BMR for Women Age 18-24 at 100 kg — Mifflin-St Jeor

Basal metabolic rate (BMR) is the energy your body uses at complete rest — the calories needed to keep you breathing, keep your heart beating, maintain body temperature and run every other background process, before you move, eat or exercise at all. For a woman age 18 to 24 weighing 100 kg (220 lb) and 165 cm tall, the Mifflin-St Jeor equation gives a BMR of 1,765 kcal/day. That is the floor your daily intake is built on: everything else — walking, working, training, digesting food — is added on top of it.

This page shows the full working, not just the answer. You will see the formula, each number substituted in, the BMR table across body weights from 50 to 100 kg, and the total daily expenditure at all five activity levels. The result is an estimate from a published equation, not a personal prescription: real resting metabolic rate varies by roughly ±10% around the prediction, so treat the figure as a starting point and check it against two to three weeks of real weight data.

At 18 to 24, BMR is at its lifetime peak for a given weight and height, because the body is still carrying the metabolic cost of growth and because muscle mass and daily activity tend to be highest in these years. The age term in the equation is small at this age — only about 105 kcal a day subtracted for a 21-year-old — which is why the figures in the table below are the highest you will see for any age band at the same weight.

Calculate your own BMR

Your weight, height and age are almost certainly different from the example on this page, so run your own numbers with the BMR calculator. It uses the same Mifflin-St Jeor equation shown below. If you want the full daily figure including activity, the TDEE calculator multiplies your BMR by your activity level, and the calorie calculator turns it into cutting, maintenance and bulking targets.

Related pages and tools:

The formula: Mifflin-St Jeor (1990)

The Mifflin-St Jeor equation is the most widely validated formula for estimating resting energy expenditure in adults. It was published by Mifflin et al. in the American Journal of Clinical Nutrition in 1990 (Mifflin MD et al., Am J Clin Nutr 1990;51(2):241–247), and it is the equation most calculators and many clinical settings use as a starting point. For women it is:

BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161

For men the final term is +5 instead of −161. The equation needs only three measurements — weight, height and age — which is both its strength and its weakness: it is easy to use, but it cannot see how much of your weight is muscle and how much is fat.

Working through the example for a woman age 18 to 24, 100 kg, 165 cm, using the midpoint age of 21 for the band:

  • 10 × 100 kg = 1,000 kcal
  • 6.25 × 165 cm = 1,031.25 kcal
  • 5 × 21 years = 105 kcal
  • Constant: −161 kcal

So BMR = 1,000 + 1,031.25 − 105 − 161 = 1,765.25 ≈ 1,765 kcal/day. That is the energy this example body would use lying still all day, before a single step, meal or workout. It is the largest single part of daily expenditure — typically 60 to 70% of total calories for a sedentary person — which is why getting it roughly right matters more than any other single number in calorie planning.

Your BMR at a glance

Figure Value
Sex Women
Age band 18 to 24 (midpoint 21)
Weight 100 kg (220 lb)
Height 165 cm
BMR (Mifflin-St Jeor) 1,765 kcal/day
Safety floor 1,200 kcal/day (women)

BMR across body weights

The same woman at the same height and age would have a different BMR at a different weight, because the equation adds 10 kcal a day for every kilogram. Here is the full picture for age 18 to 24 at 165 cm:

Weight BMR
50 kg 1,265
60 kg 1,365
70 kg 1,465
80 kg 1,565
90 kg 1,665
100 kg 1,765

Each 10 kg step adds exactly 100 kcal a day to BMR. That linearity is a property of the equation, not of the body: in reality, gaining 10 kg of muscle raises resting expenditure more than gaining 10 kg of fat, because muscle is metabolically more active than fat. The table is therefore most accurate for people near an average body composition, and it overstates BMR for people with a high body-fat percentage and understates it for very muscular people.

From BMR to TDEE: every activity level

BMR is only the resting part. Total daily energy expenditure (TDEE) is BMR multiplied by an activity factor — 1.2 for sedentary, 1.375 for lightly active, 1.55 for moderately active, 1.725 for very active and 1.9 for extremely active. For a BMR of 1,765 kcal/day:

Activity level Multiplier TDEE
Sedentary 1.2 2,118
Lightly active 1.375 2,427
Moderately active 1.55 2,736
Very active 1.725 3,045
Extremely active 1.9 3,354

The range is wide: 1,236 kcal a day separates the sedentary and extremely active figures for the same woman. That spread is why activity level is the input people most often get wrong, and why it is worth being honest about a typical week rather than your best week. A desk job with three gym sessions a week is lightly active, not moderately active, and using the higher factor inflates every target built on it.

