When you calculate your TDEE you first calculate your BMR to get your calories burned if you just lay in bed all day. Then you multiply it by your activity level, sedentary 1.2x, lightly active 1.375x, active 1.55x, very active 1.75x, extremely active 1.9x.
There is a good chance that you, like I did, think you should choose a higher activity level than the calories you actually burn.
A 6′ tall 30 year old man who weighs 200 lbs has a BMR of about 2000 calories. If he is sedentary his TDEE is 2400, so he should be burning 400 calories through movement. That doesn’t sound like much, but if you work a desk job, do no exercise, and walk less than 4 miles a day (~8,000 steps) it is likely too high.
Similarly if he set it to ‘lightly active’ that would be 750 calories. In order to run off 750 calories he would need to run 6 miles at 10 min/mile pace. I don’t think anyone would consider a person who runs 6 miles a day ‘lightly active’.
During this self-isolation time I have been going on another cut after a winter of lifting and clean bulking. I track everything I eat and drink with MFP and wear a Fitbit Ionic to calculate calories burned. I then use a google spreadsheet I put together a couple years ago to compare my expected weight loss using 3,500 calories = 1lb fat loss vs actual weight loss (If you want to use that spreadsheet click here, simply enter your weight, calories consumed, and exercise calories every day).
I work a desk job (software engineer) but I run between 2 and 4 miles every day, I use resistance bands and do calisthenic exercises every other day, and walk another ~2 miles. If you asked me what I thought my activity level was I would guess either active or very active. For me those would be 1,050 and 1,425 calories burned respectively. But according to my Fitbit I only burn an extra 741 calories per day on average.
This is where my spreadsheet comes in handy. It adds up my calories consumed, subtracts BMR and calories burned, then estimates my expected weight loss. It will do the same thing with the calories consumed TDEE. Well check this out! My actual weight is within 0.3 pounds of both my BMR estimated weight and my TDEE (set as lightly active) weight!
What does this tell me?
My actual TDEE should be set as ‘lightly active’ despite all of the exercise I get every day.
Also, I am pretty good at counting calories apparently, I might be underestimating a little but not by much.
TL;DR: The multipliers on the TDEE calculation result in higher than expected calories burned through activity. There is a good chance you are not burning as much as you think. My scientific study with a n=1 shows that calorie counting and TDEE set to the right level (or BMR plus burned calories) and 3,500 net calories = 1 pound of fat can estimate weight loss to within 3% of the amount lost.
P.S. I have thought about taking what this spreadsheet does and making a webapp version that will automatically pull all of the information from Fitbit and give the same info without any user input data needed. Is anyone interested in a tool like that?