Why Your Fitness Watch Doesn't Match The Treadmill

You jump on a treadmill and settle into your run.

Ten minutes later, you glance at the treadmill display and see you're running at 10km/h. Then you look at your watch, and it says 4:48/km.

A few minutes later, the numbers still don't match.

So which one is wrong?

It's a question we get asked regularly at The Training Ground, and it's become even more common as wearable technology continues to improve. Whether you're using a Garmin, Apple Watch, Coros, Polar, Fitbit, or another fitness tracker, you've probably noticed that the numbers don't always align with what the treadmill is displaying.

The answer is surprisingly simple:

Neither device is necessarily wrong.

They're just measuring different things.

The Treadmill Measures The Machine

When you step onto a treadmill, the machine calculates pace based on the speed of the belt moving beneath you.

If the treadmill is set to 12 km/h, it assumes the belt is travelling at exactly 12 km/h and calculates your pace accordingly.

The treadmill doesn't know:

• How long your stride is

• How efficiently you're running

• How much your arms are moving

• Whether you're holding the handrails

• How hard the effort feels

It simply measures the machine.

This is one of the reasons treadmills can be such useful training tools. Assuming they are properly maintained and calibrated, they provide a consistent environment where speed and pace can be controlled very precisely.

That said, treadmills aren't perfect either.

Like any piece of equipment, they experience wear and tear over time. Belt tension, motor performance, servicing schedules, and calibration can all influence how closely the displayed speed matches the actual speed of the belt.

Your Fitness Watch Measures You

Your wearable takes a completely different approach.

When running outdoors, most fitness watches use GPS to estimate speed and distance.

Indoors, GPS is usually unavailable, so the watch relies on a combination of:

• Accelerometers

• Cadence data

• Arm movement

• Stride length estimates

• Historical running data

Instead of measuring the treadmill, your watch is trying to estimate how far your body is moving.

This distinction is important.

The treadmill measures the machine.

Your watch measures your movement.

Both are producing pace data, but they're collecting that information in completely different ways.

Why The Numbers Don't Match

There are several reasons why your watch and the treadmill can report different paces.

1. Your Running Mechanics Change Indoors

Most runners don't move exactly the same way on a treadmill as they do outdoors.

Without even realising it, people often:

• Shorten their stride

• Increase cadence

• Change their posture

• Alter their foot strike pattern

Your watch uses movement patterns to estimate pace, so any change in running mechanics can affect the result.

If your watch has learned your running style from hundreds of outdoor kilometers, it may not perfectly interpret your treadmill running mechanics.

2. Arm Movement Influences Watch Accuracy

   
   

Your watch sits on your wrist, which means arm movement plays an important role in how it estimates pace and distance.

Research has shown that altering arm movement can influence the accuracy of wrist-based activity tracking. This is why wearable devices often struggle when people:

• Hold treadmill handrails

• Push prams

• Carry shopping bags

• Use walking aids

The less natural the arm movement, the harder it becomes for the device to accurately estimate movement.

3. The Treadmill May Not Be Perfectly Calibrated

Most commercial treadmills are reasonably accurate, but even small calibration differences can create noticeable discrepancies.

A treadmill displaying 12 km/h may not be moving at exactly 12 km/h.

Over a short interval, the difference may be insignificant. Over a longer run, however, those small discrepancies can add up and create a meaningful difference between the treadmill's reported distance and your watch's estimated distance.

What Does The Research Say?

The good news is that modern wearables are actually quite impressive.

A 2020 study examining several popular wearable devices found step-counting accuracy during treadmill walking and jogging was generally very good, with many devices producing errors of only 1–3% under controlled conditions.

The challenge isn't counting steps.

The challenge is estimating speed and distance from those steps.

To calculate pace indoors, your watch must make assumptions about stride length, movement efficiency, and running mechanics. As those variables change, so can the accuracy of the estimate.

Even GPS-based measurements aren't perfect. Research investigating running watches has demonstrated that GPS devices can produce slightly different distances and paces on repeated runs of the exact same route.

In other words, every device has limitations.

The goal isn't to find a device that's perfect.

The goal is to understand what each device is actually measuring.

So Which One Should You Trust?

The answer may surprise you.

In most cases, you shouldn't become overly concerned with which number is "correct."

Instead, focus on consistency.

If you're completing all of your treadmill sessions on the same treadmill, using the same setup, under similar conditions, that data becomes extremely useful for tracking progress.

Likewise, if you're consistently using the same wearable device, trends become far more important than individual readings.

Ask yourself:

• Is my pace improving?

• Is my heart rate lower at the same speed?

• Does the session feel easier than it did six weeks ago?

• Am I recovering better between efforts?

Those questions matter far more than whether your watch says 4:52/km while the treadmill says 12.3 km/h.

The Bigger Lesson

One of the biggest mistakes people make with fitness technology is assuming every number needs to be perfectly accurate.

It doesn't.

Training isn't about collecting perfect data.

It's about creating meaningful progress.

Technology should help guide your decisions, not become the focus of your training.

The athlete who consistently trains for six months will almost always outperform the athlete who spends six months obsessing over which device is correct.

At The Training Ground, we encourage our members to use technology for what it does best:

identifying trends, measuring effort, and helping create accountability.

Use the treadmill speed.

Monitor your heart rate.

Pay attention to your perceived effort.

Track your progress over time.

Do those things consistently, and you'll get the information that actually matters.

Not whether your watch and treadmill agree, but whether you're becoming fitter, stronger, and more capable than you were yesterday.

References

• Montes J, et al. (2020). Step Count Reliability and Validity of Five Wearable Technology Devices While Walking and Jogging in Both a Free Motion Setting and on a Treadmill.

• Fuller D, et al. (2020). Reliability and Validity of Commercially Available Wearable Devices for Measuring Steps, Energy Expenditure, and Heart Rate: Systematic Review.

• Welch WA, et al. (2013). Classification Accuracy of the Wrist-Worn GENEA Accelerometer.

• Connolly G, et al. (2022). Comparison of Wrist-Worn and Hip-Worn Activity Monitors During Treadmill Exercise and Daily Living.