Can you trust the running stats on your fitness tracker or smartwatch? – Which? News – Which?

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If you're a regular runner – or have been inspired by the London marathon to get serious about training – you'll want to make sure that the wearable on your wrist is giving you accurate information. But we've tested a number that seriously disappointed us with their inaccurate metrics. 
Having an accurate smartwatch or fitness tracker is vital to pacing yourself properly, be it for marathon prep or just tracking progress and goals over time. 
Yet we've uncovered wearables that overestimate or underestimate how far you've run by 40% or more in our tests. 
If you're using one of these, you might think you've covered the full 26.2 marathon miles in your training when you haven't – or else worry that you can't match the marathon length when you've actually exceeded it. 
We explain what to look for to ensure you've got the stats you need to succeed.
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Some wearables are extremely accurate at tracking how far you've run, while others are shockingly poor. The worst we've found was out by nearly 47%. 
Opting for onboard GPS, which many running watches have, is a strong start to picking an accurate tracker. 
Wearables that don't have standalone GPS rely on your phone's GPS to track your running route. But that means you'll need your smartphone along for the ride if you want to track your stats. 
Your other option is to do without GPS altogether and allow your device to base your distance measurements on steps. But we've found that steps-based distance trackers tend to be less accurate than those with GPS. 
Wearables often struggle to log steps correctly, missing steps or mistaking any swing of your arm for a step. 
Even if they're accurate at counting steps themselves, things can go wrong when they attempt to translate this into a steps-based distance measurement.
Built-in GPS isn't a guarantee of accurate distance tracking, though. Some devices with on-board GPS that we've tested miscalculated distance by nearly 17%. 
Not sure which smartwatch features you need? Get straight to our top picks and get choosing advice in our guide to the best smartwatches for 2023
Based on our testing, you're more likely to get into strife with a cheaper watch. Most of the wearables we've tested recently that have been poor at distance tracking have been from cheaper brands. 
It's not all bad news, though – a couple of cheaper smartwatches for around the £70 mark proved accurate at tracking distance, but you'll need to pick your wearable wisely.
Big brands don't always get it right either. We were surprised to find two models from a normally impressive brand that misjudged running distance by more than 16%. 
If you're scouting for the very cheapest smartwatches or fitness trackers (the kind costing less than £20 or so online), there are other risks to consider, too. We security-tested 12 of the cheapest wearables in 2022 and all had concerning security flaws.
If you're looking for a low-cost device, invest well with our guides to the best cheap smartwatches and best fitness trackers.
Fitness tracker vs smartwatch – find out which is right for you
In addition to built-in GPS, there are some other useful features to consider when choosing the best smartwatch for running training.
These include dedicated running metrics, covering things such as your pace, elevation and estimated recovery time, plus heart-rate monitoring. 
While all smartwatches tend to offer basic heart-rate monitoring, some have advanced insights such as what 'heart rate zone' you're in, to help you fine-tune performance.
Other useful additions include competitive running to give you extra motivation, mapping and navigation tools to help you focus on the job at hand without getting lost, and energy tracking, which can estimate your energy levels to help you pitch your workout at the right pace.
Onboard music storage is a nice-to-have for extra motivation (and allows you to leave the smartphone at home).
We've rounded up some advanced GPS running watches at different prices to kickstart your search. Click on the links to get our verdict on how accurate they are at measuring distance and heart rate, and how good they are overall vs rivals (full reviews available to Which? members only):
We go the extra mile when it comes to testing smartwatches and fitness trackers. In addition to distance accuracy, we test heart-rate monitoring, step-counting accuracy, ease of use, screen quality, battery life and security. Find out more about how we test smartwatches.
*Prices correct as of 19 April 2023
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