Watch tool
Watch size calculator
Enter your numbers to see the result.
On this page (6 sections)
How to measure your wrist properly
Wrap a soft tape measure, or a strip of paper you can mark and lay flat against a ruler, around your wrist just above the wrist bone, on the arm the watch will sit on. Pull it snug but not tight: aim for the tension a bracelet has once you have stopped noticing it. The box above defaults to inches, so read to the nearest sixteenth and type it as a decimal. Sixteenths convert cleanly: 6 3/4 in is 6.75, 7 1/8 in is 7.125. Switch the unit if you would rather read millimeters. Four mistakes produce most of the wrong answers.
- Running the tape over the wrist bone. Cross the head of the ulna, the bump on the outside of the wrist, and the reading comes out several millimeters high, enough to move the suggested diameter by a full millimeter.
- Pulling the tape tight. Soft tissue compresses, and you meet the difference again at the last hole of every strap you buy.
- Measuring in the morning. Wrists swell through the day, so an evening measurement sizes for the worst case.
- Measuring the forearm. An inch further up the arm is noticeably larger, and that is not where a watch sits.
Most adult wrists fall between 6 and 8 inches, which is 15.2 to 20.3 cm. That range covers almost every watch on sale, which is why rules like "40 mm is the perfect size" fall apart on a real wrist. The calculator refuses anything outside 110 to 260 mm, roughly 4.3 to 10.2 inches, so typing 17.5 with inches selected returns "Check the units" rather than a nonsense answer.
A worked example, from tape to shortlist
Say the tape reads 7.00 inches, the unit is left on inches and the proportion on Balanced. The calculator converts to millimeters first, then works from that one number.
| Step | Result |
|---|---|
| Wrist as measured | 7.00 in, which is 17.8 cm or 177.8 mm |
| Diameter midpoint: 177.8 multiplied by 0.225, plus 0.6 | 40.6 mm |
| Range shown, the midpoint plus or minus 2 mm | 39 to 43 mm |
| Lug to lug ceiling: 177.8 multiplied by 0.2, plus 13 | about 49 mm |
| Comfortable thickness | under 13.5 mm |
| Usual lug width | 20 to 22 mm |
Choosing Vintage moves the midpoint down 2 mm, so the range becomes 37 to 41 mm; Modern moves it up to 41 to 45 mm. The lug to lug ceiling does not move at all, and that is the lesson of the page: proportion is taste, but the flat top of your wrist is a physical dimension that does not care what you like.
Apply it. An Omega Speedmaster Professional is 42 mm across with 50 mm lug to lug: inside the diameter range, 1 mm over the length ceiling, so it is wearable but at the limit and worth trying on first. A Hamilton Khaki Field Mechanical 38 is 38 mm across with 47 mm lug to lug: below the range on diameter, comfortably inside on length, so it reads as the smaller watch and sits dead flat. The diameter alone gives you neither conclusion.
Why lug to lug matters more than diameter
Diameter is the number brands print and buyers argue about. Lug to lug is the number that decides whether the watch sits flat. It measures the full length of the case including the lugs, and it has to fit inside the flat top of your wrist. If the lugs hang over the edge, the strap has to bend immediately at the case and the watch rocks, whatever the diameter says.
Two watches with an identical 40 mm diameter can differ by 6 mm in lug to lug, which is the difference between a watch that disappears on the wrist and one that never settles.
| Watch | Diameter | Lug to lug |
|---|---|---|
| Hamilton Khaki Field Mechanical 38 | 38 mm | 47 mm |
| Omega Speedmaster Professional | 42 mm | 50 mm |
| Seiko Prospex Turtle | about 45 mm | about 47 to 48 mm |
The widest case there is also the shortest: the Turtle is seven millimeters wider than the Hamilton and about the same length, because its cushion case has no lugs to speak of. Seiko does not publish a lug to lug figure for it, so that row is a measured number rather than a specification, which you will meet repeatedly while shopping.
What the number should decide
- Whether to open the listing. A published lug to lug more than about 2 mm over your ceiling is a no. A diameter inside the range does not rescue it.
- What to ask a seller. Rolex and Seiko, among others, do not publish lug to lug. Ask for a caliper measurement from lug tip to opposite lug tip.
- Which strap to buy. Straps are bought by lug width, never by case diameter. The 20 to 22 mm here predicts what watches in your range usually take; measure the real gap and use the strap and lug width finder.
