How to find the lug width of your watch

The number the watch does not tell you

Lug width is the distance across the gap in the watch case where the strap sits, measured in millimetres. That distance is the only number that decides whether a conventional replacement strap will fit. The name on the case tells you the model. It does not tell you the lug width. You can own an Apple Watch, a Galaxy Watch, a Garmin or a Fitbit and the model name will not replace the measurement you need to match a strap that fits the case.

Many watches use a pair of holes in the case for a spring bar. The strap fits between those holes. The measurement you take across the slot is the lug width. If the bar sits in a different position or the case uses a moulded pin the measurement method changes. Apple Watch does not use lug width at all. Fitbit trackers use moulded pins rather than a lug. Garmin and Samsung models use standard lug widths across many examples, and that makes replacement straightforward once you have the measurement.

You will see straps sold by their lug width in millimetres. That label is the single fact you need for fitting on most watches. It will tell you whether a strap will go on at all. After you have the number you will check other details such as material, length and clasp, but none of those replace the need for the correct lug width.

Taking the measurement, with the strap off

Remove the strap before you measure. If the watch has a quick-release spring bar there will be a small lever at one end of the bar. Push that lever with your fingernail to release the strap without using a tool. If the bar is a conventional spring bar you will need a narrow tool to compress it and free the strap. Once the strap is off you will see the gap in the case where the bar sits.

Place a ruler or a pair of callipers across that gap and read the distance in millimetres. Read to the nearest millimetre. If you use callipers, close them lightly so you do not bend the bar holes. If you use a ruler, align the zero with the inside edge of the slot and view the scale straight on to avoid parallax. Take the reading a couple of times to be sure.

If your measurement falls between two numbers, measure again. Try the other side of the case and compare. Lug widths on these watches are even numbers, so a reading of twenty-one means the ruler moved rather than that the watch is unusual. Look up the model instead: the width is a property of the case, and every watch named on this site has one stated width. If you own an Apple Watch, remember it does not use a lug width at all. Check the case size in the Watch app under General, About, or read the engraving on the back of the case.

Garmin: four widths, and which watch takes which

Garmin models divide neatly into four lug widths across the range. The smallest 18mm width is used on the small Venu models. If you have the Venu 2S or the Venu 3S then you will need an 18mm strap. The most common width in the lineup is 20mm. Vivoactive 3, 4, 5 and 6 all use 20mm. The Venu, Venu 2, Venu 3 and Venu Sq also use 20mm. Several Forerunner models share that width too: the Forerunner 55, 245, 645 and 745 take 20mm straps.

A step up comes with 22mm. Instinct and Instinct 2 use 22mm. The mid-size Fenix cases use 22mm as well. At the top end, the largest Fenix cases require 26mm. The large Fenix models include the 5X, 6X, 7X and the 51mm Fenix 8, and these all use 26mm. The progression is clear: 18mm on the small Venu variants, 20mm across many Vivoactive, Venu and Forerunner models, 22mm on Instinct and the mid-size Fenix, and 26mm on the large Fenix cases.

When you own a Garmin, knowing which model you have lets you match the width quickly. The model name is engraved on the back of the case under the sensor window. Use that engraving to confirm which of the widths you should measure against before buying a strap.

Samsung: one width, six years of watches

If you own a Samsung Galaxy Watch from the Watch 4 onward you can expect a 20mm lug. Samsung has kept a single width across several generations. Galaxy Watch 4, Watch 5, Watch 6, Watch 7, Watch 8 and Watch 9 all use 20mm lugs. That includes the Classic variants and the FE models where they exist. The Galaxy Watch 5 Pro is on the same 20mm width.

A single width across multiple models makes it easy to change straps between those watches. You can measure should you doubt which model you own, but many owners find a 20mm replacement will fit any of the listed generations. When you remove the strap and measure the slot you will see 20mm between the inner faces where the spring bar sits.

Check the case for the model engraving if you need to confirm which generation you have. The engraving is a reliable place to verify the model when the name on the original box or paperwork is not to hand. With Samsung models in this range the 20mm rule covers most cases and saves you from having to match several different sizes.

