# How does Strava calculate calories?



## KneesUp (3 Dec 2019)

I’m not expecting the calories figure on strava to be anything like accurate, especially as I have no sensors, but I’d expect it to be consistent. I’ve done the same commute the last two days with times 18 seconds apart over 2.6 miles, but yesterday it guessed at 254 calories and today it went for 30 calories - which is quite a difference. It’s all downhill, pretty much, if that helps?


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## PaulSB (3 Dec 2019)

I can't answer your question but a better comparison could be the uphill ride home. You'll be working then whereas on the downhill you may be coasting more at times than you realise.


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## Heltor Chasca (3 Dec 2019)

I think it’s nonsense. It doesn’t account for panniers, heavy bikes, road surface, weather etc. The other day it calculated I used up 10000 calories on a 10 hour ride 😮


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## Tenkaykev (3 Dec 2019)

Over the years I've used a general "rule of thumb". 

100 kilo calories per mile for running, 50 for walking ( not strolling) 

From my perceptions of effort whilst exercising I'd tend towards 50 for cycling ( not caning it) but a steady effort. 

I've found it a handy tool for working out my post exercise rehydration strategy. A 10k run = 3 pints of Tanglefoot or equivalent.


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## Venod (3 Dec 2019)

This any good I haven't read it.

https://support.strava.com/hc/en-us/articles/216917097-Calorie-Calculation


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## Dogtrousers (3 Dec 2019)

We know how it calculates power 
https://support.strava.com/hc/en-us/articles/216917107-How-Strava-Calculates-Power

Power is effectively calories per second. So at a guess they sum the amount of work done in joules based on the above power calculation. Then convert it to a daft unit, calories.

Try comparing the Strava power estimates for the two rides, see if you get a similar disparity.

Edit cross post with @Venod


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## vickster (3 Dec 2019)

My rule of thumb, 30-40 calories per mile. Less if a lightweight rider, or very easy terrain. More if a heavy rider, and/or tough terrain (uphill a lot)


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## swansonj (3 Dec 2019)

vickster said:


> My rule of thumb, 30-40 calories per mile. Less if a lightweight rider, or very easy terrain. More if a heavy rider, and/or tough terrain (uphill a lot)


Doesn't it depend considerably on how fast you cycle?


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## vickster (3 Dec 2019)

swansonj said:


> Doesn't it depend considerably on how fast you cycle?


Ok, you could add effort but speed depends on terrain too? It's a guesstimate like any other calorie calculation done outside a lab!


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## si_c (3 Dec 2019)

Strava power has calculated for me quite consistently when I've used it, that being said I know it's not accurate.

As a guideline I use about 40 to 50cals per mile, depending on how hilly it is, bearing in mind I weight 95ish kg.

This is backed up by my power meter, but if you ride faster then expect to burn considerably more.


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## roubaixtuesday (3 Dec 2019)

According to the link "Strava uses your power output and a coefficient for human efficiency. " Checking a recent ride of mine, the coefficient used is about 0.25 ie you burn 4* as many calories as you put out power.

I find the Strava calories pretty consistent and reliable. In essence I think they work by using an aerodynamic coefficient, a constant resistance and a velocity dependent resistance. There's a description of the type of model used for this sort of thing here.

https://ridefar.info/wp-content/uploads/positions.jpg

Strava also takes into account the power gained or lost from gravity, so will subtract (or add) the energy gained or lost in climbing/descending.

254kcal in 2.6 miles sounds like a *lot*, and it's on top of the energy gained from descending so I'm guessing there is some glitch associated with that figure. It equates to 100 calories/mile, which is double what I clocked on a recent hilly ride. Have a look at the analysis. 30 kcal sounds realistic for a downhill 2.6 miles.

On a generally downhill ride, you can expect the figures to fluctuate a lot from ride to ride, because Strava is essentially guessing what your resistance is, and if it's close to your freewheeling speed, a small amount either way will make a big difference to net calories.


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## Milkfloat (3 Dec 2019)

They do it using one of these.


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## vickster (3 Dec 2019)

si_c said:


> Strava power has calculated for me quite consistently when I've used it, that being said I know it's not accurate.
> 
> As a guideline I use about 40 to 50cals per mile, depending on how hilly it is, bearing in mind I weight 95ish kg.
> 
> This is backed up by my power meter, but* if you ride faster then expect to burn considerably more*.



Unless going downhill


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## swansonj (3 Dec 2019)

vickster said:


> Ok, you could add effort but speed depends on terrain too? It's a guesstimate like any other calorie calculation done outside a lab!


Yeah, agreed, it's all approximate.

