# IIFYM vs Scooby - why such a big difference?

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Posts: 14,261 Member
edited July 2015
To preface my question, I know that all of the different TDEE calculators are estimates so I don't expect them to hit my TDEE perfectly. I'm just wondering what is going on that two calculators can be so different.

In the past I felt like the IIFYM calculator was pretty accurate for me but now it gives me a number that is just completely off. This is what I am entering:

Female
38 yrs
5'5.5"
148 lbs
Leaving it at Mifflin-St Jeor
5 times/wk exercise
Results are BMR of 1354 and TDEE of 1980

I put the same info in this Scooby calculator and get the following at these activity levels:
3-5 hrs/wk - BMR 1364, TDEE 2114
5-6 hrs/wk - BMR 1364, TDEE 2353

Based on my experience calorie counting, I maintain somewhere between 2400 and 2500 so the last Scooby number is closest to reality for me, especially when I factor in tracking errors and calorie miscounts for high protein foods. So I'm not changing anything based on these calculators but I do have to scratch my head at how the same input is coming up with such different output when the calculators appear to be built on the same model (i.e., Mifflin-St Jeor formula.)

## Replies

• Posts: 9,532 Member
edited July 2015
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It's the exercise assumption. The "5 times/wk" in the top one is a lot less than "5-6 hrs/wk" in the bottom one. Change the top one to "5 times/wk (intense)" and they're a whole lot closer.
• Posts: 495 Member
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I use Scooby's calculator, and he's done me well...and I don't feel like I'm starving...and he's got that geeky engineering mind...he's better at math than me. LOL
• Posts: 30,886 Member
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IIFYM site is screwed up every time I've tried to use it lately, but it's always given me a number substantially lower than Scooby -- I think the difference is just different estimates for what "lightly active/3-5 days a week" and the like involve. My numbers also seemed to be closer to Scooby when I had results to compare them with.
• Posts: 2,254 Member
edited July 2015
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I have wondered this as well. Someone in another thread suggested using "katch-mcCardle" for the IIFYM calculator -- but you have to know your body fat %. When I use Mifflin St. Jeor, IIFYM and Scooby are still very far apart, with exercise for both set to 3-5 times a week and goal set at losing 25%. For me, Scooby is much lower.
• Posts: 874 Member
edited July 2015
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They all give out different readings, so i just add them all up and divide them which gives the average number.... and i just use that as my TDEE.
• Posts: 529 Member
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Mr_Knight wrote: »
It's the exercise assumption. The "5 times/wk" in the top one is a lot less than "5-6 hrs/wk" in the bottom one. Change the top one to "5 times/wk (intense)" and they're a whole lot closer.

^ This ^

Anything other then sedentary as an activity level for any calculator is going to be based on an assumption/opinion of what the activity level actually is.

I always calc based on sed anyway, and then track my exercise and let MFP add to baseline for my exercise days.
• Posts: 30,886 Member
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They all give out different readings, so i just add them all up and divide them which gives the average number.... and i just use that as my TDEE.

Once you've been logging for a while you can figure out a more accurate one based on your real numbers (and how you log).
• Posts: 14,261 Member
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Mr_Knight wrote: »
It's the exercise assumption. The "5 times/wk" in the top one is a lot less than "5-6 hrs/wk" in the bottom one. Change the top one to "5 times/wk (intense)" and they're a whole lot closer.

Aha!

I recently joined an IIFYM group on FB. A lot of the women have had serious transformations in a fairly short amount of time, which they attribute solely to hitting their macros rather than eating at a deficit. One of their rules to live by is using Mifflin-St Jeor and only counting lifting sessions, no cardio, when using the IIFYM calculator. I don't want to completely underplay the benefit of getting enough protein and fat but based on all of that, my guess is that a good portion of the transformations are due to eating quite a bit below their actual TDEEs rather than being religious about macros. If I ate at 20% less than my IIFYM figure, I'd be in a 900 calorie daily deficit, which would mean pretty fast weight loss even at my current weight.
• Posts: 2,561 Member
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I'm all about Scooby's calculator. If I used the IIFYM calculator, I'd be eating 1850 calories a day for maintenance...which is the amount of calories I was eating to lose most of the weight I've lost. Scooby's got me closer to 2150, which is what I've actually maintained on in the past.
• Posts: 4,925 Member
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If I remember correctly, the IIFYM calculator translates 1 time of exercise as thirty minutes of exercise.
• Posts: 14,261 Member
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If I remember correctly, the IIFYM calculator translates 1 time of exercise as thirty minutes of exercise.

Well that would certainly make a difference. I usually consider a workout to be an hour.
• Posts: 8,485 Member
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Very interesting. I have always used IIFYM and have found it very accurate when losing and maintaining even though all my workouts are 60 min.
Cheers, h.