Did I really burn 706 calories in 70 minutes of HIIT?

This is a topic I talk about all of the time. How many calories did I actually burn during that workout? It is an excellent question with a particularly convoluted answer. Calorie expenditure is related to age, weight, heart rate during exercise and sex. In general, male-bodied people burn more calories for similar effort than female-bodied people and heavier people burn more calories for similar effort as lighter people.

The Journal of Sports Sciences provides a calorie expenditure formula for male and female.

Males use this formula:

Calories Burned = [(Age x 0.2017) – (Weight x 0.09036) + (Heart Rate x 0.6309) — 55.0969] x Time / 4.184.

Females use this formula:

Calories Burned = [(Age x 0.074) – (Weight x 0.05741) + (Heart Rate x 0.4472) — 20.4022] x Time / 4.184.

I am a 41 year old female who weighs 115 pounds. My average heart rate over this 70 minute period was 143bpm**, so my calculations look like this:

Calories Burned = [(Age{41} x 0.074) — (Weight{115} x 0.05741) + (Heart Rate{143}x 0.4472) — 20.4022] x Time{70} / 4.184.

41 x 0.074 = 3.034

115 x .05741 = 6.60215

143 x 0.4472 = 63.9496

(3.034) – (6.60215) + (63.9496) – (20.4022) x 70min          = 669 cal burned
4.184

So I did not quite burn 706 calories. The treadmill had some help with these calculations because I did check in with it and take my heart rate several times. If I hadn’t, the number would be even more off. In fact, I’ve had these numbers be as much as 30% too high!  In general, when you haven’t programmed the equipment with your specifics and it’s not keeping track of your heart rate, the number of calories burned will be over-estimated. Now you know.

**How I determined my average heart rate: My heart rate monitor isn’t working so I took my heart rate on the treadmill at intervals throughout the session, which comprised 10 minutes of warm up, 30 minutes of 1 minute sprints followed by 1 minute of active recovery, and finished with 30 minutes of active recovery at approximately 60% of my maximum heart rate, for a total of 70 minutes.

125bpm x 40min = 5000 (warm up and recovery)
163bpm x 20min = 3260 (beginning on intervals)
173bpm x 10min = 1730 (end of intervals)
(5000+3260+1730)/70min total = average 143bpm