How is Cardio load calculated?

[patilu]

In response to a prompt, I indicated I wanted to improve my cardio load. The first few cardio load scores in the '40s based on treadmill workouts seemed reasonable. Then yesterday I got a score of 1 for a 30 minute  intermittent interval spinning workout where my average heart rate was 132 which is well into the vigorous range for me and more challenging than my typical spinning workout. According to Fitbit, My cardio fitness score is 39 to 43 which is considered excellent for my age. I don't understand the why discrepancy between the treadmill scores and the spinning score.

Moderator edit: updated subject for clarity

[YachatsMike]

Had the same kind of questionable results
Cardio Range  42 - 66
Yesterday:  Spin for 40 minutes, 120 BPM, Cardio Load 15
Today:  Row for 40 Minutes, 127 BPM, Cardio Load 73

Hard to justify the jump.

[alexthecat · Accepted solution]

@patilu It takes a few weeks for the Cardio Load feature to fully calibrate. During that time, the numbers you get may seem less meaningful. It may be best to do what you normally would and let the app figure things out for itself. 

One thing to understand about Cardio Load is that it is affected by your Readiness Scores. When your Readiness Score is higher, it can take a lot more effort to reach your Target. Conversely, if your Readiness Score is low, it may be better to exercise at lower intensity, so you don't blast right by your Cardio Load Target. 

[DavidFitbit]

Hi @YachatsMike, @alexthecat and welcome @patilu to the Fitbit Community! It's great to have you here!

I'm glad to explain what may be the reason of your cardio load results.

Fitbit calculates your target load using several factors: Training target, Recent activity and Daily readiness as @alexthecat says.

By looking at how well your body has recovered from previous workouts and the stressors of everyday life, daily readiness plays an important role in determining your target load. If your readiness is low, you’ll receive a lower target load, guiding you to balance rest and recovery. For more information on how daily readiness is calculated, go to What's my daily readiness score in the Fitbit app?

I hope this information will be helpful. Have a nice day!

[patilu]

Your answer doesn't seem to apply to my question. How is the cardio load value that appears in the information about a specific workout (along with distance, calories, zone minutes, etc), calculated?  I understand the readiness value which has been available in the Fitbit app for some time now. And I understand how readiness relates to the target load range that I should shoot for on a given day to improve my fitness.

[LZeeW]

@patilu   Calculating cardio load isn't arithmetic, like calculating active zone minutes.  It is a more complicated algorithm that uses several factors, like @alexthecat and @DavidFitbit described.

[patilu]

It doesn't appear that I'm going to get an explanation for my original question: How I could be getting a cardio load values of 30s to '40s for the first few workouts that I did (which were on  the treadmill) after the cardio load function was added to my Fitbit app, but only got cardio load of 1 (That's not a typo-it was one)  for the first spinning workout I did, which was a 30 minute intensive HIIT workout with my heart rate in the vigorous zone. My guess is it was a malfunction.  But thank you to all who responded.

[Alohatim23]

I also felt "cheated" when my watch didn't seem to be counting my cardio load points correctly for certain exercises on certain machines. I pedalled hard for about 45 minutes, my heart rate mostly in the "vigorous" range, and I got 3 cardio points!  I started to notice that I always accumulated cardio load points when I used machines where my hands were moving, the treadmill and the elliptical. When I used the stair climber and the stationary bike, I kept my hands on the grips and my hands didn't move very much.  When I started swinging my watch hand as I exercised, I noticed that I began accumulating points!  I use a pixel 3 watch.  I think this is a defect in the functionality of the watch.  When I'm in "stair climber" or "spinning" exercises, why can't the watch ignore my hand movements?

[Alohatim23]

That backs my theory that it is all about the hand/watch movements. Try swinging your watch hand while you spin.  I think you will start accumulating cardio load points.  I completely stopped using the bikes and the stair climber because I could not accumulate any cardio points, even though I was racking up zone minutes. 

[patilu]

Thanks. I appreciate the validation of my experience with spinning workouts. My treadmill workouts continue to earn cardio loads in the 40's but I did another spinning workout just yesterday which earned 53 zone minutes but zero cardio was attributed to the workout. It would be helpful if Fitbit would at least acknowledge that there's a problem. I've only had access to the cardio load feature in the Fitbit app for a couple of weeks but as I understand it it has been appearing in the app for some folks going back to the summer. You and I surely aren't the first people to encounter this problem.

[AD65]

I understand that it might take a few weeks for the data to settle but I am getting zero cardio loads on all my cycling. I rode for 3 hours and burned over 4000 calories in the day, the day before 2 hours and the day after 2 hours of riding and the system is giving me zero for my cardio load for all my rides despite my heart rate data saying otherwise. I am assuming this is a data feed issue because my cycling data comes from peloton but all my heart rate data is available on my fitbit so I dont understand why I get higher cardio scores from walking my dogs than I do from cycling for hours at 30km/hr. 

