How we test fitness trackers

In-depth Which? tests put fitness trackers through a wide range of real-world scenarios to find out exactly how capable they are.

We only recommend fitness trackers or activity trackers that are comfortable to wear, easy to use and accurate at tracking your fitness day in day out. 

Fitness trackers from all the big brands, including Fitbit, Garmin, Huawei and Polar go through a raft of tests in and out of our lab. The best trackers provide in-depth and accurate data on the device or an easy-to-use app, as well as helpful tools for motivation. They'll be comfortable to wear, and have a range of useful features. The worst we've found will overstate or understate steps and distance, struggle to measure your heart rate and lack important features.

Each fitness tracker is tested in different scenarios to find out how they perform – from day-to-day activities to exercise. We put them through several rounds of testing too, so we can be sure our fitness tracker reviews can reliably answer your key questions, including:

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Video: what makes a best buy fitness tracker

How accurately does a fitness tracker capture steps?

'Walking is simple, free and one of the easiest ways to get more active, lose weight and become healthier', says the NHS. 

Whether you're specifically going for a walk, trying to incorporate more walking into your daily routines (walking to work in the morning, for example), or milling about the house, a fitness tracker could help you track how many steps you clock up across the day. 10,000 steps is a popular daily target, but most fitness trackers will allow you to set a personalised goal. 

But fitness trackers can often miscalculate steps, wrongly registering any movement of your hand as a step or failing to register genuine steps. 

Our testers wear each fitness tracker on a 10-minute run and a 10-minute walk, and while carrying out a range of household activities, such as:

• sitting reading a magazine
• loading and unloading a dishwasher
• sweeping the floor
• going up and down stairs
• carrying shopping bags

We compare the data from the fitness tracker to that from a trusted reference step-counter. 

Consistency is just as important as accuracy. If a tracker is accurate on some days but not others, you won’t know whether you’re improving. So we repeat our walking test to see if the trackers consistently overstate, understate or hit the accuracy mark.  

How well does it track sports, exercise and heart rate?

Steps are important, but modern fitness trackers are capable of far more advanced tracking. 

If you're a runner looking to track the length of your run, we've got you covered. Whether the fitness tracker is relying on GPS to track distance, or relying on steps, we check if it can tell us correctly how far you've travelled. We take each fitness tracker on routes of known distances (both walking and running), including open sky areas and woodland areas to add the challenge of potential loss of GPS signal. 

More and more fitness trackers now measure heart rate. We test how accurately a fitness tracker measures heart rate during high-intensity running and while resting. If the fitness tracker gives no heart-rate data, or abnormal heart-rate data, we take a second reading. We use chest belts to compare the results – these use an electrical measurement method known to be more reliable, so we know we can trust their data. 

Can a fitness tracker help me exercise more and stay motivated?

The best fitness trackers are easy to use and offer motivational tools to help keep you engaged, such as customisable goals, online competitions against friends, ‘badges’ that you can earn, and daily and weekly or monthly workout summaries. Some are clunky and hard to navigate, don't offer much in the way of motivation or send you so many reminders to get moving that you feel nagged and hassled. 

Our testers spend weeks getting stuck into using each tracker and accompanying phone app. Our reviews can tell you how easy it is to navigate and customise, and its ability to keep you interested. 

Is it comfortable and will I want to wear it?

A fitness tracker could be super-accurate, but if it rubs your skin, digs into your arm or is just plain heavy and ugly, you’re not going to wear it.

You can’t tell from looking at an activity tracker in the box or in a photo what it will be like to wear. So our male and female testers rate each tracker for how comfy it is to wear for sports and daily life. We'll tell you in the reviews if it's likely to be too big for a small wrist. 

Blood oxygen (aka pulse oximetry or SpO2) monitoring

A growing number of wearables can report on your blood oxygen saturation: the amount of oxygen being carried around your system by your blood cells, expressed as a percentage. 

Some fitness trackers and smartwatches can take on-demand measurements. Some can do this overnight, when you're asleep. And some can do both. 

If a wearable is able to take an on-demand blood oxygen measurement, we test it out and compare its readings to a medical pulse oximeter which we use as a reference device, so we can tell you how accurate it is. 

We don't factor this into our test score, as it's not a feature that most people will need. However, we can tell you whether it falls within the threshold of accuracy tolerated by the FDA (US Food and Drug Administration). We use this because there's no UK or EU standard for pulse oximetry features on wearable devices at the moment, as there is for medical pulse oximeters. 

