Garmin Forerunner 645 (No music) – Review

My Forerunner 645 History

I purchased the Forerunner 645 in March 2018 and since then have ran several hundred times with it as my primary running watch, covering some several thousand kilometres.

These have ranged from training runs and commutes, through races ranging from 5km Park Runs and 100km+ ultra marathons. In other words, I’ve put it through most scenarios that any runner is likely to want to use it for.

(*Note – the Forerunner 645 is waterproof up to 5 metres ATM and comes with a number of features for swimming, including automatic lap recognition. It also has cycling features built in. While I have swam with it on a number of occasions (without issue other than the occasional odd number of laps being counted), this review is purely from a running perspective)

Waterproof to 5ATM and auto-lap recognition

Design overview

The Forerunner 645 doesn’t break the mould when it comes to design. It looks, for the most part, like most Forerunners that have come before it as well as subsequent releases.

The screen has a 1.2″ colour display and packs 240×240 pixels. Not really pushing the envelope on resolution then, but given the size the low pixel density isn’t really noticeable. Made with Gorilla Glass, the screen is tough and has in my over a year of wearing it, yet to obtain a single scratch. Easily visible even in the sun and with a back-light for when it gets dark, I’ve never had any issue with being able to view what I’m looking for. When the sun is bright the screen can show some reflections, but nothing that impairs its visibility.

Some reflection in bright light but perfectly readable

Around the screen is a stainless steel bezel. It’s robust and gives a nice feel to the watch, more so than some of the plastic models. The bezel has picked up a few scratches and so on over the previous year, but given the amount I put it through and the number of times I’ve scraped it, that’s only to be expected.

Weighing in at 42.2g, it’s weighs virtually the same as its plastic predecessors (such as the Forerunner 235 at 42g) and considerably less than some of Garmin’s biggest hitters (the Fenix 5S Plus manages to reach 65g). I’ve personally never had any issue with its weight and usually forget I’m even wearing it.

There’s no touch screen so operation of all functions is through the five buttons surrounding the bezel. They work perfectly well and even after a year I’ve never found any of them to prove sticky or difficult. Three buttons down the left hand side are usually used for scrolling through menus, with two buttons on the right hand side are used to select or go back one screen.

Small buttons down the side are used to operate the watch

Finally, there’s the strap. Made of silicon, the strap is soft and comfortable. It can attract sweat underneath it when running (what doesn’t) but is easily washed or wiped clean.

What I have found is that the little loops the band tucks through to keep the watch in place have a habit of breaking away over the course of a few months. Garmin sell replacement straps for the absurd price of £45, but alternatives are available on Amazon from third parties for £15+. I’ve been through two straps in the last year, one Official and my current third party one, both of which have done the same thing. Maybe it’s the material they’re made from or just the way I wear them (although how many ways are there…?)

My ability to break watch straps


The Forerunner 645 comes with a number of features and pre-set activities, for running, cycling and swimming, as well as miscellaneous sports.

Activity options

Running is obviously the focus of this review. A click of the select button bricks up the activity menu, with “Run” right at the top (the list can be edited and rearranged as you prefer). Clicking select again takes to you to activity screen and the watch begins to search for a GPS lock. Once found, a green circle appears around the watch face and you’re good to go.

From then on the select button acts as both start and pause, with the directional buttons on the left hand side scrolling through the various screens of data.

Those screens are fully customisable, so I have a number of screens showing different data for different running scenarios:

Races up to Marathon – Distance, time and average pace
Interval training – Heart rate, lap pace, previous lap pace and distance
Recovery runs – A single screen showing my heart rate, with zones
Ultra marathons – A route marker showing my general position along the course, with average pace and distance remaining.

That kind of customisation is essential. The ability to show distance remaining for ultra marathons rather than distance travelled is a good example. Over the course of 100km discrepancies inevitably occur and looking at the distance you have travelled, you may think you should have reached the next checkpoint by now, which can be a big mental drain.

