BMR Calculator (Mifflin vs Katch)
Estimate resting calories before you plan daily intake
BMR and daily calorie needs FAQ
What does BMR actually measure?
Basal metabolic rate (BMR) is an estimate of the calories your body uses at complete rest to keep you alive: breathing, circulation, organ function, temperature control and other essential processes. It does not include walking around, training, work or digestion.
Is BMR the same as my maintenance calories?
No. Maintenance calories are usually higher than BMR because they include everyday movement, exercise and digestion. To estimate maintenance, most people take BMR and apply an activity multiplier in a TDEE calculator, then fine-tune based on weight trends and how they feel.
Why use Mifflin–St Jeor as the default?
Mifflin–St Jeor performs well in many modern adult samples and only needs age, sex, height and weight, which most people know. It is widely used in clinical and coaching settings as a practical default equation for resting energy.
When is Katch–McArdle more useful?
Katch–McArdle can be helpful if you have a reasonably accurate body fat percentage and your lean body mass is very different from average. It may track changes better in people who gain or lose a lot of muscle. If your body fat estimate is just a guess, Mifflin–St Jeor is usually safer.
How accurate are BMR equations?
Even good equations can be off by 10–20% for an individual. Sleep, hormones, medications, past dieting, genetics and measurement error all matter. That’s why it’s better to treat BMR as a starting estimate and adjust based on longer-term changes in weight, performance and lab work.
Should I eat exactly my BMR for weight loss?
Most guidelines do not suggest eating right at or below BMR for long periods. Very low intakes can be hard to sustain and may not be appropriate for your health. Many people instead use a modest deficit from estimated maintenance, ideally with professional support, enough protein, and attention to how they feel.
How often should I recalculate my BMR?
You usually only need to recalc BMR when your weight or body composition changes by a few percent, or when your routine shifts a lot (for example a new job, big training change, or major life event). Day-to-day fluctuations are usually not a reason to change the base number.
Who should be extra careful with DIY BMR tools?
Anyone with major medical conditions, a history of disordered eating, pregnancy or breastfeeding, or medications that affect appetite or metabolism should be extra cautious. In those cases, your healthcare team’s guidance is more important than any online equation.
How to use this BMR calculator in a simple plan
This page gives you a resting calorie estimate that you can plug into a TDEE, macro or meal plan without needing a lab test. You can keep things simple: use Mifflin–St Jeor as the base number, check Katch–McArdle if you know body fat %, then base your next steps on real-world feedback.
1. Enter honest, current measurements
Use your current height and weight, not your goal numbers. Pick the unit system you think in. In US mode, enter height in feet and inches and weight in pounds. In metric mode, use centimetres and kilograms. Add age and sex, and, if you have a decent estimate, body fat % from a reliable method.
2. Read the Mifflin and Katch results
Hit Calculate BMR to see:
- A Mifflin–St Jeor BMR in kcal/day.
- Energy per hour based on that number.
- A Katch–McArdle BMR if you gave body fat %, plus the % difference vs Mifflin.
If the two methods are within roughly 10% of each other, they are broadly in the same ballpark. Bigger gaps are a sign to look closer at the inputs or ask a professional.
3. Combine BMR with activity to plan intake
BMR is just the base. To estimate daily maintenance calories, most people multiply BMR by an activity factor that reflects how much they move. From there, they set a small deficit for fat loss or a small surplus for gain. A dedicated TDEE and macro calculator can walk you through those steps with clearer targets for protein, carbs and fats.
4. Adjust based on trends, not single days
Once you are using a BMR-based plan, track weekly averages for weight, performance, sleep and hunger. If things are drifting the wrong way for a few weeks, adjust calories up or down in small steps (often 5–10%) instead of making huge jumps. Bring your numbers to any doctor or dietitian you work with so they have a concrete starting point.
Remember that equations are tools, not verdicts. How you feel, what your labs show, and what is sustainable in daily life matter more than hitting a precise BMR number.
How the BMR equations work under the hood
The calculator keeps the math transparent: it converts inputs into metric units when needed, applies the standard Mifflin–St Jeor equation for everyone, and, if you provide body fat %, adds a Katch–McArdle estimate based on lean body mass.
1. Convert US measurements to metric
In US mode, height in feet and inches is converted to centimetres and weight in pounds is converted to kilograms so both equations can use the same units:
Total inches = (feet × 12) + inches
Height (cm) ≈ total inches × 2.54
Weight (kg) ≈ weight (lb) ÷ 2.2046
2. Apply the Mifflin–St Jeor formula
For adults, Mifflin–St Jeor is:
BMR (kcal/day) = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + s
where s = +5 for males and s = −161 for females. This gives a baseline resting energy estimate most people can use without extra testing.
3. Add Katch–McArdle when body fat is known
If you provide body fat %, the tool estimates lean body mass (LBM) and then applies Katch–McArdle:
LBM (kg) = weight (kg) × (1 − body fat % ÷ 100)
BMR (kcal/day) = 370 + 21.6 × LBM (kg)
Because it focuses on lean mass, this equation can be more responsive in people who gain or lose a lot of muscle, as long as the body fat estimate is reasonably accurate.
4. Keep the result easy to use
The tool rounds BMR to whole calories and shows a per-hour value as a simple mental anchor. The key idea is not perfection but a clear, adjustable starting point you can take into a broader nutrition or training plan with your coach or healthcare team.
References and further reading on BMR and energy needs
These resources explain where the equations and ranges in this tool come from:
- Mifflin MD et al. — A new predictive equation for resting energy expenditure in healthy individuals — original paper describing the Mifflin–St Jeor equation often used for adult BMR.
- Frankenfield D et al. — Comparison of predictive equations for resting metabolic rate — review comparing several equations, including Mifflin–St Jeor and others, in different adult groups.
- American College of Sports Medicine — Estimating resting metabolic rate — overview of common BMR methods and how they fit into practical nutrition planning.
Use these as background reading and pair them with individual guidance from your healthcare professionals, especially if you have existing health conditions or complex nutrition needs.