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How to measure a watch movement (lignes and millimetres)
Movement size is quoted in lignes and millimetres, where one ligne equals 2.2558mm, so an 11.5-ligne calibre measures about 25.6mm across.
The ligne and its conversion
The ligne is the traditional Swiss unit for movement diameter, written with three prime marks, as in 11.5 lignes. One ligne is exactly 2.2558mm. So an 11.5-ligne movement is about 25.6mm across, and a 13-ligne movement is close to 30mm. Datasheets usually give both the ligne size and the millimetre figure, and it is worth carrying the millimetre value because that is what you compare against a case.
The ligne describes the round diameter. For a shaped movement the makers quote length and width in millimetres instead.
Diameter is only half the story
A movement has to fit the case in two directions. Diameter decides whether it drops into the case opening. Height, or thickness, decides whether the caseback closes over it and whether the dial and hands sit at the right level. An ETA 2892-A2 at roughly 3.6mm high and an ETA 2824-2 at roughly 4.6mm high share almost the same diameter but sit a full millimetre apart in thickness, which changes the case depth they suit.
Always read both figures off the datasheet. Two movements that match on diameter can be wrong on height.
The other measurements that decide a fit
Beyond diameter and height, a genuine fit depends on the stem position and stem height, so the crown lines up with the case tube, and on the hand-hole sizes for the hour, minute and second hands, so your existing hands press on. Dial-foot positions and the date-window location matter too, because the dial has to locate on the movement and any calendar has to show through the right aperture.
None of these appear if you only note the ligne size, which is why a swap needs the full datasheet, not just the headline diameter.
Read it, do not guess it
Where a case is already empty, a caliper across the mainplate gives you the diameter in millimetres, which you can divide by 2.2558 to recover the ligne size. But the reliable route is the manufacturer technical sheet, which lists diameter, height, stem height and hand sizes together. Our per-calibre pages carry those figures so you can line up a donor without hunting through PDFs.
Look up any calibre's real dimensions
Open the calibre size toolDiameter in lignes and millimetres, height, stem and hand sizes per calibre.Find watch movement caliber on eBayaffiliateRelated
Sources: Ranfft movement archive ยท ETA/Sellita/Miyota/Seiko Instruments manufacturer technical sheets