Apple: no lug width, two runs of case size

Apple Watch does not use a lug width. It fits straps on a proprietary rail that is cut for one of two runs of case size. The small run comprises 38, 40, 41 and 42mm. The large run comprises 44, 45, 46 and 49mm. A band made for one run does not go on a watch from the other run.

If you own an Apple Watch do not measure a lug because there is none. Instead check the case size. The case size is engraved on the back of the case under the sensor window. You can also open the Watch app on the paired iPhone, then go to General and About to see the case size listed there. Match the strap to the correct run and you will find the connector aligns with the rail.

Apple Watch bands are labelled by the run they fit rather than by a millimetre lug width. That means a strap marked for the small run will slide into the proprietary rail on any 38, 40, 41 or 42mm case. A strap for the large run will slide into 44, 45, 46 or 49mm cases. Do not attempt to fit a strap from one run to a case from the other because the connectors are not compatible.

Fitbit: pins, and the name of the tracker

Fitbit does not use a conventional lug width. Its trackers use moulded pins and specific fittings, and you must match the strap to the tracker fitting rather than a measurement in millimetres. Charge models share fittings with their close successors. The Charge 5 and Charge 6 use the same fitting between them. Charge 3 and Charge 4 share another fitting. The Versa and Sense family share a separate fitting pattern. Versa 3, Versa 4, Sense and Sense 2 use the same fitting. Versa 2 is on an older fitting that is not the same as the later Versa models.

When you own a Fitbit, identify the tracker model name. That name is the key to choosing a strap that will attach to the moulded pins. You can read the model name on the device or check any documentation you have. For many Fitbit models the fitting changed between generations so do not assume that a band for a later model fits an earlier one.

Because the pins are moulded you cannot measure a lug for a millimetre value. Instead choose a strap that is explicitly listed for your tracker model. If you see a strap that states it fits the Charge 5 and Charge 6, for instance, you can use it on either of those trackers. Match the tracker name and the fitting to ensure correct attachment.

What a millimetre out feels like before it fails

A strap that is off by a millimetre will usually go in when you force it but it will not sit flush. One edge will sit proud. At first you will notice the strap feels slightly uneven against the case. It may rub or catch on clothing when you move. The watch will not feel as solid on the wrist. It will not be a dramatic failure at once. Instead you will notice small shifts and a slight wobble when you gesture.

A proud edge changes how the spring bar bears load. Ordinary movement then works the bar sideways. The spring bar can back out a hair each time you move, and that adds up over days. The bar may slowly withdraw until the strap can move enough to free itself. Before the bar fails you will feel the strap move a little under pressure, or you may see a tiny gap between the strap end and the case. That is the most reliable early sign.

When the bar works loose the strap can fall free suddenly. The time from the first small movement to failure depends on how much you move your wrist and how the strap is loaded. Choosing the correct lug width avoids the slow loosening that eventually throws the spring bar. A strap that matches the case keeps the contact even and the bar seated properly, so you do not get the small, early signs that precede a failure.

Changing a strap, and the mistake people make

Release the old strap first. If the strap uses a quick-release spring bar push the small lever with your fingernail and the strap will drop away without a tool. If the bar is conventional compress it and free the strap carefully. When you fit the new strap make sure the spring bar sits fully in the holes on both sides before you let go. Press the strap in and feel for the bar to seat.

Pay attention to direction. One side of the strap carries the buckle and the other side carries the tail. The buckle side fits the top lug of the case as you wear the watch, and the tail side fits the bottom lug. The short end with the buckle does not belong on the same side as the longer tail or you will end up wearing the watch with the buckle the wrong way. The one mistake many people make is putting the buckle half in the wrong slot so the strap sits backwards.

After seating the spring bar check visually that the end of the strap sits flush with the case on both sides. Try a gentle tug away from the case to make sure the bar is secure. If the strap has quick release check the lever has returned to its resting position and the bar is not visible. A correctly fitted strap will sit even, point the buckle the right way and not shift when you move your wrist.