I answer a fair number of questions about calculations, so I though I would consolidate my answer into one post that I can refer to more conveniently.
Calculating your maintenance calories
There are two important concepts here: BMR (basic metabolic rate) and NAMR (normal activity metabolic rate). Your basic metabolic rate is the rate at which you burn
calories by just living. This is an important number, because it will account for most of your calorie expenditure. There are many calculators online for BMR, and here is one I use:
BMR Calculator.
Use any one you like; they all have pretty much the same formula, and just because you have your BMR, doesn't mean you're done! The real challenge is to calculate your NAMR, i.e. the
calories you expend every day doing what you do normally,
without exercise. A lot of calculations take your BMR and use a multiplier for the level of activity you have. A good example of this is the
Harris Benedict Equation. The problem is, as you can see if you look at this link, it includes your exercise activity.
What I do is this: I take my BMR and use the "sedentary" multiplier of 1.2. In my case, my BMR is 2,059
calories. Using the multiplier of 1.2, I get 2,471
calories. Since I have a sedentary job, this is just about right for me. However, if you have a job that requires a lot of walking, e.g. a school teacher, you would probably bump up this number a bit. My wife walks around 5 miles a day in her job as a teacher, so I would multiply her BMR by 1.2, and then add another 500
calories to that.
Now we want to get a weekly NAMR, so we multiply 2,471
calories x 7, which is 17,297
calories a week.
Next step: calculating your exercise
calories. The best way to do this is with a
heart rate monitor. Polar makes one that will also measure your
calories, based on your
heart rate, for around $80 US. It's much more accurate than the machines. If you can't get an HR monitor, then you'll have to use online tables or the machines. Just keep in mind that machines typically overestimate calorie expenditure, except in the case of heavy interval or
HIIT cardio, in which case they typically underestimate it.
The key here is to get your average exercise calorie expenditure per week. For me, a typical week looks like this:
Day 1:
HIIT Cardio, 600
calories
Day 2: Weight Training, 550
calories
Day 3: Interval
Cardio, 1000
calories
Day 4: rest
Day 5: Weight Training, 550
calories
Day 6: Interval
Cardio, 1000
calories
Day 7:
HIIT Cardio, 600
calories
-------------------------------------------
Total: 4,300
calories
Now I have my weekly calorie expenditure: 21,597
calories per week.
This gives me a daily maintenance expenditure of 3,085
calories. In other words, if I consume about 3,000
calories a day, and keep exercising the way I do, I won't lose weight and I won't gain weight.
Since I want to lose weight, but not lean muscle mass, I want to set the right calorie deficit level. For this, I use
body fat percentage.
Calculating your Body Fat Percentage
I think this is one of the most critical components of your plan. Forget about
BMI, which is just a convenient index for insurance companies, and one that will penalize you severely for holding onto your muscle. (I'll give you an example of this later). Short of immersing yourself in a tank of water and paying for a scientific test at a lab, there are many less exact ways to do it.
One way that is a complete waste of time is buying a scale that measures
body fat. These scales work by sending a light current through your legs. If you are male, and don't have a lot of
fat on your legs, you will appear to be totally ripped. Another way is to get a cheap pair of plastic calipers online, and measure your skinfolds. This works well if you are consistent. I've actually found that the handheld devices give me a fairly accurate reading.
The key here is that you'll have to find a method that works for you, and then stick with it.
Once you know your
body fat percentage, you can use it to calculate two important things: your "ideal" calorie deficit, and your target weight.
Currently, my
body fat percentage is 25.8%. Not good, I know, but that's what it is. From this, I can get my lean
body mass and my
fat body mass, using my current weight of 237 lbs.
Lean
Body Mass = 237 x (1 - 0.258) = 176 lbs.
Fat Body Mass = 237 * 0.258 = 61 lbs.
My formula for a safe deficit: 10
calories per
pound of
fat body mass. In my case, this would be 61 x 10, or 610
calories. In other words, if I set my average daily deficit at 610
calories, this will give me a healthy rate of
weight loss that does not cut into my lean muscle mass, and one that especially doesn't trigger the "starvation" response in my metabolism.
This formula scales well, too. As you get closer to your target weight, your deficit will have to go down a bit; if you are really obese, it allows you to set a much higher deficit level, at least in the beginning.
Calculating your "Ideal" Weight
As I said, forget about
BMI. Instead, use your Lean
Body Mass (LBM) and your Target
Body Fat Percentage (TBFP). My
goal is to get to 12%
body fat. (If I were a woman, this would be higher, of course) The reciprocal of this is my Target Lean Mass Percentage (TLMP). In my case, this is 88%. My target weight is my current lean
body mass (LBM), divided by TLMP:
LBM/TLMP = 176 lbs. / 88% = 200 lbs.
I think you would agree that a 58 year old, 6 foot, male with 12%
body fat would be pretty darned good, right? Well take a look at the
BMI for this: it's 27.1, right in the middle of the "overweight" category. This is a good example of how worthless
BMI is in setting your
goal, and how silly it is for anyone who has any muscle mass whatsoever.
Fine-Tuning As You Progress
One of the reasons I go to all the trouble of getting these calculations is that they're good measures to have as you go along. For example, I change my Normal Activity Metabolic Rate into a rate per
pound, which in my case is calculated as:
NAMR / Weight = 2,471 / 237 = 10.4
calories per
pound. This is important, because as you lose weight, you have to adjust your NAMR. For example, when I weighed 270 lbs., my NAMR was 2,808
calories per day; when I am at my target weight, my NAMR will be 2,080
calories a day, a difference of 728
calories a day!
This, by the way, is one of the reasons people "plateau." They forget to adjust their NAMR after they've lost 20 or 30
pounds, and the difference of 300
calories a day, which comes to 3
pounds a month, causes them to maintain rather than lose weight.
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