But when I was commuting before retiring, my morning journey took anything from 32 minutes to 45 minutes, depending partly on traffic but largely on whether I was in a mood to push myself or not. 50% difference in time equates to considerably more than 50% difference in power, and still more than 50% difference in calories per mile, once you're into the speed regime where wind resistance dominates?


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## PK99 (3 Dec 2019)

swansonj said:


> Doesn't it depend considerably on how fast you cycle?



Per mile, the difference is small - high effort for short time vs low effort for longer time


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## vickster (3 Dec 2019)

swansonj said:


> Yeah, agreed, it's all approximate.
> 
> But when I was commuting before retiring, my morning journey took anything from 32 minutes to 45 minutes, depending partly on traffic but largely on whether I was in a mood to push myself or not. 50% difference in time equates to considerably more than 50% difference in power, and still more than 50% difference in calories per mile, once you're into the speed regime where wind resistance dominates?


I don’t really care that much how many calories I may or may not have burnt


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## si_c (3 Dec 2019)

vickster said:


> I don’t really care that much how many calories I may or may not have burnt


I do. A lower number means I have to moderate my cake intake. I don't like that.


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## vickster (3 Dec 2019)

si_c said:


> I do. A lower number means I have to moderate my cake intake. I don't like that.


Makes no real difference to me, I’m overweight regardless


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## si_c (3 Dec 2019)

vickster said:


> Makes no real difference to me, I’m overweight regardless


Same here, but I have aspirations towards being as skinny as I was at eighteen.


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## vickster (3 Dec 2019)

si_c said:


> Same here, but I have aspirations towards being as skinny as I was at eighteen.


I’m probably lighter now than I was at 18!


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## swansonj (3 Dec 2019)

vickster said:


> I don’t really care that much how many calories I may or may not have burnt


No problem- it's just that, as a former and still part-time physicist, I just can't resist these challenges😋


PK99 said:


> Per mile, the difference is small - high effort for short time vs low effort for longer time


That works if speed is roughly proportional to power, which is broadly true at low speeds. But as soon as the speed is high enough for wind resistance to be significant, then even more so when it becomes dominant, i suggest that we're into a regime where speed is not proportional to power, and it ceases to be true - you spend more calories beating the wind than you gain by the shorter time.


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## roubaixtuesday (3 Dec 2019)

swansonj said:


> No problem- it's just that, as a former and still part-time physicist, I just can't resist these challenges😋
> 
> That works if speed is roughly proportional to power, which is broadly true at low speeds. But as soon as the speed is high enough for wind resistance to be significant, then even more so when it becomes dominant, i suggest that we're into a regime where speed is not proportional to power, and it ceases to be true - you spend more calories beating the wind than you gain by the shorter time.



For speeds where wind resistance dominates (maybe 15mph), resistance is proprtional to speed squared, and energy (calories) to resistance* distance.

So you'd expect the number of calories to be proportional to the square of speed for a given distance ie quadrupling from 15mph to 30mph (should you be able to maintain 30mph!).


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## si_c (3 Dec 2019)

roubaixtuesday said:


> For speeds where wind resistance dominates (maybe 15mph), resistance is proprtional to speed squared, and energy (calories) to resistance* distance.
> 
> So you'd expect the number of calories to be proportional to the square of speed for a given distance ie quadrupling from 15mph to 30mph (should you be able to maintain 30mph!).



Almost, calorific burn for any given distance can be calculated as (effort to overcome resistance) * time. So for a doubling of speed you might see a quadruple of effort, but a half the time, so calorific burn to be around double.


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## roubaixtuesday (3 Dec 2019)

si_c said:


> Almost, calorific burn for any given distance can be calculated as (effort to overcome resistance) * time. So for a doubling of speed you might see a quadruple of effort, but a half the time, so calorific burn to be around double.



That's not actually correct.

Air resistance (force, units Newtons) is proportional to velocity squared (drag coefficient multiplied by fluid density multiplied by velocity squared)
Power (units Watts) is force multiplied by velocity, proportional to velocity cubed
Calorific burn, energy (Work, units of Joules or Newtonmetres) is force multiplied by distance, so is proportional to velocity squared

All assuming that only air resistance is important.


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## PK99 (3 Dec 2019)

roubaixtuesday said:


> For speeds where wind resistance dominates (maybe 15mph), resistance is proprtional to speed squared, and energy (calories) to resistance* distance.
> 
> So you'd expect the number of calories to be proportional to the square of speed for a given distance ie quadrupling from 15mph to 30mph (should you be able to maintain 30mph!).



Try punching some numbers in here : http://bikecalculator.com/

using the standard parameters:

100W gives 15.04mph. 3.99mins per mile =23 calories
200W...……...19.74mph 3.04mins...………….=35 calories


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## Dogtrousers (3 Dec 2019)

None of this really explains the OP's conundrum of a set distance with similar times returning very different figures.
Like the OP I'd have expected Strava's simplistic calculations to be quite consistent.

