1. The letters tell you the type
SR means silver oxide, the chemistry that keeps a watch keeping time well. The extra letters matter too: SW is the low-drain version for analog watches, SL is the high-drain version for digital ones, and a plain W means no drain indication.
2. The numbers are a size name
They’re legacy codes rather than measurements. An SR626SW is actually 6.8 mm across and 2.6 mm thick. The letters and numbers together name one exact cell.
3. The ones you’ll see most
The SR626SW (6.8 × 2.6 mm) and the SR920SW (9.5 × 2 mm) cover a lot of watches. The SR621SW (6.8 × 2.1 mm) is the thinner one, and the SR521SW (5.8 × 2.1 mm) is the smallest common size.
4. SR versus LR
An LR44 and an SR44 are the same size, but the SR is silver oxide and the LR is alkaline. Silver oxide holds a steadier voltage, so a watch keeps better time with one.
5. Check the old battery, not the watch model
Watch models change their batteries over the years. The code on the old battery is the only reliable guide, so read it before you buy.