Start with the empty cell or module
The safest answer is the code printed on the dead cell (CR2016, 377, SR920W, etc.). On many Casios, the caseback also shows a module number — that module is what joins the model to the correct cell.
Retail packs often print 377 for SR626SW or 371 for SR920W. Those are silver-oxide watch sizes — not the same as alkaline lookalikes.
Common watch sizes on this site
CR2016 — thin 20 mm lithium coin (many Casio digitals such as F-91W / A158).
CR2025 — same diameter, thicker than CR2016 (some Casio databank / AE models).
CR1616 / CR1216 — smaller-diameter coins on some Casio models.
SR626SW (377) and SR920W (371) — silver-oxide button cells; chemistry matters vs alkaline twins.
Casio vs other Japanese brands
Casio is our published watch pilot (model → module → battery). Seiko, Citizen and Orient are planned next collections — we will not invent batteries from brand alone.
Until those pages exist, use the code on the cell, the brand’s module/caliber docs, or measure diameter and thickness and use the size matcher.
Water resistance
Opening a water-resistant watch yourself can void the seal. Prefer a watchmaker when the case is gasketed or screw-back and you need the watch to stay water-resistant.