Polar M400 review

This sports watch with activity-tracking smarts gets the Wareable treatment
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Polar M400
By Polar
Bang for your buck, the Polar M400 is a fantastic option for anyone looking for a wearable that offers a complete picture of everyday activity and training. If you’re a serious cyclist there are better products out there and some runners might like to see more running form data but for anyone who’s about to take on their first marathon or just embarking on the couch-to-5km, then this is as good as any device you’ll find right now.

  • Full 360 tracking
  • USB charging
  • Decent battery life
  • No built in HR monitor
  • Activity tracking is basic
  • Lack of accessory support

The wallet-friendly younger sibling to the Polar V800, the M400 is one of the first fitness devices to bridge the gap between the fitness band and the GPS sports watch. For £169/$179, it offers full activity tracking from your wrist, whether you're walking up the office stairs or speeding through your local park on a 5km training run.

Polar M400: Design and build

While it does have some multisport credentials (it will track the GPS related stats for cycling and even horse riding) the M400 is built to be a running watch with added intelligence.

Let's face it, being the younger relation to a multi-talented piece of tech like the V800 was always going to be difficult but the M400 stands up remarkably well.

Guide: Using Polar Flow

Unlike the V800 it won't pair up with quite as many accessories and cycling cadence sensors are not an option, but on the plus side, the design is actually better than its rather pricier stablemate.

For starters the strap is more comfortable on the wrist. Added to that the watch face is a little smaller and more ergonomic.

Then there's the simple USB charging that does away with the need for a unique charging cable or dock; a bone of contention with many other GPS watches, including the V800. Despite this, it still manages to be fully water-resistant in water up to 30m, taking the activity tracking element off of dry land.

There's also a high-contrast display that makes it easier to see your stats even in direct sunlight.

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It's a real shame that it doesn't have the built-in heart rate monitoring that some running watches like the Adidas miCoach Smart Run or the TomTom Cardio Runner now offer, but it can be paired with a chest strap for added accuracy andthe Polar staple heart rate zone training.

Weighing in at 56.6g and measuring 11.5mm thick, it's also not the bulkiest of the running watches, which adds to the overall comfort.

Polar M400: Features

Let's start with what the M400 tracks. There is, of course, the basics including distance, pacing, altitude and calorie burn. But where the M400 cuts a new path is in its ability to deliver data for the times when you're not running.

The internal accelerometer of the unit will track steps, activity and sleep. It'll also give you a nudge if you've been stationery for a long period just like you'd get from a Jawbone UP24 or a Fitbit Flex.

The Activity Guide feature is basically like having a Polar Loop built into your running watch.


While the activity and sleep tracking don't offer the depth of some of these devices, having all your data pushed to a single platform is extremely appealing to the point that you're happy to forgo some of the deeper data.

In terms of running, one of the features we found most useful was the Back to the Start feature that guides you home via the most direct route using a GPS marker stored at the start of each run. It's brilliant for running in a foreign city, or for people who just tend to get lost easily.

There are also a range of smart coaching features just like those we've seen on the V800 and competitors like the Garmin Forerunner 220 or 620.

Running guide: Using your running watch for interval training

Running Estimator calculates how long it'll take you to cover a distance at the pace you're currently clocking, while PB support gives you feedback on any records you've broken during a run.

There's also handy post-run feedback on the overall training benefit from the work you've just done. This helps you determine whether you've been burning fat, improving cardio and what effect this has had on your body. Running Index gives you a window into how your running is improving based on heart rate and speed. This is great if you're training towards a specific goal.

The Polar smart calorie counter is far more accurate than you would get with a regular fitness band as it takes into account your height, weight, age, gender and activity levels to form a much more accurate estimate off the calories you are using during exercise.

The Polar fitness test has been developed from years of trial and error, and is now so accurate it rivals any lab-based sub-maximal VO2 test. Using heart rate data and fluctuations at rest it will provide you with a VO2 max figure, that you can use to track your performance in just 5 minutes of testing.

If you're not entirely sure what this means, check out our GPS running watch jargon buster.

Notifications have recently been added to the M400 mix. The bad news is it's iOS only for now, but the good news is that all smartphone notifications will appear in your iPhone's Notification Center; that's WhatsApp, Facebook, Instagram, Twitter, the whole shebang.

You can choose whether notifications make a sound or just appear on screen, and there's a handy 'do not disturb' function as well.

Polar M400: Syncing, apps and web tools


Thanks to Bluetooth Smart technology, all the data from your M400 can be easily, and instantly, synced to a connected smartphone. Your vitals end up in the Polar Flow app and web tool, which both offer a competitive range of options to enhance your training. From customisable training plans to elite athlete level data, Polar Flow is one of the best tracking tools we've come across.

Running guide: How to stay injury free with wearables

It might not have the gloss and social features that Nike+ Running has, or the range of off-the-shelf training plans you get with Adidas miCoach, but it's a great tool for runners.

Polar M400: Battery life

A full charge of the M400 will get you around eight hours of GPS tracking. On that front it's competitive with most of the other running watches in this category.

When it comes to the day-to-day tracking or general watch mode you're looking at closer to 21 days. That's a lot more than most fitness bands.

How we test

Kieran Alger


Kieran is a world record-setting runner and one of the UK's most experienced running journalists.

A constant tester of the latest fitness technology, he's always hunting for innovations that can make him run faster, further and generally be in better shape.

Kieran is often found wearing four GPS running watches at once. And to date he's tracked more than 50 marathons, 13 ultras and countless half marathons - including the Marathon Des Sables.

In 2022, he became the first person to run the river Danube from sea to source, a measly 1,830 miles in 66 days. And still had time to test running gear.

Kieran regularly takes running tech to the extremes for Wareable and the likes of Runner's World, Mens Health and Wired.

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