- Whether size is the problem at all. A watch that feels wrong is often too thick, or on a bracelet that will not size to the half link you need. Our guide to straps and bracelets and the note on shortening a bracelet cover that fix.
Cases the calculator cannot see
- Integrated bracelets and cushion cases. When the bracelet starts at the case rather than at lugs, the first link sets the effective length, and a stiff one behaves like a longer case.
- Vintage watches. A 34 to 36 mm case from the 1960s often carries around 44 mm of lug to lug and sits under 10 mm thick, so it wears larger than its diameter suggests, and its 18 mm or odd 19 mm lugs narrow your strap choices.
- Bracelets without micro-adjustment. The gap between two link positions can be 5 to 10 mm of circumference, more than the daily swelling you measured for. Half links or a clasp with adjustment holes solve what no case dimension can.
- Rectangular cases and unusual wrists. A Tank has no diameter, so compare length to your ceiling first. A flat wrist offers a wider shelf than its circumference suggests, a round one less.
What this calculator does and does not claim
It converts your wrist measurement into a proportional starting range, then adjusts it for the look you say you want. It is a shortlist filter, not a verdict. Dial opening, bezel width, case shape, thickness and how far the lugs turn down all change how a given diameter wears. A 40 mm diver with a thick bezel wears smaller than a 38 mm watch with a wide open dial.
There is also no standard for watch size. International standards govern water resistance, shock, magnetism and chronometer rate, but nothing defines a correct diameter for a wrist, so the coefficients here are a proportional rule fitted to conventional practice rather than a measurement of your arm. Treat the diameter range as soft guidance and the lug to lug figure as the one hard constraint, use the range to narrow what you try on, then trust your own wrist. Our guide to watch sizing and fit covers the same measurements at length, and the comparison of 38 mm, 40 mm and 42 mm shows how little the headline diameter settles.
Common follow-up questions
What size watch is right for a 7 inch wrist?
A 7 inch (17.8 cm) wrist is comfortably served by roughly 39 to 43 mm, with lug to lug under about 49 mm. That is the middle of the market, so nearly everything is on the table, and thickness and lug to lug will decide comfort long before diameter does.
Does the calculator work for women's watches?
Yes. The arithmetic is proportional to wrist circumference, not to who is wearing it. Watches marketed to women are often 26 to 36 mm, which is what the calculator suggests for smaller wrists, but there is no rule against a larger case if you like how it sits. Our note on the best watches for a small wrist goes further.
How much lug overhang is acceptable?
Ideally none. One or two millimeters each side is livable on a case whose lugs curve down hard. Beyond that the strap angle fights the wrist, the watch rocks, and the clasp ends up in the wrong place. The lug to lug explainer covers how the measurement is taken.
My wrist falls between two ranges. Do I round up or down?
Round toward the smaller case if the watch is thick and toward the larger one if it is thin. A tall case lifts the mass away from the wrist and lets the watch pivot, so a 42 mm watch at 10 mm thick often wears better than a 40 mm watch at 15 mm.
How do I size a rectangular watch?
Ignore diameter, which does not exist for a rectangle. Check the case length including lugs against the lug to lug ceiling above, then the width against the diameter range as a sanity test. Rectangular cases are narrow for their length, so length is almost always the binding constraint.
Should I measure again before sizing a bracelet?
Yes, in the evening. Case dimensions decide whether the watch fits at all; bracelet sizing decides whether it sits still. If it lands between two links, ask about half links or a clasp with micro-adjustment rather than accepting one that slides.
Sources and further reading
- Hamilton published specification page for the Khaki Field Mechanical 38 mm, reference H69439931, for the 38 mm diameter, 47 mm lug to lug, 9.5 mm thickness and 20 mm lug width.
- Omega published specification for the Speedmaster Moonwatch Professional, for the 42 mm diameter, 50 mm lug to lug and 20 mm lug width.
- Seiko published specifications for the Prospex Turtle, which give case width, thickness and lug width but no lug to lug figure. The 47 to 48 mm length quoted here is a measured figure, and different people measuring the same watch will differ by a few tenths of a millimeter.
- Manufacturer specification pages generally, which are where diameter, thickness and lug width should be read. Retailer listings routinely round, omit lug to lug, or repeat a figure from a different reference in the same family.
Last reviewed 4 September 2026. Spotted an error? Tell us and we will fix it in public.