Showing distance remaining is therefore a much more accurate representation of your position, as it will compare you’re existing position on the pre-loaded route (you’ll need to upload a GPX file for the route in advance, available from most ultra marathon race organisers) and calculate how much of that route is actually left.

Recovery estimates – of debatable use

After your run, the Garmin will use a series of algorithms, taking into account running from the previous days, general pace and distance etc, to give you an estimate of how long you need to recover. This doesn’t mean it is suggesting that you shouldn’t run at all during that time, just that this is how long it estimates until you’ll be back at fully recovered. I’m sceptical of the estimate if I’m honest. I find listening to your body and running according to feel tends to be a much better strategy.

VO2 Max estimates

The watch will also calculate your V02 Max. V02 Max is essentially a measurement of the maximum amount of oxygen your blood can absorb during high intensity exercise. Without delving into too much detail (there are pages upon pages dedicated to this on the Internet), the higher the better and numbers are relative to gender and age.

While the most accurate V02 measurements are reached through a visit to a dedicated lab (expensive!), Garmins calculate the figure through an equation involving pace and heart rate over a number of runs. Not 100% accurate, but a useful and affordable indication.

Race time predictor – of questionable accuracy

Garmin will also analyse recent runs, going through your distances and paces etc in order to predict your race times for a number of distances. This is also one of the features I find to be of limited use.

The figures are of course approximates only, but the above predict finish times for me at 5k, 10k and half marathon distances substantially below my PBs. That doesn’t necessarily make them wrong. They are estimates of your times if you were to run those races now, rather than what you are capable of overall. So if you’ve done a number of slower training runs recently for example, or have backed off for other reasons, then the lower estimate may be fair.

However, I just don’t find them helpful and never use them. They don’t tell me anything I don’t already know and of course don’t take into account the particulars of the course that the race will be held on. The time taken to run a hilly marathon will likely be substantially different than a flat course, but the race predictor has no way of knowing which you’ll be running and so cannot adjust accordingly.

Step counts

In-built into the 645 is a pedometer, counting steps as you walk. You can set a target to try and keep yourself on track and set alerts that let you know if you’re staying still for too long during the day. This can be turned off (it uses more battery) but otherwise happens in the background by default and doesn’t only activate when you turn on an activity, such as running for example.

Heart rate monitor

The 645 includes a built-in heart rate monitor. Wrist based heart rate monitors are broadly accepted as being less accurate than a chest-strap monitor (which Garmin also sell and which can send its data directly to the watch if you would prefer to use that), but I’ve found them to be perfectly accurate for general usage.

Heart rate can be viewed in real time during activities and once an activity is done, the heart rate data can be overlayed in the app over your elevation gains, pace etc to see which elements of a run have caused any changes.

For me, the heart rate is essential as when I’m doing a recovery run I will run by heart rate zone alone, ignoring pace and time and simply trying to keep myself within a specified zone to focus on that element of my fitness, for example increasing aerobic endurance.

Battery Life

If you’re running ultras, get used to doing this on the run

Ah, battery life. How important this is to you will depend on what you demand from your watch. If you’re running primarily consists of short distances, you’ll be able to get through those with no problem at all with multiple days between charges.

If you’re a marathon runner then again, you’ll get around the course no problem, but you’ll want to be charging it up after. It’s when you push into ultra marathon territory that the problems start.

Garmin’s official specifications for the Forerunner 645 say the following:

Smartwatch Mode: Up to 7 Days
GPS Mode: Up to 14 Hours

The words “up to” are important. Technically, 5 hours is “up to” 14 and, while the battery performance has never been as dire as that, it’s fallen far short of the 14 hours suggested.