Note that, even if a wearable can take accurate pulse oximetry measurements, our testers are healthy people, so we can't guarantee a device will pick up signs of a medical problem. And these trackers are not intended to be medical devices; manufacturers say they're designed for 'recreational' use only. In other words, they're more like toys for people who are generally healthy and are interested in learning more about how their bodies work. Get medical help if you become concerned about your health. 

If you're an endurance athletes or alpine enthusiast, a pulse oximetry-monitoring wearable might help you spot signs of over-exertion. Or, should you notice any unusual measurements in your overnight pulse oximetry measurements, it could be a sign of sleep apnoea, a sleep disorder in which your breathing stops and starts. 

As we said before, please don't take this as a diagnosis: discuss it with your doctor. 

Are fitness trackers secure?

We put every fitness tracker and smartwatch we review through a privacy and security test to check for vulnerabilities that could be exploited by cybercriminals. 

You might wonder why anyone would be interested in hacking your fitness tracker. But wearable devices can gather plenty of sensitive personal data about you beyond your daily step count and how many calories you’ve burnt during your workout: data such as your usual bedtime, when you’re normally in and out of the house, where you go during the day, perhaps even your ovulation cycle. 

All this data adds up to a detailed profile of you. If it isn’t protected, it could be used against you, whether by an individual with a grudge or an organisation trying to harvest data to sell on.

In 2017, we reported on an investigation by the Norwegian Consumer Council (NCC) that found children’s smartwatches vulnerable to hackers.

A fitness tracker that isn’t secure could also act as an entry point to your home network, putting other devices such as laptops and smartphones at risk of being compromised. 

When it comes to smart products, including wearables, there’s no such thing as 100% security. However, we do check for the most common vulnerabilities and flag any significant issues with manufacturers so they can be rectified. The wearables we recommend had nothing that worried us at time of testing. 

If a wearable can be used by both Android and iPhone users (as most can), we test both the Android and iOS versions of the app. 

Data encryption

Our test looks at whether your personal data is encrypted - scrambled so that it can only be read by someone with a decryption key. We also check where your data is being sent, particularly if that’s outside the EEA (European Economic Area). 

Firmware

Firmware is the software programmed into the hardware of your fitness tracker. If firmware is out-of-date, it can leave holes through which a hacker could access your fitness tracker. Has the manufacturer stated how long they will provide updates for?

Data transparency (permissions)

Can you opt out of individual data permissions? If you do, what are the consequences? We check what data the app is asking for, and the reasons behind it, to make sure the data being collected is strictly necessary. 

Password policy

We check for weak default passwords, and whether fitness trackers force you to set unique, strong passwords. We also look to see whether there’s any option to access your account using biometric methods, such as Face ID or a fingerprint. Find out more about setting a secure password.

Wi-fi

If the fitness tracker has wi-fi connectivity, we check to see if it can join a wi-fi network that isn’t secure.

Man-in-the-middle attacks

We check if fitness trackers are vulnerable to a man-in-the-middle attack. This is where a third party eavesdrops on data being passed between your fitness tracker and the cloud.

Decommissioning

At some point you’ll probably want to replace your fitness tracker. If you’re donating it, recycling or selling it on, you need to be sure none of your data is still attached. 

New forms of cyberthreat

The threats posed by hackers are constantly evolving. We’ll update our testing as new threats appear. 

We’ve tested most of the wearables you’re likely to see on sale from popular brands. If you buy a wearable we haven’t tested, stick with a well-known brand. A famous name is no guarantee of good quality – as our testing has shown time and again across all the product areas we test. But a brand with a good name has more to lose if it doesn’t take privacy and security seriously.

Be wary of ultra-cheap fitness trackers and smartwatches and do your research before buying from a brand you've never heard of, even if the device in question looks temptingly like it's from Apple or another famous brand. Our snapshot test of ultra-cheap Apple, Fitbit and Samsung lookalikes bought from online marketplaces found all had worrying security flaws, with some collecting shocking levels of personal data on users.

Should I buy it?

Each of our tests makes an impact on our overall test score – the overall percentage score we award to each fitness tracker on test.

Some factors are more important than others, so we weight each result differently to ensure the fitness tracker features you’re likely to use most are the ones that contribute the highest proportions to the total test score. 

The scoring breakdown is slightly different for fitness trackers and smartwatches, reflecting the fact that, if staying connected was your primary concern, you'd opt for a smartwatch rather than a fitness tracker. 

A fitness tracker needs to score at least 72% in our tests to become a Best Buy. Smartwatches need 78% to be a Best Buy. Fitness trackers and smartwatches that score 45% or less are Don’t Buys.

Fitness trackers score breakdown

• 45% fitness
• 20% ease of use
• 10% app
• 10% battery
• 5% smart functionality
• 5% build quality
• 5% features

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