BMR Calculator

Basal Metabolic Rate Calculator for Australia

Your basal metabolic rate (BMR) is the energy your body uses at rest. This basal metabolic rate calculator estimates it in both kilojoules and calories using five equations: Mifflin-St Jeor (the headline figure), the revised Harris-Benedict, Katch-McArdle if you add a body fat percentage, and the Schofield and Oxford equations for adults. It then multiplies your BMR by an activity factor to show a rough range of daily energy needs.

Seeing the equations side by side matters because they never agree exactly. The spread between the lowest and highest result is a useful reminder that any single BMR number is an estimate.

BMR Equations and Formulas

Worked Example: Woman, 35, 165 cm, 68 kg

Using Mifflin-St Jeor: 10 × 68 + 6.25 × 165 − 5 × 35 − 161 = 1,375 kcal per day, which is about 5,754 kJ. The other equations give the revised Harris-Benedict at 1,436 kcal (6,008 kJ), Schofield at 1,398 kcal (5,850 kJ) and Oxford at 1,355 kcal (5,668 kJ). That is a spread of about 80 kcal, from 1,355 to 1,436 kcal (5,668 to 6,008 kJ).

Multiplying the Mifflin-St Jeor figure by activity factors gives roughly 1,650 kcal (6,905 kJ) if sedentary, 2,132 kcal (8,919 kJ) if moderately active and 2,613 kcal (10,933 kJ) if extra active. Remember that these are broad estimates, not targets.

Which BMR Equation Should You Use?

For most healthy adults, Mifflin-St Jeor is a sensible default. If you know your body fat percentage from a reliable measurement, Katch-McArdle accounts for lean mass, which can matter for people with a body composition far from average. Schofield and Oxford are weight-only, so they are handy when height is unknown and they are commonly referenced in nutrition work in Australia and New Zealand. None of them is a measurement. If the results differ by a few hundred kilojoules, that is normal.

Frequently Asked Questions

What is BMR, and what does BM R stand for?

BMR stands for basal metabolic rate: the energy your body uses at complete rest to keep you alive, covering breathing, circulation, temperature control and cell repair. It is sometimes typed as BM R or called resting energy expenditure. In Australia it is usually expressed in kilojoules (kJ) per day, with calories (kcal) shown alongside; 1 kcal is 4.184 kJ.

What is the basal metabolic rate formula?

There is no single formula. The most widely used basal metabolic rate formula for the general public is Mifflin-St Jeor: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5 for men, or − 161 for women. Other equations include the revised Harris-Benedict, Katch-McArdle (which uses lean body mass), and the weight-based Schofield and Oxford equations. This calculator shows them side by side.

What is the equation for BMR if I only know my weight and age?

The Schofield and Oxford (Henry) equations estimate BMR from weight, age and sex alone, with separate coefficients for age bands (18–30, 30–60 and over 60). For example, the Schofield equation for a woman aged 30–60 is 0.034 × weight (kg) + 3.538 in megajoules per day. They are intended for adults aged 18 and over, so this calculator leaves them blank under 18.

What is the Oxford equation for BMR?

The Oxford equations, published by Henry in 2005, were built from a larger and more varied dataset than the original Schofield equations and are weight-based, with coefficients by sex and age band. The result comes out in megajoules per day, which the calculator converts to kJ and kcal. Like every predictive equation it is an estimate rather than a measurement.

What is the Schofield equation, and why do dietitians use it?

The Schofield equations (1985) estimate basal metabolic rate from weight, age band and sex, and they underpin the energy estimates in the FAO/WHO/UNU reports. They have long been used in nutrition work, including in Australian and New Zealand nutrient reference material, so you may see them in dietetic practice. Some studies suggest they can overestimate for some groups, so treat the result as a starting point.

What is the most accurate BMR formula?

Mifflin-St Jeor is often considered the most accurate of the common equations for healthy adults in many studies, which is why it is the headline figure here. Even so, all of these are estimates and can be 10% or more away from your measured rate. If you know your body fat percentage, Katch-McArdle may suit very lean or muscular people better, and the spread across equations gives a sense of the uncertainty.

Is this an Australian basal metabolic rate calculator?

It is built for Australian users, with kilojoules shown alongside calories and metric units by default, and it includes the Schofield and Oxford equations that are familiar in Australian and New Zealand nutrition. The equations themselves are international and are not specific to Australians. An accredited practising dietitian can interpret the numbers for your circumstances.

How is a rest energy calculator different from a daily calorie calculator?

A rest energy calculator, like this one, estimates only what your body burns at rest. Your total daily energy use is higher because you also move, work, exercise and digest food, which is why the calculator multiplies BMR by an activity factor from 1.2 to 1.9. Those daily figures are rough, so treat them as a guide rather than a target.

Why is my BMR different from my friend's, or from a smartwatch or scale reading?

BMR depends mainly on lean mass, body size, age and sex, and people of the same weight can differ by a few hundred kilojoules for reasons the equations cannot see, such as genetics, hormones and body composition. Smartwatches and body-composition scales use their own estimates, so their numbers will not match exactly. If you need an accurate figure for a medical reason, ask your GP or dietitian about measurement by indirect calorimetry.