To put this into a real-race context, I wore the Forerunner 645 for the Centurion North Downs Way 100 Mile Ultra. I’d placed my portable charger in my drop bag for the 50 mile checkpoint, figuring I would have plenty of time to get there, collect the charger and give it a boost before setting out for the second leg. Reaching Henley in 9:32:50, the battery was at 3%. I barely had time to get the charger out and get it plugged in before I risked loosing my progress.

85km and just over 9:30 hours through – with approximately 3% battery remaining

Nor was this with all features activated. To preserve battery I had gone into settings before the race and turned off everything except the GPS. That meant no heart rate monitor, no step tracking, no other measurements. Just pure GPS. When you’re pushing through an ultra, the last thing you need is the extra urgency of worrying about a battery.

To make matters worse, Garmin still insist on using a proprietary charging cable, which clamps on to the watch like a pair of crocodile jaws. This makes the cable more bulky to carry around and more expensive if you need to source a replacement.

I wish companies would drop proprietary cables

It’s ultimately the battery issue which has pushed me into buying the new Garmin Forerunner 945, which is advertised as having up to 30 hours GPS. I’ll post a review of that once it’s arrived and I’ve had the chance to put it through its paces.


GPS on the Forerunner 645 is fairly strong. Options wise it has standard GPS, GPS + GLONASS (the Russian GPS system, which provides additional satellite coverage, in theory providing a better signal) and “UltraTrac mode”.

The latter is in my view best ignored. It’s designed for battery saving over long distances and severely reduces the number of times the watch pings the satellites. The reduced frequency of contacting the satellites gives an equivalent saving in battery usage. Unfortunately it also decimates GPS accuracy, adding or losing significant distances over relatively short periods. Over a 5km run, I’ve seen it cause inaccuracies of up to 500 metres. Extrapolate that to a 100km ultra, that’s 10km.

Be prepared to stare at this little red band for a long time while waiting for GPS signal

Instead, I opt for the GPS+ GLONASS combination. There are two parts to this. The first is acquiring a GPS lock. The second is accuracy once you’ve found that signal.

On the former, the 645 is a real disappointment. I can stand outside in completely clear surroundings, with no buildings or tree coverage and the 645 can take up to three minutes to “go green” and find a complete signal. That is fairly rubbish and not the way you want to start a run.

Worse, I used to have an old Garmin 235, which is several years old and only costs £199 new. I gave that old 235 to my fiancee who wears it when we go running together. It finds a signal almost immediately despite us standing right next to each other and the 645 struggling on for several minutes. Not really what you want when the 645 costs a good £150 more.

Once you have that GPS connection however, I’ve found it to be reliable. I’ve never lost a signal having acquired it, even when running under heavy tree cover. I’m not one of those who will trawl through my GPS data, matching routes to try and find the smallest discrepancies. The important question for me is whether the GPS is accurate enough so that I can have confidence in the distance I’ve run and the pace I’m running at.

Below is the GPS data from my run at this year’s Brighton Marathon. Zoomed out, the data appears to have tracked the route pretty much dead on. The overall distance the 645 recorded was 42.48km, approximately 260 metres further than pure marathon distance. That discrepancy is unlikely to be purely down to the Garmin. All marathons of this size inevitably involve a degree of weaving to and fro around other runners, whereas the marathon route itself is measured purely along the racing line.

Brighton Marathon GPS map from my Forerunner 645

Zooming in on the route gives a bit more perspective. For the most part the data follows the route accurately, although there are a few instances where the exact positioning must be off. The below image for example shows that I ran in the park at one instance, rather than on the road parallel to it and then, at one point, through a building. The difference in distance is likely that of a few metres and no GPS watch I have ever worn (and likely ever will wear) gives 100% accuracy.

Slight discrepancies – in this case from my Brighton Marathon route, it thinks I ran slightly in the park and then through a building

The below data is perhaps also illustrative. It’s the GPS data recorded on my 645 from when I spent almost five hours running an ultra marathon around a 400 metre race track (surprisingly enjoyable). Given the course consisted of 125 laps of the same track, the data should broadly be one single overlay.

