Fitbit stepcount is wrong ??

Introduction

Ever been walking around a park for the 50th time, trying to hit that mystical 10k arbitrary step target. Your legs trembling, from the weight of performative wellness expectations on Instagram. Feeling like you have spent your day, literally orbiting around.

Step count (Pedometer) accuracy

Well, I too have extended my route before, in order to meet self-imposed fitness targets. I must thank my sports watch, that nursed me through the final stages on each and every occasion. Providing that feedback on how close I was.

What if you never actually got there though ?

If the watch lied to you …. Betrayal !!!

What if the walk you uploaded on 12002 steps, was actually 11998 steps !! Shock horror; Or maybe you didn’t obliterate that target after all.

But in all honesty, how do we know if what our trusty device tells us, is totally true.

Calculating GPS accuracy

Yesterday, I ordered myself an “Amazfit Bip”, tracker watch. I wanted it to tell the time, and at the same time, show my step count on longer walks.

I also wanted to know, if those sparse negative reviews, claiming the step count is inaccurate, were actually correct. Like many readers here though, I wasn’t confident about uncovering this.

P.s All research used to find this out is linked below, under “GPS accuracy research”.

So here goes … The process

Step 1: Find out a measured distance with a tape measure, or a study that uses that  method.

Step 2: Compare the measured distance, with what other devices and apps say it is.

Step 3: Calculate the error margin in the device or apps distance.

Step 4: Calculate error in the value stated. e.g If something says 1, then it is less precise, that if it says 1.11112

Step 5: Convert distance units to steps/ convert error distance to error steps.

Step 6: Calculate total error for step count/ list the minimum number of steps your device needs to say, to guarantee you hit your step target accurately.

So here goes …. Example 1 (Google maps)

Step 1: Study states known “measured with tape measure” distance, of 1000 feet.

Step 2: Google maps app states this distance is 1006/1007 feet.

Step 3: 1007-1000 = 7 feet error.

(Calculating this error as a percentage of the correct value) = (7/1000) times 100 = 0.7% error or 0.007 as a decimal.

Step 4: Google states distances to 1 decimal place, e.g 4.1 miles.

This means: (error margin can be more or less by 0.05 miles).

This error is always the same, no matter what distance you record up to.

Step 5: 1 mile on average for an adult is 2000 steps, so

0.05 miles = (0.05 times 2000) = 100 steps error.

Step 6: Minimum number of steps will vary depending on distance.

Let’s take the 10,000 step example …

Error from Step 5 must be 100 steps, as it is always the same.

Measuring error from Step 3, must be (0.7% or 0.007) times total number of steps, in this example: 10,000 steps.

So total error =

(0.007 times 10000) + (100) = +- 170 steps

Conclusion

You would need 10000+170 steps to make sure you hit the 10,000 target, assuming this was calculated from distance travelled on Google maps.

E.g. If you did a 5 mile route on Google maps at 2000 steps per mile, (average for an adult), you would actually need  (170/10,000) =  0.017 times 5 miles = 0.085 miles extra, to make sure you hit 10,000 steps, or 5 miles. This equates to 5.085 miles that need to be walked.

Google maps would only state 5.1 miles though, which could be as low as a 5.05 mile route. This means you would need to calculate a 5.2 mile route on maps, to guarantee this error margin is covered.

The lowest number you could have walked after following the 5.2 mile route, would be 5.15 miles. This is well above the 5.085 mile cutoff for guaranteeing 10,000 steps.

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So enough examples …

I think you get the idea of the process from the above example, so using an example of me walking to my local shop, with a recorded stepcount from my current GPS watch of the Garmin Forerunner 645, I then calculated how many steps I should have done using Google, and the error above.

1. My Garmin Forerunner 645 GPS watch stated: 2374 steps
2. Using Google maps as above, Google stated the route as 1.2 miles

1.2 miles times 2000 steps per mile = 2400 total steps

3. So total error in google = (0.007 times 2400) + (100) = +- 116.8 steps

4. Google states my step count was between 2400-116.8 and 2400+116.8 so

2283.2- 2516.8

5. This means my Forerunner 645 was at least as accurate as Google maps, sitting well within this range at 2374.

Summary

Apologies for all the numbers.

Hopefully this gives you some idea of the accuracy of step counts from Google maps, and the Garmin Forerunner 645 GPS watch.

More importantly, it provides the process to calculate accuracy for any device or app, including Fitbits, and other trackers. I think this clearly shows, no matter what technology is used, there will always be an error margin.

Don’t fret about accuracy when buying any device, as this can be calculated and adjusted for afterwards. If you need to do an extra 3000 steps to guarantee 100,000 steps, it isn’t that massive in the grand scheme of things.

I’ll use my error formula again, to update you on how my new watch, and tracking apps compare in accuracy, next week. Any issues with any of my calculations, please let me know in the comments below. Thanks.

Products I have/ have bought

My new watch ( Amazfit Bip) https://www.amazon.co.uk/Amazfit-smartwatch-Activity-Monitoring-Bluetooth/dp/B08B52LW3R

My current GPS cycling and walking watch ( FR 645) https://www.amazon.co.uk/gp/offer-listing/B078J6795M/ref=dp_olp_0?ie=UTF8&condition=all&qid=1592560535&sr=1-5

GPS accuracy research

4 ( 2000 steps per average mile ) https://www.healthline.com/health/stride-length#calculate-step-and-stride-length

Fitness challenges / products

My 50,000 steps in a day challenge using FR 645: https://reasea.org/2020/04/10/50000-steps-or-25-miles-walked-in-one-single-day/

My brand new healthy recipes ebook with cuisine from the healthiest nations on earth ( First 3 recipes for free just click look inside): https://www.amazon.com/Live-Longer-Ingredients-long-life-ebook/dp/B087TNJQHK/ref=sr_1_3?dchild=1&keywords=Live+longer&qid=1588325227&s=digital-text&sr=1-3

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