What your BMR means at age 18-24

This is also the age where fad diets do the most damage. Very-low-calorie diets in the late teens and early twenties can disrupt hormones, bone density and menstrual health, and the weight lost is usually regained. If your goal is weight loss, the floor of 1,200 kcal a day for women is a hard minimum, and a moderate deficit of 300 to 500 kcal below TDEE — not a crash diet — is the approach the evidence supports.

At 100 kg and 165 cm, this example's body mass index is 36.7 kg/m², which the World Health Organization (WHO) classifies as Obese (underweight below 18.5, healthy weight 18.5–24.9, overweight 25–29.9, obese 30 and above). BMI is a screening tool only: it cannot tell muscle from fat, so a muscular woman can score as overweight while carrying little body fat, and a woman with low muscle can score as healthy while carrying a high fat percentage. Use it alongside waist measurements and body-fat estimates, never on its own. For a height of 165 cm, the healthy-weight range by BMI is roughly 50.4 to 67.8 kg.

What this number does not tell you

A BMR estimate is a starting point, not a measurement. The Mifflin-St Jeor equation was validated on population averages, and individual resting metabolic rate varies by roughly ±10% around the prediction because of differences in muscle mass, genetics, thyroid function and health. Two women with identical age, weight and height can have BMRs a few hundred calories apart. The equation also uses total body weight, so it is less accurate for very muscular people and for people with a high body-fat percentage, because muscle burns more energy at rest than fat does.

Nor does BMR describe your whole day. It excludes the energy of movement, the energy of digesting food (the thermic effect of food, roughly 10% of intake), and the energy of exercise — all of which the activity multipliers in the table above approximate rather than measure. And it says nothing about health on its own: a low BMR is not a diagnosis, and a high BMR is not a clean bill of health. Treat the 1,765 kcal figure as a hypothesis: use it to set a target, follow it for two to three weeks, watch the weekly weight average, and adjust by 100–200 kcal based on what actually happens.

How to use your BMR

Start from TDEE, not from BMR alone. At the moderately active level, TDEE for this example is 2,736 kcal/day. A maintenance target is that figure; a weight-loss target is roughly 300–500 kcal below it, which here gives about 2,336 kcal/day; a muscle-gain target is roughly 300–350 kcal above it, about 3,086 kcal/day. Note the floor: no weight-loss target for a woman should go below 1,200 kcal/day without medical supervision, and any target near that floor should be short-term and supervised.

If your goal is weight loss, aim for 0.5–1% of body weight per week — for a 100 kg woman, roughly 0.5–1.0 kg a week. Faster loss usually costs muscle, and muscle is the tissue that keeps BMR up. Keep protein at roughly 1.6–2.2 g per kilogram of body weight a day when dieting or training — about 160–220 g a day at 100 kg — spread across meals, and keep resistance training in the week. If your goal is muscle gain, the same protein range applies, with the surplus spent mostly on carbohydrate around training.

Recalculate when things change. BMR falls by about 50 kcal a decade from age alone, and it falls with weight loss too — a lighter body burns fewer calories — so a target set at your old weight gradually stops working. Recalculate every 5 kg of weight change, or every year, and always check the new figure against real weight data rather than trusting it blindly.

Frequently asked questions

What is the BMR for a woman age 18 to 24 at 100 kg?

Using the Mifflin-St Jeor equation with a height of 165 cm and the midpoint age of 21, BMR is 1,765 kcal/day. Your own BMR will differ if your height, weight or exact age differ; use the BMR calculator with your own measurements.

How was the 1,765 kcal figure calculated?

It is 10 × 100 + 6.25 × 165 − 5 × 21 − 161 = 1,765.25 ≈ 1,765 kcal/day, using the women's Mifflin-St Jeor equation published in 1990. The result is an estimate, not a measurement of your individual metabolism.

What is the TDEE for a woman age 18 to 24 at 100 kg?

It depends on activity. At the moderately active multiplier of 1.55, TDEE is 2,736 kcal/day. Sedentary it is 2,118 kcal/day and extremely active it is 3,354 kcal/day. Use the TDEE calculator with your own activity level.

Does BMR fall with age?

Yes, slowly. The equation subtracts 5 kcal a day for every year of age, so BMR falls by about 50 kcal a decade when weight and height stay the same. In practice it often falls faster, because muscle mass and daily activity tend to decline with age — which is why strength training is the most effective way to protect your BMR.

Is 1,765 kcal a day enough to eat?

No — BMR is what you would burn lying still all day, not a food target. Your daily intake target should be based on TDEE, which is higher. And no woman should eat below 1,200 kcal a day without medical supervision. If a calculated weight-loss target falls below that floor, hold it at the floor and talk to a healthcare professional.

Why is my BMR different from another calculator's?

Different calculators use different equations (Mifflin-St Jeor, Harris-Benedict, Katch-McArdle) and different assumptions about height and age, so results commonly differ by 100–300 kcal. That spread is normal. Treat any single figure as a starting estimate and adjust from two to three weeks of real weight data.

Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.