twice-a-day training, or heavy manual labour combined with training. Rare, and easy to overestimate.

The single biggest source of error in TDEE estimation is picking the wrong multiplier — and most people overestimate. Three gym sessions a week plus a desk job puts you in the 1.375 tier, not the 1.55 tier, no matter how hard the sessions felt. Rule of thumb: if you sit seven or more hours a day, you cannot be more than "lightly active" regardless of gym work. Structured exercise burns less than people assume — a 60-minute weights session nets 250-350 extra kcal, not the 700 the machine claims.

Worked example: 30-year-old man, moderate activity

Take a 30-year-old man, 75 kg, 175 cm, training three or four times a week and walking to the shops. Plug those into the Mifflin-St Jeor equation:

BMR = 10 × 75 + 6.25 × 175 − 5 × 30 + 5
BMR = 750 + 1,093.75 − 150 + 5 = 1,698.75 kcal/day

That is his resting metabolism — the cost of being alive. Multiply by the moderate activity factor of 1.55 and you get TDEE:

TDEE = 1,698.75 × 1.55 = 2,633 kcal/day

Activity burn — everything on top of resting metabolism — is TDEE minus BMR, or about 934 kcal a day. To maintain 75 kg he eats 2,633 kcal a day; to lose half a kilo of fat a week, 2,133; to gain muscle, about 2,900. The TDEE calculator runs the arithmetic for any combination of sex, age, weight, height and activity level and reports BMR, TDEE and activity burn side by side.

What TDEE does not include

Exercise-Associated Thermogenesis (EAT)

Structured exercise is already inside the activity multiplier. A common mistake is to run the TDEE calculation, pick the "moderate" multiplier because you go to the gym, and then add the calories your Apple Watch says you burned in a workout on top. That double-counts the workout. Pick one method: TDEE with the multiplier or BMR plus a per-day exercise log. Not both.

Non-Exercise Activity Thermogenesis (NEAT)

NEAT is the calories you burn from unconscious movement — fidgeting, shifting posture, standing up. It is also inside the activity multiplier. NEAT varies more between people than almost any other energy component; two people the same size, doing the same structured exercise, can differ by 800-1,000 kcal a day just in NEAT. That is why some people gain weight easily on a "small surplus" and others cannot gain weight on 4,000 kcal. NEAT also drops during diets, which is one reason weight loss slows.

Thermic Effect of Food (TEF)

Digesting food costs energy. Protein costs the most (20-30 % of its calories), carbohydrates around 5-10 %, fat 0-3 %. On a mixed diet TEF averages roughly 10 % of total intake. The activity multipliers assume a typical Western diet — around 15 % protein — so TEF is baked in. High-protein diets (25-30 % protein) push TEF above the assumption and effectively raise TDEE by another 40-80 kcal a day.

Using TDEE to lose or gain weight

A kilogram of body fat stores about 7,700 kcal (or 3,500 kcal per pound). That gives you the exchange rate for weight change:

• Maintenance: eat at TDEE. Weight should hold within ±0.5 kg over 2-3 weeks.
• Slow cut (recommended): eat 300-500 kcal/day below TDEE. Loses 0.3-0.5 kg per week. Preserves muscle and training performance.
• Aggressive cut: eat 700-1,000 kcal/day below TDEE. Loses 0.7-1 kg per week. Higher risk of muscle loss, worse gym performance, more hunger. Reserve for short blocks (4-8 weeks) with plenty of protein (1.6-2.2 g/kg).
• Lean bulk: eat 200-400 kcal/day above TDEE. Gains ~0.25 kg per week, most of which is muscle if you are training hard. Beyond that, extra weight is fat.
• Aggressive bulk: eat 500+ kcal/day above TDEE. Faster scale weight, worse body composition. Only justified for underweight teenagers or serious athletes.

These are targets, not laws. Everyone's TDEE is a moving average — it drifts with the seasons, with sleep, with stress, with training load. Track your weight for 2-3 weeks at the calculated intake, then adjust ±100-200 kcal based on what the scale actually does. The equation gives you a starting point; your body gives you the correction.

Why the Mifflin-St Jeor equation misses individuals

Body composition

Mifflin-St Jeor uses total weight. It cannot tell whether that weight is muscle or fat. Muscle burns roughly three times more calories per kilogram at rest than fat tissue, so two people at 75 kg with 15 % versus 25 % body fat will have measurably different BMRs. If you know your body-fat percentage, the Katch-McArdle equation (BMR = 370 + 21.6 × lean-body-mass-kg) is more accurate. If you do not, stick with Mifflin-St Jeor.

Age drift

Metabolism slows with age. The equation captures this via the −5 kcal/year term, which totals about −150 kcal between age 20 and age 50. Recent Pontzer et al. work in Science (2021) suggests adult BMR is more stable than assumed between 20 and 60 and drops more sharply after 60. Mifflin-St Jeor slightly overestimates BMR for over-60s and slightly underestimates for 20-40 year olds by that measure — but by less than the ±10 % background error.

Adaptive thermogenesis

Prolonged calorie deficit lowers BMR beyond what the equation predicts — a survival response that resists further weight loss. Biggest Loser contestants showed BMR suppression of 500 kcal/day six years post-show, well below what their new weight predicts. Aggressive dieters have to periodically re-feed to reset TDEE, or their calculated deficit stops working.

