TDEE Calculator with Mifflin-St Jeor

What this calculator does

It estimates how many calories your body burns per day at rest (Basal Metabolic Rate) and during a typical 24-hour cycle including movement (Total Daily Energy Expenditure). The output drives every nutrition prescription downstream: cutting target, surplus target, macronutrient split, energy balance projection over weeks.

The math is two equations stacked. The first, Mifflin-St Jeor, is the most accurate published predictor of resting metabolic rate across healthy adults aged 19-78, beating Harris-Benedict by roughly 5% in head-to-head comparisons [1]. The second is a simple multiplier reflecting how active the rest of your day is. We use the standard ACSM-adopted multipliers, the same ones working trainers use on intake forms.

The formula, no abstraction

BMR (Basal Metabolic Rate) is 10 × weight_kg + 6.25 × height_cm − 5 × age + s, where s = +5 for males and s = −161 for females.

TDEE (Total Daily Energy Expenditure) = BMR × Activity Multiplier

A common mistake: doubling up the activity multiplier with a separate “exercise calorie” tracker in MyFitnessPal. The multiplier already includes training. If you’re going to manually log exercise burn on top of TDEE, set the multiplier to 1.20 (sedentary), then add training calories separately. Otherwise stick with the multiplier and skip exercise logging.

Worked example

Take a 28-year-old male, 178 cm, 68 kg, training 4 times per week (moderate).

BMR = 10×68 + 6.25×178 − 5×28 + 5
    = 680 + 1112.5 − 140 + 5
    = 1657.5 kcal/day, round to 1658

TDEE = 1658 × 1.55 = 2570 kcal/day

That 2,570 figure becomes the maintenance budget. From there:

• Cut target (−20%): 2,056 kcal. Projects ~0.5 kg/week loss assuming 7,700 kcal/kg conversion
• Surplus target (+10%): 2,827 kcal. Projects ~0.25 kg/week gain at the conservative end

The 7,700 kcal/kg conversion is approximate; in practice some of a deficit comes from glycogen and water in the first week, not fat. We model the multi-week version of this with the energy balance projection tool.

When this calculator is enough, and when it isn’t

The Mifflin-St Jeor equation is calibrated against people of healthy body composition. If your client sits well outside that range, the equation under- or over-estimates predictably:

• Body fat above 30%: Mifflin overestimates BMR by 5-10% because it weights total body mass rather than lean body mass. Use Katch-McArdle if you have a body fat percentage: BMR_Katch-McArdle = 370 + 21.6 × LBM_kg, where LBM = total weight × (1 − body fat fraction). Our body fat calculator (Navy method) feeds this.

• Highly trained athletes: Mifflin tends to underestimate for advanced trainees with above-average lean mass. Bump the multiplier one level up, or use Katch-McArdle.

• Pregnant or breastfeeding clients, recovery from illness, eating disorder history: out of scope for a calculator. Refer out, do not prescribe macros from these numbers.

• Day-to-day variance: real BMR fluctuates ±3-5% based on sleep quality, ambient temperature, recent meal composition, menstrual phase. The number you get is a population estimate, not a reading.

How it differs from Harris-Benedict and Cunningham

The 1919 Harris-Benedict equation overestimates BMR by 5% in modern populations because activity patterns and average lean mass have shifted. It’s still common in older nutrition software but is no longer the recommended primary equation [2].

Cunningham (1980, refined 1991) is similar to Katch-McArdle in using lean body mass, slightly higher predictions for athletic populations. Use it if your client is a competitive strength athlete and you have a DEXA-grade body composition number.

For a typical online coaching client with no DEXA, Mifflin-St Jeor with sensible activity multiplier classification is the working default.

What you do with TDEE inside Coach Sheet

In the Profile tab of the Coach Sheet workbook, you enter Age, Sex, Height, Weight, and Activity once. The Quick Stats tab below auto-calculates BMR (Mifflin), TDEE, and three macro variants (maintenance, cut at −20%, surplus at +10%). Each value links back to the formula in a hover tooltip. You can verify the math without leaving the sheet, and edit the multiplier or the formula directly if you prefer Katch-McArdle for a specific client.

When weight changes, every downstream calculation updates: BMR, TDEE, macros, the projected weight trajectory chart on the energy balance tool. No manual recompute. That’s the data ownership argument from the manifesto, in formula form.