01Resting energy is an estimate, not a measurement
Mifflin-St Jeor estimates resting energy expenditure from weight, height, age and the sex constant used by the equation. The simplified male equation is 10 × weight kg + 6.25 × height cm − 5 × age + 5; the female equation ends with −161. These are the published coefficients, not a sex or gender assessment. Source: Mifflin et al., 1990, PMID 2305711.
The original research measured resting energy expenditure in 498 healthy adults aged 19 to 78. It did not measure a permanent metabolism for every individual, and it was not designed for children, pregnancy or clinical nutrition treatment. This calculator therefore labels the output resting energy rather than promising an exact basal metabolic rate.
A 2005 systematic review compared predictive equations with measured resting metabolic rate and cautioned that equation errors can matter. The review supports Mifflin-St Jeor as a useful general option, not as an individual accuracy guarantee. Source: Frankenfield et al., PMID 15883556.
Indirect calorimetry measures energy expenditure. An online equation only estimates it.
02Three equations for three input choices
Mifflin-St Jeor is the default when you know weight, height, age and the applicable sex constant. Revised Harris-Benedict is available as a comparison using the Roza and Shizgal coefficients from PMID 6741850. Different equations can legitimately return different estimates; that spread is not a confidence interval.
Katch-McArdle instead uses lean body mass: weight × (1 − body-fat percentage), followed by 370 + 21.6 × lean mass kg. The equation is presented in McArdle, Katch and Katch, Exercise Physiology. Use it only when you have a body-fat estimate you trust. Measurement error in that percentage flows directly into the result, so lean-mass mode is not automatically more accurate.
The selected method is always printed with the result and included in PDF reports. Switching methods never changes the displayed formula silently.
- Mifflin-St Jeor: general default with four body-details inputs.
- Revised Harris-Benedict: an alternative published estimate, not an average of the other methods.
- Katch-McArdle: opt-in lean-mass method requiring body-fat data.
03Activity, maintenance and the calorie target
Estimated maintenance, often called TDEE, is resting energy multiplied by an activity factor. The five choices run from 1.2 to 1.9 and describe both everyday movement and purposeful exercise. These multipliers are coarse planning categories. Choose the description that matches a typical week rather than your most active day.
Goal defaults are transparent assumptions: Fat Loss −20%, Maintain 0%, Lean Bulk +10% and Aggressive Bulk +20% of estimated maintenance. They are starting scenarios, not prescribed rates of weight change. Advanced options allow a custom percentage from maintenance or a direct daily calorie target you already have.
The result shows the daily calorie gap from estimated maintenance. It deliberately does not convert that gap into a promised weekly weight loss or gain. Real energy expenditure and body weight adapt over time. NIH/NIDDK offers a separate dynamic Body Weight Planner for goal-weight and time-course planning.
A static calorie gap is arithmetic, not a forecast of what the scale will do next week.
04How calories become macro grams
Protein and carbohydrate use 4 kcal per gram, and fat uses 9 kcal per gram. These are the general Atwater conversion factors listed in USDA nutrition resources. The calculator reconciles the three energy-yielding macros back to the calorie target before displaying a valid plan.
Protein-first mode sets protein from grams per kilogram and fat from a percentage of calories; carbohydrates receive the calories that remain. If requested protein and fat already exceed the calorie target, the tool shows the excess and asks for an adjustment. It does not hide the problem by silently forcing carbohydrates to zero.
Percentage mode offers balanced, higher-protein, higher-carb and lower-carb presets. In a custom split, you set protein and fat while carbohydrates remain as the visible share, so the three percentages always total 100%. Presets are preferences, not rankings of diets.
- Protein kcal = protein grams × 4.
- Carbohydrate kcal = carbohydrate grams × 4.
- Fat kcal = fat grams × 9.
- Feasibility check = protein kcal + carbohydrate kcal + fat kcal compared with the calorie target.
05Protein, AMDR ranges and fiber in context
The adjustable protein-first default is 1.8 g/kg. The International Society of Sports Nutrition reports that 1.4–2.0 g/kg/day is sufficient for most exercising individuals to build or maintain muscle, with higher intakes useful in some resistance-trained, energy-restricted contexts. That is not a universal target for every sedentary adult or every medical condition. Source: Jäger et al., 2017, PMID 28642676.
The adult Acceptable Macronutrient Distribution Ranges are broad references: protein 10–35%, carbohydrate 45–65% and fat 20–35% of energy. A preset or custom plan can fall outside one of these ranges. The tool labels that fact without turning it into a health grade. Food quality, micronutrients, medical history and adherence are not captured by three percentages.
Fiber is shown separately as 14 g per 1,000 kcal, consistent with the FDA Daily Value of 28 g on a 2,000 kcal diet. It is a reference, not a fourth energy-yielding macro added to the calorie equation, and not a personalized treatment target.
The Assessment dial checks energy reconciliation only. It does not score health, diet quality or protein adequacy.
06Using the two-to-six-meal split
Equal meal targets can make a daily plan easier to track. Choose two, three, four, five or six eating occasions to see protein, carbohydrate and fat grams for each one. The labels are organizational aids; they do not prescribe meal timing or tell you which foods to eat.
Daily grams are rounded to whole grams first. The allocator then distributes any remaining whole grams across meals so the meal rows add back to the displayed daily grams exactly. No grams disappear through independent rounding.
Meal calories are calculated from those rounded grams with the 4/4/9 factors. The small difference from the unrounded daily calorie target is printed below the meal grid rather than hidden.
07Calibrate carefully and know the scope
Use the result as a starting estimate. Consistent tracking and longer-term trends can help you decide whether the activity choice and calorie target match your actual response. A single daily weigh-in does not validate or invalidate an energy equation.
Recalculate when your weight, activity, goal or preferred eating pattern changes. If you already have a clinician-provided calorie target, direct-calorie mode lets you use that number while still checking whether your selected macros fit within it.
This tool is for general education in healthy adults aged 18 and over. Children, pregnancy or breastfeeding, eating-disorder care and medical nutrition therapy require individualized guidance. Kidney disease, diabetes, medicines affected by food and gender-affirming treatment can also change which inputs or targets are appropriate; consult a qualified clinician or registered dietitian.
No macro split can diagnose a condition, guarantee a body-composition outcome or replace individualized care.