Ethnicity

Mifflin-St Jeor was validated on a mostly-Caucasian US sample. Small ±3-5 % offsets have been reported in East Asian and African populations — statistically real, practically negligible against the ±10 % background variability.

Common mistakes

Picking "moderately active" because you go to the gym

A desk job with three weekly gym sessions is "lightly active" (1.375), not "moderately active" (1.55). The difference is roughly 300 kcal/day at typical body sizes — the exact size of a mild deficit. Overestimating activity is the number-one reason people "can't lose weight while eating so little."

Adding workout calories on top

TDEE already includes exercise via the multiplier. Adding the Apple Watch estimate for a workout double-counts. If you want to track workouts separately, drop the multiplier to sedentary (× 1.2) and add a per-day exercise burn on top — that gives you the same total by a different route.

Chasing week-to-week scale noise

Scale weight can swing ±1-2 kg on water, glycogen, salt and gut contents. A single low reading does not mean you need to cut further; a single high reading does not mean the diet failed. Track a 7-day rolling average and let it run for at least three weeks before adjusting intake.

Using TDEE as a hard budget

TDEE is an estimate, not a budget you must not exceed. If your target is a 500-kcal deficit and you eat 400 under one day and 600 the next, you have hit the weekly average. Metabolism does not reset at midnight.

When to seek professional advice

A calculator estimates energy expenditure for a healthy adult with typical body composition. It is not the right tool if you are pregnant or breastfeeding, under 18, over 65, recovering from an eating disorder, managing diabetes, thyroid disease, PCOS or other conditions that affect metabolism, taking medications known to alter appetite or weight (steroids, SSRIs, some antipsychotics), or planning a very-low-calorie diet under 1,200 kcal/day. For any of these, a registered dietitian, GP or endocrinologist can order tests — resting metabolic rate via indirect calorimetry, thyroid panel, HbA1c — that a general equation cannot replicate. This article, like the calculator, is educational; it is not a treatment plan.

Related tools on Calc Dragon

The TDEE calculator is the headline tool, but a full diet plan needs more than calories. Use the calorie calculator to turn TDEE into cut, maintain and bulk targets. The BMR calculator gives you the resting metabolism number in isolation. The protein calculator sets your daily protein floor from body weight and training goal. The BMI calculator and body-fat calculator give context on where the weight sits. The weight converter switches between kilos, pounds and stones for anyone tracking in different units.

Frequently asked questions

What is TDEE and how is it different from BMR?

BMR is the calories your body burns to keep you alive at complete rest — breathing, circulation, organ function, brain activity. TDEE is BMR plus everything else: walking around, fidgeting, working out, digesting food. For most people TDEE runs 20-90 % above BMR depending on how active they are. BMR is fixed by biology; TDEE is what you can actually influence.

Which activity multiplier should I choose?

Be honest. Most desk workers who go to the gym three times a week are "lightly active" (1.375), not "moderate". Moderate (1.55) suits people doing structured exercise 4-5 hours a week plus incidental movement. Very active (1.725) is for athletes training daily. Extra active (1.9) is reserved for manual labourers or twice-a-day training. Overestimating activity is the single biggest source of calorie-tracking failure.

How accurate is a TDEE calculator?

Group accuracy of Mifflin-St Jeor is roughly ±10 % for healthy adults aged 19-78. The bigger error source is the activity multiplier, which is a coarse band rather than a measurement. Treat the TDEE figure as a starting estimate: track weight for 2-3 weeks at that intake, and adjust ±100-200 kcal/day if you drift up or down from your goal.

Does TDEE change day to day?

Real daily energy expenditure fluctuates by a few hundred kcal depending on how much you moved, how well you slept and how much you ate. The number the calculator returns is an average — the value you should aim to eat around over a week, not on any specific day. The metabolism smooths short-term intake very effectively.

Should I recalculate my TDEE as I lose weight?

Yes. BMR falls as you lose weight — a lighter body costs less to run. As a rough guide, recalculate every 4-5 kg of weight change. Someone who starts at 100 kg with a 2,900 kcal TDEE will settle at roughly 2,600 kcal when they reach 85 kg, even at the same activity level. Ignoring this is why long diets stall.

Does TDEE include exercise?

Yes. That is the whole point of the activity multiplier. If you also log workouts in a fitness tracker and add those kcal on top of TDEE you will double-count and overestimate intake. Pick one method — TDEE-with-multiplier or BMR-plus-tracked-workouts — not both.

Why is my TDEE lower than a friend's of the same weight?

Height, age, sex and lean-body-mass all affect BMR independently of scale weight. A 175 cm 30-year-old weighing 75 kg burns more calories at rest than a 160 cm 50-year-old at 75 kg, even before activity is considered. Body composition matters even more — 75 kg of muscle burns more than 75 kg with a higher fat share.

Is this medical advice?

No. This is a general educational estimate based on a published equation. It is not a substitute for advice from a registered dietitian, GP or sports physician — particularly if you are pregnant, breastfeeding, under 18, over 65, recovering from an eating disorder, or managing a metabolic condition such as diabetes or thyroid disease. Use the figure as a starting point and seek qualified guidance for prescriptive plans.