What size GPS watch should I buy: measure the wrist, then decide what the extra battery is worth

The short version

Outdoor watch sizing is unusual, because the case size is not a style choice the way it is on a dress watch. On a GPS watch, a bigger case contains a bigger battery and, on the solar models, a bigger solar lens, and the maker publishes the difference in hours. So the question is not "what looks right"; it is how many extra hours am I getting, and is that worth wearing a larger watch for ten years.

For most people the honest answer is the smaller size, because the smaller case still carries the feature that matters most on this site: the solar claim. A 45 mm Instinct 3 Solar says unlimited with solar in exactly the same way the 50 mm does.

How to measure, properly

Two minutes with a soft tape measure or a strip of paper and a ruler:

1. Wrap the tape around your wrist just past the wrist bone, towards the hand, which is where a watch actually sits. Not over the bone.
2. Snug, not tight. Note the number in millimetres.
3. Compare against the case width the maker publishes, not the lug-to-lug of a dress watch, because rugged GPS watches have integrated or short lugs.

Rough guidance, which is guidance and not a rule:

One caveat worth more than the table: if you will wear the watch over a jacket cuff or under a glove, size down. A case that fits bare skin comfortably is a case that catches on a sleeve.

What the bigger case actually buys

Here is the part nobody puts in a sizing guide, from the makers' own battery tables:

Read that honestly and two conclusions fall out. On the fenix, the step from 47 mm to 51 mm is large: nine extra days of smartwatch use, 28 extra GPS hours and more than double the max-battery figure with solar. If you do multi-day trips, that is a real argument for the big case.

On the Instinct, the step from 45 mm to 50 mm buys twelve days and twenty GPS hours, on a watch whose smaller size already says unlimited with solar. Unless you are routinely recording 40-plus hour activities, the smaller Instinct gives up almost nothing you will notice. The whole solar argument is in are solar watches worth it.

Thickness matters more than width

Everyone shops on diameter and then complains about thickness. Case height is what decides whether a watch catches on a cuff, bangs on a door frame and sits comfortably when you sleep in it, and the makers publish it.

For scale, using the figures on this site: the G-Shock GA-2100 is 11.8 mm, the GMA-S2100 is 11.2 mm, the COROS APEX 2 Pro is 14 mm, the COROS NOMAD is 14.8 mm, the COROS VERTIX 2S is 16 mm, the Casio Rangeman GW-9400 is 18.2 mm and the G-Shock GBD-H2000 is 19.4 mm. That is an eight-millimetre spread, and in use the difference between an 11 mm watch and a 19 mm watch is far more noticeable than two millimetres of diameter.

If you work with your hands, wear long sleeves, or have ever caught a watch on a rope or a door handle, treat height as the first number you check. The thin-case argument for Casio is in best G-Shock for work.

If you have a small wrist

The rugged outdoor category is sized for large wrists and there is no point pretending otherwise. Three honest options:

The 45 mm Instinct 3 Solar is the best answer in GPS watches. It is a genuinely smaller case with the full solar claim, 10 ATM, the MIL-STD-810 line and the flashlight, giving up only battery hours.

The 47 mm fenix 8 Solar is the smaller mapping option, with sapphire and a titanium bezel and a 40 m dive-rated case. It is still a substantial watch; the 51 mm is a lot of watch.

A smaller Casio if you do not need GPS. The GMA-S2100 is 46.2 by 42.9 by 11.2 mm and 41 g, which Casio describes as its small case size and which is the smallest, lightest tough watch we rank. The reasoning is in best G-Shock for small wrists.

One more option people forget: the strap. A nylon strap on a COROS or Suunto saves real weight, 17 grams on the VERTIX 2S and 13 on the APEX 2 Pro by COROS's own figures, and nylon conforms to a thin wrist far better than thick silicone.

The picks

The rules, in order

1. Measure first. Past the wrist bone, towards the hand, snug. Everything else follows from that number.

2. Check thickness before diameter. An 11 mm watch disappears; a 19 mm watch announces itself. Both are published.

3. Buy the size whose battery table you need, not the longest one. On the Instinct, the smaller case already says unlimited with solar. On the fenix, the big case genuinely earns its size on long trips.

4. Size down if you wear sleeves or gloves. The watch that fits bare skin is the watch that catches on everything else.

5. Consider the strap as part of the size. Nylon saves grams and conforms better than silicone.

6. The watch you keep wearing wins. A 51 mm case in a drawer has a battery life of zero. This is the whole reason we rank the 45 mm and 47 mm sizes as the default recommendation, and it is consistent with the ranking logic in best rugged GPS watches.