[Delboy]

I use fitbit charge 6 but don't get cardio load when using stationary gym bike even when I record the ride as eliptical I still get no score.

Why?

[Missifit]

Same!  I got 90 active mins on a 45 min spin workout and got 0 cardio load.  But then somehow in the hours following the workout, I got a cardio load of 110.  I'm not into this new thing.

[Grozzzer]

My cardio score is 47, it dropped from 55 when the new app kicked in, my average cardio load for the year is 116, for today it was 145 but today was a cardio day 

[amackayj]

I'm also getting weird scores.

I use a variety of different measuring devices and apps. Garmin Edge 540, Strava android app and fitbit sense 2

On 21.12.2025 I rode 32 miles for 3 hours, ave HR 142 bmp readiness 81 and got a  score of 496

On 19.12 I rode 30 miles for 6 hours, ave HR 102 and got 219 readiness 90

26.12 very similar to 21.12: 35 miles average HR 129 for 3.5 hours, readiness 68 and 308 cardio load

29.12 I rode 45 miles in 5.5 hours average HR 106 readiness 68 got 259

I've set an increasing fitness target, but I'm really struggling to match those earlier values and consistently under training

I'm hoping that the app is still calibrating, so I'm just going to keep doing what I can and maybe it'll stabilise?

[Rshute95]

I came to this forum for the same reason. I had my first cardio load yesterday and got a score of 1 for a workout where I had 54 cardio zone minutes from a spin class. This morning I did a full body strength workout that got 24 fat burn minutes, 0 cardio minutes, and got a 17 cardio load. This afternoon I did spin again with 30+ cardio minutes and got a score of 3. Clearly the Fitbit platform is using steps/watch movements to calculate the score, despite its many explanations saying it's only based on heart rate, essentially. It's very frustrating because the majority of my cardio workouts are on the bike, so I guess my cardio load will just be inaccurate? Hopefully they fix this.... 

[phatbitzr]

@alexthecatCardio Load is not affected by Readiness Score.  It is an absolute number based on how long and vigorous your activity is throughout the day.  The Target Load would be affected by the Readiness Score.  If you are fatigued w/a low Readiness Score, fitbit will give a low Target Load so you will take it easy for the day.

As far as how Cardio Load is calculated, it is probably flawed, as it seem to take the average heart rate over the exercise period.  It will give a higher cardio load if someone do a zone 2 ride (pretty low intensity) the whole time, compared to a workout that consists of nothing but 1 minute max intensity (>150% vo2max) interspaced with 4 minutes of recovery.  The zone 2 ride would barely doing anything to improve your cardio fitness.

[alexthecat]

@phatbitzr I have been using the Cardio Load feature since September and, based on my experience with it, your assumptions about how it is calculated are incorrect. Perhaps, you are confusing it with Active Zone Minutes? 

It is entirely possible to get a high Cardio Load Target and a lower Readiness Score. That means that you are being prompted to do a longer workout at lower intensity. On days when you have a high Readiness Score and high Cardio Load Target, you will need to push yourself a little harder to reach that Target, because exercising at a lower intensity will not get you there. 

Here is some info about Cardio Load if you'd like to learn more about it. Unless you have a Pixel Watch 3, you are probably new to this feature. I think you will find that it works a bit differently once you have a little more experience with it. 

[phatbitzr]

@alexthecatCardio Load and Target Load are 2 different things.  Cardio Load based on the TRIMP method.  There are many different ways to calculate that number, but they all are based on heart rate, whether it is averaging, dividing it into zones, second by second calculation, and applying different weight to different intensities.  What some do is rely on heart rate reserve, which is based on HRmax and RHR.  RHR could fluctuate, so that might be the only reason an exact exercise could result in different Cardio Load, thats on the assumption Fitbit uses that model, and it would not vary much.

Target Load, on the other hand, is a recommendation of much exercise one should do for the day.  That is probably some proprietary algorithm based on Readiness Score and past activities.  This is where every training app say their algorithm is better than others.

I've had TRIMP score on various other exercise apps, so its not new, but those are only for the duration of the exercise.  With the fitbit watch, it measure throughout the whole day.  We just don't know the algorithm behind it, since anything above RHR should increase the Cardio Load, unless they are only counting HR that reaches zone 1 or 2.

[alexthecat]

@phatbitzr I think it would be helpful for you to check out the link I shared earlier. It might clear up some of the confusion that you seem to have about this feature.