Maybe one of your rides had one of those inexplicable 800mph "blips" in it or something?

@KneesUp We need more than two data points! Was one of your two an outlier? (As things stand they are _both_ outliers!)


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## Milkfloat (3 Dec 2019)

Dogtrousers said:


> None of this really explains the OP's conundrum of a set distance with similar times returning very different figures.


How very dare you, my dartboard explains it all . Seriously, Strava's figures on pretty much anything are educated guesses at best.


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## roubaixtuesday (3 Dec 2019)

PK99 said:


> Try punching some numbers in here : http://bikecalculator.com/
> 
> using the standard parameters:
> 
> ...



They give roughly the same ratios. The relationship I quoted is purely for air resistance and is exactly that used in the calculator, see here http://bikecalculator.com/what.html

They also take into account rolling resistance, hence there is some discrepancy


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## si_c (3 Dec 2019)

roubaixtuesday said:


> That's not actually correct.
> 
> Air resistance (force, units Newtons) is proportional to velocity squared (drag coefficient multiplied by fluid density multiplied by velocity squared)
> Power (units Watts) is force multiplied by velocity, proportional to velocity cubed
> ...


My understanding is your description of air resistance is correct but in your original post you said that calorific burn would quadruple for a doubling of speed. That was incorrect.

For example, if you travel 20miles at 10mph and burn 320 calories you would expect to burn 1300 if that was the case, but in reality it works out around 750ish according to bike calculator.


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## roubaixtuesday (3 Dec 2019)

si_c said:


> My understanding is your description of air resistance is correct but in your original post you said that calorific burn would quadruple for a doubling of speed. That was incorrect.
> 
> For example, if you travel 20miles at 10mph and burn 320 calories you would expect to burn 1300 if that was the case, but in reality it works out around 750ish according to bike calculator.



No, it is exactly correct, assuming air resistance dominates as I said. Read the formulae, it's just algebra. 

At 10mph air resistance will not dominate, rolling resistance will. 

On the bike calc numbers, using defaults in there (I suspect you may have used kmh).

20 miles at 10mph= 834 calories, 29 W
20 miles at 20 mph = 1182 calories, 82 W
20 miles at 40 mph = 2577 calories, 358W
20 miles at 80 mph = 8147 calories, 2263W
20 miles at 160 mph = 30433 calories, 16907W

Note how the increase in calories approaches the square as velocity increases, and the increase in watts approaches the cube. It's almost exact at the ludicrously high final step. 

The rolling resistance from bike calc is a greater proportion at sensible cycling speeds than I would have guessed, but the principles remain.


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## si_c (3 Dec 2019)

roubaixtuesday said:


> No, it is exactly correct, assuming air resistance dominates as I said. Read the formulae, it's just algebra.
> 
> At 10mph air resistance will not dominate, rolling resistance will.
> 
> ...



The increase in watts should be proportional to the square of the velocity not the cube. I used my own values for the bike and rider weight, and a fixed distance of 20miles for the values.

Rider weight: 210lbs
Bike weight 24lbs

Using a fixed distance of 20miles, I set the speed to ~10mph, giving 
Time: 119.7mins, 46W average power, 316kcal

Setting the speed to ~20mph gives
Time: 59.9mins, 220W ave power, 756kcal

Which doesn't track with your estimates.


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## roubaixtuesday (3 Dec 2019)

si_c said:


> The increase in watts should be proportional to the square of the velocity not the cube. I used my own values for the bike and rider weight, and a fixed distance of 20miles for the values.
> 
> Rider weight: 210lbs
> Bike weight 24lbs
> ...



Yes, as I have explained, because rolling resistance will dominate at low speeds.

Note every single post I have made has included the caveat _assuming air resistance dominates._

Also, even moving from 10 to 20mph the change far exceeds your _"So for a doubling of speed you might see a quadruple of effort, but a half the time, so calorific burn to be around double. " _As you go to higher speeds, the relationship tends to what I've posted.


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## Dogtrousers (3 Dec 2019)

Here's another fun wattage calculating thingy. 
https://www.gribble.org/cycling/power_v_speed.html


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## Twilkes (11 Jan 2020)

Not calories but power - I know Strava power is an estimate but when I change the bike setting from my 10kg bike to my 14kg bike the power estimate for most rides almost doubles, from 137 watts to 268 watts in one particular case. Surely 4kg can't make that much difference? Rider weight is 100kg in both cases.

Apart from that I've found power estimates fairly consistent for similar conditions, no idea how accurate they are though as I don't have a power meter.


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