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I've been looking at BMR calculators, and they ask for weight as one of the inputs to determine how many calories you might burn simply by existing on any given day - but it's widely stated that muscle is more expensive to maintain than fat in terms of a caloric goal.

So therein lies the question - do you input the weight of lean body mass, or total? And if it's the latter, how do you account for whatever arbitrary percentile of that total being fat?

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Cunningham's formula is a pretty good formula to estimate RMR, especially in athletes

RMR = 370 + 21.6 * FFM

FFM is fat free mass, as you might expect. Many online sites just use other formulas, or deduce your FFM from your total weight and height using for example your BMI (weight/height^2) and Deurenberg's formula.

So, it depends. If a site is asking you for your weight and not your fat free mass (which is usually what people have handy), it is likely asking for your weight and using some heuristic as the one I've shown above.

If you are training and have access to your FFM via a DEXA scan for example, you can calculate that using the formula directly and don't need any site to calculate it.

A final very important point is that this number is not really important as you should take it only as a starting point to achieve your goals.

Then you should measure your progress and adjust your intake based on your progress. For example, you've determined you expend 2300 kcals daily based on your RMR, your activity multiplier, your training expenditure, and your food thermogenesis.

Then you try eating just 2300 kcals if you want to maintain, about 10% more than that if you want to gain or about 10% less than that if you want to lose. Do it for some time (1 or 2 weeks) and measure. If your measurements are consistent with your goals (maintaining, gaining, or losing), it means the numbers were calculated properly for you. If the measurements don't match, ADJUST the TARGET by another 5 to 10% and measure again in one or two weeks.

This method does two things:

1.- It avoids making you have to go periodically back for a heuristic that may or may not match your body.

2.- It is perfectly matched to your circumstances and reality. So if you gain a lot of muscle, for example, you'll be able to adapt your target automatically.

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  • As mentioned above, unless you can afford dexa scans, the numbers don't matter as long as you use the same tools and methods, what matters is the progress you make over a period of time using those same methods. It's a trial and error process, best way is to check your lean mass, body fat, weight after each week and adjust calories from there.. it's incredibly difficult unless you have an Olympic nutritionist to get an exact number for everything
    – user32213
    Commented Aug 6, 2021 at 16:34
  • Totally, as you say it is difficult but most impòrtantly they don't matter at all for a normal person trying to control intake.
    – user35666
    Commented Aug 6, 2021 at 19:56
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If they ask you for weight, that almost certainly means total body weight, and they're most likely estimating your body fat percentage from other parameters, such as gender, age, and possible activity level.

There will be slight differences in BMR between two people with the same fat-free body mass if one of them also has substantially more fat mass, as body fat is metabolically active, just to a much lesser degree than muscle. Also, body fat will made a huge difference to TDEE (BMR plus energy used in activity), because the person with more body fat will be heavier, and will need to carry that extra mass around with every step that they take throughout the day.

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