# RMR Calculation Guide: Mifflin and Katch-McArdle Estimate resting metabolic rate with the exact DTC formulas, compare Mifflin-St Jeor with Katch-McArdle, and understand the limits. --- - **Canonical URL:** https://dothecalculation.com/blog/health/rmr-calculation-guide - **Category:** Health - **Author:** Do The Calculation Team - **Published:** 2026-06-06 - **Last updated:** 2026-07-01 - **Reading time:** 19 min read - **Publisher:** Do The Calculation (https://dothecalculation.com) - **Methodology:** https://dothecalculation.com/methodology --- Resting metabolic rate is the energy your body uses while resting under less restrictive conditions than a formal basal metabolic rate test. The Do The Calculation RMR Calculator does not measure that energy directly. It estimates it with Mifflin-St Jeor and, when body-fat percentage is entered, Katch-McArdle. This guide reproduces both formulas and explains exactly which result the page highlights. ## Quick Answer - Without body-fat input, the DTC calculator highlights the Mifflin-St Jeor estimate. - With body fat above zero, it also calculates Katch-McArdle and highlights that result as RMR. - Mifflin uses weight, height, age, and a male/female formula constant. - Katch-McArdle uses estimated lean body mass, so body-fat measurement error flows directly into the result. - RMR is not TDEE or a daily calorie target; activity and food processing are not included. - Predictive formulas can be meaningfully high or low for an individual and cannot diagnose a metabolic disorder. > **Medical and editorial trust note** — This article and calculator are educational, not medical or nutrition advice. Resting energy can be affected by illness, recovery, pregnancy, breastfeeding, medications, endocrine conditions, and major weight changes. Seek qualified care for unexplained weight change, persistent fatigue, temperature intolerance, menstrual changes, eating-disorder concerns, or other symptoms. Last reviewed for calculator logic and source alignment by the Do The Calculation editorial team on July 1, 2026. ## What RMR Means RMR is a resting-energy concept. It represents energy used for breathing, circulation, nervous-system activity, temperature regulation, tissue maintenance, and other functions while the body is at rest. A laboratory measurement uses respiratory gases to estimate energy expenditure under controlled conditions. A web calculator uses a prediction equation instead. In everyday calculator content, BMR and RMR are often used loosely or interchangeably. Strict measurement protocols differ: basal testing generally requires more controlled fasting, sleep, environmental, and rest conditions. The practical point is that both are resting baselines, not full-day expenditure. ## Why an RMR Estimate Matters A resting estimate gives calorie planning a defined baseline. It helps separate the energy associated with basic body function from the extra energy associated with movement and food processing. That distinction is useful when comparing calculators, evaluating a maintenance estimate, or understanding why a sedentary day still uses substantial energy. The estimate is also useful as a reasonableness check. If two tools return very different full-day calorie numbers, compare their resting formulas first and their activity assumptions second. A small resting difference can become larger after multiplication, but activity selection often explains more of the gap than formula rounding. **Questions an RMR estimate can support** | Question | What RMR contributes | What remains unknown | | --- | --- | --- | | Why is TDEE higher? | Provides the resting baseline before activity | Whether the activity factor matches real behavior | | Why do formulas disagree? | Makes different model inputs visible | Which estimate is closer for the individual | | Did weight change alter the estimate? | Recalculates the weight-dependent model | Adaptive, medical, and behavioral effects | | Is metabolism medically abnormal? | Nothing diagnostic | Requires symptoms, history, examination, and appropriate testing | _[Figure: RMR Estimate, BMR Estimate, and TDEE]_ ## Mifflin-St Jeor Formula Used by DTC **Mifflin-St Jeor** ``` Male option: RMR = 10W + 6.25H - 5A + 5 Female option: RMR = 10W + 6.25H - 5A - 161 ``` - W = weight in kilograms. - H = height in centimeters. - A = age in years. - The DTC core clamps negative weight, height, and age inputs to zero before calculation. Mifflin-St Jeor was published in 1990 as a predictive resting energy expenditure equation. Prediction means it estimates the average relationship found in study data; it is not a metabolic measurement for the person entering the form. ## Katch-McArdle Formula and Lean Body Mass **Katch-McArdle implementation** ``` Lean body mass (kg) = weight kg x [1 - body fat % / 100] RMR = 370 + (21.6 x lean body mass kg) ``` - The calculator uses this formula only when body fat is greater than zero. - Body-fat values above 100 are capped at 100 inside the core function. - The page highlights Katch-McArdle whenever it is available. Katch-McArdle replaces height, age, and the sex-specific constant with estimated lean body mass. That can be useful when body composition is known, but the formula is only as credible as the body-fat input. Home scales, calipers, circumference methods, and scans can disagree. _[Figure: Which Result Does the RMR Page Highlight?]