A blog by runners. For runners.

Why Does my Nike+ Show More Calories than the Treadmill?

I recently answered a question on Yahoo Answers where someone wanted to know why the treadmill shows different calories burned than their Nike+:
“My Nike plus shows I went a farther distance and burned more calories than the treadmill?
I entered my height and weight on both the treadmill and Nike + the Nike plus shows I ran 1/4 more 1/3 more calories burned”

The Trouble with Treadmills

This isn't uncommon. In fact if you tried 3 treadmills by different manufacturers I'm sure you'd find 3 different calories reported! Treadmills use your weight and the distance you travel and a formula to calculate the calories burned but these are are averages across the population of the world. The percentage of body fat and your metabolism play a part in reality.
Some calculations include estimates of “after burn” effect of running where your body continues to burn calories at an accelerated rate after you finish working out. How many? It depends on how fast you were running and can depend on the time of the day. So all in all, it's really pretty complicated just for treadmills!

Are Your Legs the Nike+ Average Length?

The Nike+ uses your weight and what it believes was the distance you ran but as you pointed out the nike+ showed a further distance than the treadmill. The Nike+ calculates distance based on your cadence (how often your foot strikes) multiplied by your average stride length. It uses an average stride length until you calibrate it so unless you have an average stride length, you're unlikely to get an accurate report of your distance.
Add to that the complexity of treadmills altering your typical stride and the fact that motors and drive units on treadmills age and don't accurately reflect the distance traveled you've got another mess!

Calibrate

The first thing I recommend is calibrating the Nike+ to make sure your stride length is accurately estimated. I blogged about my own experiences Calibrating the iPod Nano Sport Kit/Nike+ so you might try that.
Once you calibrate you'll get a more accurate distance estimate but you're still going to be stuck with different calculations of calories.

Getting it Right

The scientific way to measure calories burned during exercise is to measure heat generated by the body during exercise and combine that with the oxygen coming into and leaving the body and the exchange of oxygen and carbon dioxide in the body.
The bottom line is that wherever you go, calories burned during exercise is less than scientific. At WalkJogRun.net we calculate calories burned by the mapped distance and your weight in pounds:
calories burned = distance in km * weight in kg * 1.036
where 1.036 represents a weighting factor for running

In the ballpark

Science aside, running and walking the same distance in your body burns approximately the same number of calories (unless you're an elite runner) so don't worry about taking your time! A 3 mile run for a 200 pound man or woman will burn around 450 calories whereas a 150 pound man or woman would burn around 350.
So next time you head out for a run and see your preferred running gadget tell you how many calories you burned, hesitate before equating that with another chocolate bar ;-)