There are some slight variations which are inevitable. The squiggles on the far right can be ignored (toilet diversions) and you would expect the recording to be a “thick” line, as it is, due to switching between lanes on the track when overtaking etc.

There are however two or three odd recordings where the watch recorded me as running either through the centre of the track, or out beyond its edges. Bearing in mind the size of the track, those odd results add up to a few metres at a time, which to my mind is perfectly acceptable.

Track Ultras turn out to be surprisingly enjoyable

In short, I’ve found the GPS to be accurate to such a degree that I’ve never had any issue with distance tracking or pacing a race, which is all I (and I imagine most others) will need. The biggest drawback is the length of time it takes to lock on to that signal – hardly fatal to the watch but a real pain nonetheless.

(*Note – Some of the new Garmin’s, as well as existing mode ls with a firmware update, now have access to the EU’s new Galileo satellite system, providing additional options and accuracy. My 945 is updated with the latest firmware but, alas, no Galileo access.)

A quick word on music

The Forerunner 645 comes in two models: standard and music. As the title of this review suggests, I opted for the non-music version. At the time of the release, the 645 supported the transfer of mp3 files from your desktop to the watch. The watch can then connect directly to your Bluetooth headphones, negating the need to carry a phone for music on your runs.

I don’t have mp3 files, but use Spotify. At the time of release, the 645 had no Spotify support, making the additional music features a waste of money for me. Since I bought the 645 a firmware update introduced Spotify support (note – you need a Spotify Premium subscription), so if either of those appeal to you, you may want to look at the music version.

Limited music control

What both versions of the 645 do include is music control. When connected to your mobile phone, the watch can be used to control any music or audiobooks that are playing on your phone. While this still means you need to carry your phone with you on the run, you can at least tuck it away somewhere and use your watch to pause, skip etc.

A quick word on the App

The Garmin app is the same for all Garmins and so I will not spend too much time on it. Rather, by way of a quick overview:

The standard welcome page of the app gives a summary of your current state as well as your day. If the 645 is connected via Bluetooth then it will give a live heart rate, as well as your current steps against target. Scrolling down will give you access to Intensity Minutes, Floors Climbed, “Stress Levels”, Calories intake and burn (if you add your food intake) etc.

Pressing on any of them will provide you with more information. The three horizontal lines in the top left brings up the full list of options, including access to your activities history and other stats.

The app’s homepage

After a run, the Forerunner 645 will sync to the app (either automatically if you have selected this, or otherwise when you open the app). GPS data, pace, heart rate and calories etc will be uploaded.

These are all easily viewed and various options are available, including expanding the map, delving into lap splits, viewing cadence and exporting data.

Post-run results

One of the more useful features for me is the gear tracker. The app allows you to add in your gear (for running, this means shoes) and assign them to particular activities. You can do this manually for each run, or set a “default”, automatically adding one pair of shoes to every run until you go and edit it.

This allows you to keep track of how far each pair of shoes has taken you and gives you an indication of when you may need to replace them (although all decisions should come down to how they feel and what they’re doing for you).

Shoe tracking


The Forerunner 645 is a great watch and sit’s approximately mid-range in Garmin’s running offerings.

Running features are accurate and wide ranging, allowing you to delve deep into the data of your run and hone in on those areas of improvements.

The battery won’t push much beyond 9 hours with GPS activated, even with other metric measurements switched off, so for those running mid- to long-distance ultra marathons, you may want to invest in a portable charger or have a look at the Forerunner 945 (hopefully I’ll have a review of that coming soon).

But at £345 (£399 for the music version), it’s considerably cheaper than the £519 Forerunner 945 and a great watch for the regular runner.

2 Replies to “Garmin Forerunner 645 (No music) – Review”

  1. Wow that’s a comprehensive review. I’m still using a Garmin230 and the strap breaks regularly on that too so obviously an issue with the range of them.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s