_ ## Worked Example Using the Page Defaults The default profile is male option, age 30, height 175 cm, weight 70 kg, and body fat 0%. With no positive body-fat input, only Mifflin-St Jeor is available. **Default Mifflin calculation** ``` RMR = (10 x 70) + (6.25 x 175) - (5 x 30) + 5 RMR = 700 + 1,093.75 - 150 + 5 = 1,648.75 kcal/day ``` - The page displays 1,649 kcal/day after rounding. - This is a resting estimate, not maintenance calories. ### Add a hypothetical 20% body-fat input **Katch-McArdle comparison** ``` Lean body mass = 70 x (1 - 0.20) = 56 kg Katch-McArdle = 370 + (21.6 x 56) = 1,579.6 kcal/day ``` - The page would highlight 1,580 kcal/day. - Mifflin would remain visible at 1,649 kcal/day. _[Figure: Formula Comparison for the Worked Example]_ ## How Body-Fat Input Changes Katch-McArdle **Katch-McArdle sensitivity at 70 kg body weight** | Body fat entered | Estimated lean mass | Katch-McArdle RMR | | --- | --- | --- | | 10% | 63 kg | 1,731 kcal/day | | 20% | 56 kg | 1,580 kcal/day | | 30% | 49 kg | 1,428 kcal/day | | 40% | 42 kg | 1,277 kcal/day | At this body weight, each 1 percentage-point increase in entered body fat lowers estimated lean mass by 0.7 kg and lowers the Katch result by about 15.1 kcal/day. A body-fat estimate that is off by several points can therefore shift the result enough to matter more than display rounding. ## How to Interpret Two Different Results Do not average Mifflin and Katch automatically. First ask whether the body-fat input is credible. If it came from a device with poor repeatability or from a method used under changing hydration conditions, the lean-mass estimate may add apparent precision without better information. - If no credible body-fat estimate is available, use Mifflin as the documented prediction and keep its uncertainty visible. - If body fat was measured consistently, compare Katch with Mifflin and record the measurement method and date. - If the difference is small, it may be less important than uncertainty in activity, intake, and normal daily variation. - If the difference is large, recheck units, body-fat entry, and measurement plausibility before changing a plan. - If a clinical decision depends on resting expenditure, ask whether indirect calorimetry or professional assessment is appropriate. > **Precision is not the same as accuracy** — The page can display a whole-number calorie result, but the underlying uncertainty is much larger than one calorie. Use rounding for readability, not as evidence that the estimate is exact. ## Factors That Can Affect Resting Energy - Body size and composition: larger bodies and more metabolically active tissue generally require more resting energy. - Age: the Mifflin equation subtracts 5 kcal for each additional year, although real age-related change is not identical for everyone. - Weight change: both formulas change when body weight changes; Katch also changes when estimated body-fat percentage changes. - Illness and recovery: fever, injury, and disease can change energy needs beyond a general calculator. - Hormonal and medical factors: thyroid and other conditions require clinical evaluation, not self-diagnosis from an equation. - Measurement conditions: food, caffeine, activity, temperature, and insufficient rest can affect laboratory resting-energy measurements. ## How to Move From RMR to TDEE TDEE estimates the energy used across a full day. A common calculator method multiplies a resting estimate by an activity factor. The DTC RMR page deliberately stops before that step. Use the TDEE calculator when the question is maintenance calories. **Separate full-day model** ``` Estimated TDEE = resting estimate x activity factor ``` - Activity factors are broad assumptions, not measurements. - Do not add exercise calories again automatically if the factor already represents typical exercise. Tool: [Estimate Total Daily Energy Expenditure](https://dothecalculation.com/calculators/tdee-calculator) — Use the separate TDEE tool when you need a maintenance-calorie estimate with an activity factor. ## Prediction Equation vs Indirect Calorimetry Prediction equations use population relationships and a few inputs. Indirect calorimetry estimates energy expenditure from oxygen consumption and carbon dioxide production under controlled conditions. A measured result can still vary with protocol and day-to-day physiology, but it answers a different question from an online formula. **Calculator estimate and laboratory measurement** | Method | Inputs | Main limitation | | --- | --- | --- | | Mifflin-St Jeor | Weight, height, age, sex formula term | Individual prediction error | | Katch-McArdle | Weight and estimated body fat | Body-fat error changes lean mass and RMR | | Indirect calorimetry | Respiratory gas exchange under a protocol | Cost, access, protocol quality, and biological variability | ## Common RMR Mistakes - Calling RMR maintenance calories: normal activity and food processing make full-day expenditure higher. - Entering pounds where kilograms are required or inches where centimeters are required. - Using an unverified body-fat estimate and assuming Katch-McArdle is automatically more accurate. - Treating the highlighted Katch result as measured metabolism simply because body fat was entered. - Eating at or below RMR solely because the number appears to be a minimum requirement. - Diagnosing a slow or damaged metabolism from a prediction equation. - Comparing two formulas without comparing their inputs and assumptions. - Recalculating after every short-term scale fluctuation instead of a meaningful change. ## How to Use the DTC RMR Calculator Enter weight in kilograms, height