General information, not medical advice. Check with a doctor or registered dietitian before a big diet change, especially if you have a medical condition, are pregnant, or have a history of disordered eating.
Fitness calculators on this site fall into two families: energy tools that estimate how many calories you burn and how to divide them into protein, fat and carbs, and body-composition tools that describe what your weight is actually made of. This page explains what each family can and cannot tell you, which one to open first for your situation, and how their outputs feed into one another so you end up with one coherent plan instead of eight unrelated numbers.
Two families of numbers, one body
Most people arrive at a calculator with a single question, such as how many calories they should eat or whether their weight is healthy. The trouble is that each tool answers only its own question, and the answers depend on one another. A macro split is only as good as the calorie number underneath it. A calorie number is only as good as the body data you fed in. And a body-weight target means little if you do not know how much of that weight is muscle.
The eight tools in this category divide neatly into two groups. The energy group covers the BMR Calculator, the Mifflin-St Jeor Calculator, the Macro Calculator, the Protein Intake Calculator and the Keto Macro Calculator. These start from your resting calorie burn and work outward to a daily intake and a set of gram targets. The body-composition group covers the Body Fat Percentage Calculator, the Lean Body Mass Calculator and the Ideal Weight Calculator. These do not tell you what to eat at all; they describe the body you are eating for.
The energy tools rest on prediction equations. The default on this site is Mifflin-St Jeor, published in 1990 from measurements of 498 healthy adults (247 women and 251 men) aged 19 to 78. In that paper, fat-free mass was the single best predictor of resting energy expenditure, which is the reason the body-composition tools matter even if you only care about calories. A 2005 systematic review by Frankenfield and colleagues compared Mifflin-St Jeor with the Harris-Benedict, Owen and WHO/FAO/UNU equations and found it was the most reliable of the four: it landed within 10% of measured resting metabolic rate in more people than the others, with the narrowest errors. The same review noted that older adults and US minority groups were underrepresented in the data, so the estimate is a starting point rather than a measurement.
Keep that framing in mind as you use any tool here. None of them measures you. They take a handful of inputs, apply an equation built on other people, and give a number that is usually close and occasionally well off. Your own weight trend over the following weeks is what confirms or corrects it.
Where to start depends on what you already know
If all you know is your height, weight, age and sex, start with the BMR Calculator. It gives your resting burn in seconds, and the page explains what that number does and does not mean. If you want to see the arithmetic behind the result, or you are checking a number from an app or a coach, use the Mifflin-St Jeor Calculator instead. It runs the same 1990 equation but lays out every term, so you can see exactly how much of your result comes from weight, how much from height and how much age takes away.
If you already know your body fat percentage, from a scan or a careful tape measurement, open the Lean Body Mass Calculator before anything else. Lean mass is weight minus fat, and it does two jobs downstream: it lets the site's calculator switch from Mifflin-St Jeor to the lean-mass formula it uses for people who enter body fat (370 + 21.6 × lean body mass in kg), and it gives a fairer basis for protein if you carry a lot of body fat. The same page explains fat-free mass index, which turns lean mass into a training-history benchmark.
If you do not know your body fat but want to, the Body Fat Percentage Calculator walks through the methods people can use at home and how far to trust each one. Treat its output as a trend tracker rather than a precise reading.
If your question is what to eat rather than how much you burn, go to the Macro Calculator once you have a calorie target. It divides calories into protein, fat and carbohydrate for cutting, maintaining or building. If protein is the only number you care about, the Protein Intake Calculator is faster and covers specific cases such as older adults and athletes. If you follow a ketogenic diet, the Keto Macro Calculator replaces the general macro tool, because keto caps carbohydrate first and lets fat fill the remaining calories.
Finally, the Ideal Weight Calculator is the one to use with the most caution. It compares the classic ideal-body-weight formulas, which were developed as a stand-in for lean weight in clinical settings rather than as a personal goal. It is useful for seeing a rough range, and the page explains why the formulas disagree with each other and with body-composition data.
One hypothetical person through five of the tools
To show how the outputs connect, here is an explicitly hypothetical person run through the formulas the tools use: a 34-year-old woman, 165 cm tall, 70 kg, with a measured body fat of 30%, lightly active (the site's 1.375 multiplier). Values are computed exactly and rounded to whole kcal or grams at the end.
Hypothetical inputs, not a recommendation for any real person. The two BMR estimates differ by 28 kcal and the two TDEEs by 39 kcal; for an average build the formulas usually sit this close. The BMI and body-fat rows are a reminder that the two measures can tell different stories about the same person.
How the outputs chain together
Read the table from top to bottom and you have the order most people should follow. Body composition comes first when you have it, because it changes everything after it. Resting burn comes second. The activity multiplier turns resting burn into total daily energy expenditure, and that is the number every plan is built on. The goal adjustment comes third: the site's calculator applies −25%, −20% or −10% for cutting, 0% for maintenance, and +10% or +20% for building. Protein comes fourth, set in grams from body weight. Fat and carbohydrate come last, filling the calories that remain.