in centimeters, age in years, and the male or female formula option. Leave body fat at zero to use Mifflin alone. Enter a positive body-fat percentage only when you understand the source and uncertainty of that estimate. The result panel rounds to whole kcal/day. Tool: [Compare RMR Formulas](https://dothecalculation.com/calculators/rmr-calculator) — Calculate Mifflin-St Jeor and optionally Katch-McArdle with the exact implementation documented here. Tool: [Calculate BMR and Activity-Adjusted TDEE](https://dothecalculation.com/calculators/bmr-calculator) — Use the BMR tool when you want the Mifflin baseline plus one of the site activity factors. ## Related Do The Calculation Resources - BMR guide: https://dothecalculation.com/blog/health/bmr-metabolic-rate - TDEE guide: https://dothecalculation.com/blog/health/tdee-calculation-guide - Daily Calorie Calculator: https://dothecalculation.com/calculators/calorie-calculator - Body Fat Calculator: https://dothecalculation.com/calculators/body-fat-calculator ## Assumptions and Limitations The calculator uses prediction equations, not respiratory measurement. It does not model pregnancy, breastfeeding, growth, fever, injury, endocrine disease, medication effects, organ-specific metabolism, ethnicity, or recent adaptive changes. The male/female equation choice is part of the published Mifflin model and may not map cleanly to every user's physiology or identity. Katch-McArdle assumes the entered body-fat percentage can be converted into meaningful lean body mass. The core caps values above 100% but the interface should still be used with realistic inputs. Neither equation determines nutritional adequacy, a safe deficit, or a diagnosis. ## Sources to Verify or Cite - Mifflin et al. original predictive resting energy expenditure paper: https://pubmed.ncbi.nlm.nih.gov/2305711/ - NIDDK Body Weight Planner for dynamic calorie-planning context: https://www.niddk.nih.gov/health-information/weight-management/body-weight-planner - MedlinePlus metabolism overview: https://medlineplus.gov/ency/article/002257.htm - MedlinePlus metabolism myths and limits of simple explanations: https://medlineplus.gov/ency/patientinstructions/000893.htm ## Frequently Asked Questions **What is RMR?** Resting metabolic rate is the energy used by the body at rest. The DTC page predicts it with equations rather than measuring respiratory gases. **Is RMR the same as BMR?** They are both resting-energy concepts, but formal measurement protocols differ. Calculator articles often use the terms loosely; neither is total daily expenditure. **Which formula does the calculator use by default?** It always calculates Mifflin-St Jeor. When body fat is left at zero, Mifflin is the highlighted RMR result. **When does the page use Katch-McArdle?** When body fat is greater than zero, the core calculates Katch-McArdle and the page highlights it as the main RMR while still showing Mifflin. **Is Katch-McArdle more accurate?** Not automatically. It may be informative when lean mass is estimated well, but body-fat measurement error can make the result less credible. **Why do the two formulas disagree?** They use different predictors. Mifflin uses total weight, height, age, and a sex term; Katch uses estimated lean body mass. **Should I eat my RMR calories?** RMR is not a food prescription. Full-day needs include activity and food processing, and nutritional adequacy requires broader context. **Can RMR diagnose a thyroid problem?** No. A prediction equation cannot diagnose endocrine disease. Symptoms or unexplained weight changes require appropriate clinical assessment. **How accurate must body-fat percentage be?** More accuracy improves the lean-mass input, but all field methods have uncertainty. Compare the source and repeatability before relying on small formula differences. **What happens if body fat is above 100%?** The core caps it at 100%, producing zero lean mass and a 370 kcal Katch result. That is software protection, not a meaningful physiological result. **Why does age not appear in Katch-McArdle?** The formula uses lean body mass as its predictor and does not include age directly. Age can still affect real physiology and the reliability of the lean-mass assumption. **How often should I recalculate RMR?** Recalculate after a meaningful change in weight or body composition, not every minor fluctuation. Use the same input method for comparisons. **Can exercise raise RMR?** Training can change body composition and energy use, but the size and persistence of any resting change vary. The calculator does not model training directly. **Can I use this during pregnancy or breastfeeding?** Do not use the generic output as a calorie plan. Pregnancy and lactation involve changing needs that require qualified guidance. **When is laboratory testing worth considering?** Indirect calorimetry may be useful when a clinician or qualified practitioner needs a measured resting estimate and prediction equations appear inadequate for the decision. ## Final Summary The DTC RMR Calculator always computes Mifflin-St Jeor and optionally computes Katch-McArdle from body fat. The page highlights Katch whenever it is available, making input quality especially important. Use either result as a resting estimate, keep it separate from TDEE, and seek professional evaluation when the question is medical rather than mathematical. --- _Source: [Do The Calculation](https://dothecalculation.com/blog/health/rmr-calculation-guide). Quote freely with attribution and a link to this page._