That order explains a mistake that shows up constantly: people pick a macro ratio first, for example a fixed percentage split, and then try to fit their life around it. A percentage split gives too little protein at low calorie intakes and more than needed at high ones, because protein needs scale with body size, not with how much you eat. The 2017 International Society of Sports Nutrition position stand puts most exercisers at 1.4 to 2.0 g of protein per kg per day, and a 2018 meta-analysis by Morton and colleagues found that the added muscle and strength from extra protein plateaued at around 1.62 g per kg per day. Setting grams first and letting fat and carbohydrate absorb the rest avoids the problem.
The keto path follows the same chain with one change at the end. Carbohydrate is fixed low first, protein is set in grams, and fat makes up the remainder. Calories still come from the same TDEE, so a ketogenic plan cannot escape the energy arithmetic any more than a high-carbohydrate one can.
The body-composition tools also loop back at the end. After eight to twelve weeks on any plan, re-measuring weight and, if you can, body fat tells you whether the calories you chose were right. If lean mass held steady while fat came down, the plan worked as intended. If weight fell but lean mass fell with it, the protein or the deficit size needs another look. This is also why the formula-based estimate should give way to your measured trend over time: a few weeks of steady intake and weigh-ins is better evidence than any equation.
Mistakes that skew fitness numbers
Most bad results come from the inputs or the order, not from the equations themselves.
Choosing an activity level from how hard workouts feel rather than how much you move across the whole day. A desk job with an hour in the gym is often closer to the site's moderately active setting (1.55) than very active (1.725).
Treating BMR as a calorie target. BMR is what you would burn lying still all day; eating at or below it while active is a large deficit, not a moderate one.
Running the lean-mass formula with a guessed body fat percentage. A rough guess can move the result more than the formula change gains you; if the number is uncertain, Mifflin-St Jeor is the safer default.
Comparing BMI with body fat percentage as if they measured the same thing. BMI is weight relative to height; it cannot separate muscle from fat, which is why the hypothetical person in the table lands in different bands on each.
Using an ideal-weight formula as a goal. These equations were built as clinical stand-ins for lean weight, and they ignore frame size and muscle.
Setting protein as a percentage of calories instead of grams per kilogram, which under-feeds protein in a deficit and over-feeds it in a surplus.
Never recalculating. Every one of these numbers moves as weight changes, so re-run the tools after a meaningful change in weight or training.
For most people, a BMR estimate is the first step, because every calorie and macro number is built on it. If you know your body fat percentage, find your lean body mass first instead, since it improves both the calorie estimate and the protein target.
What is the difference between the BMR Calculator and the Mifflin-St Jeor Calculator?+
Both use the 1990 Mifflin-St Jeor equation by default. The BMR Calculator is built for a quick answer and explains what resting burn means; the Mifflin-St Jeor Calculator shows every term of the equation worked out, which is useful for checking a number or understanding why yours is what it is.
Do I need my body fat percentage?+
No. Mifflin-St Jeor needs only weight, height, age and sex, and it was the most reliable of four common equations in the Frankenfield 2005 review. Body fat helps most if you are very lean or very muscular, because the weight-only formula cannot tell muscle from fat. If your body fat figure is only a guess, leave it out.
How much protein should I eat?+
The ISSN 2017 position stand gives 1.4 to 2.0 g per kg of body weight per day for most people who exercise, and notes that 2.3 to 3.1 g per kg may help trained people keep lean mass while in a calorie deficit. A 2018 meta-analysis found extra protein stopped adding measurable muscle gain at around 1.62 g per kg per day.
Why do different calculators give me different calorie numbers?+
They use different equations and different activity multipliers. The 1984 revised Harris-Benedict equation, for example, has a stated precision of about ±14% in normally nourished people, and the Mifflin paper found Harris-Benedict ran about 5% high against measured resting expenditure. Small differences in the activity level you pick usually matter more than the choice of formula.
Is the keto macro calculator different from the regular macro calculator?+
Yes. The general Macro Calculator sets protein and fat and lets carbohydrate fill the rest. The Keto Macro Calculator caps carbohydrate first, then sets protein, and lets fat fill the remaining calories. Both start from the same TDEE, so the total calories do not change, only how they are divided.
What does a fat-free mass index tell me?+
FFMI is lean mass divided by height squared, which lets you compare muscularity across people of different heights. In a 1995 study of 157 male athletes by Kouri and colleagues, drug-free users topped out at an FFMI of about 25, which makes it a useful ceiling for natural progress in men.
How often should I re-run these calculators?+
Re-run them after any meaningful change in body weight, training volume or daily activity. Because each estimate is a starting point, the more useful habit is to compare the calories you actually ate with your weight trend over several weeks and adjust from that, which tells you more about your real burn than any equation.