Watch Battery Sizes Guide: Reading Numbers and Stocking Up

A watch battery size is set by two things: the diameter and the height of the cell. Both are built into the reference number. A 377 is the same cell as an SR626SW, so you can find the right replacement from the number on the old cell, or from the case-back, a battery chart or a pair of callipers.

This guide is for jewellers, repairers and retailers. It explains how to read the codes on silver oxide button cells, how the Renata numbers match the IEC codes, and which common sizes are worth keeping in stock.

How do you read a watch battery number?

Most watch cells carry one of two kinds of number. Both describe the same cell.

The Renata-style three-digit number

Numbers like 377, 364 or 371 are short reference numbers. They are quick to say, quick to write on a bag and easy to match against a chart. A three-digit number does not tell you the dimensions directly. You look it up.

The IEC code (SR626SW and similar)

The IEC code does describe the cell, once you know the pattern:

• S means silver oxide chemistry (nominal 1.55 V).
• R means round.
• The digits give the size. The leading digits are roughly the diameter in millimetres, and the last two digits are the height in tenths of a millimetre. So SR626 is about 6.8 mm across and 2.6 mm high, and SR920 is about 9.5 mm across and 2.0 mm high.
• The suffix letters tell you the drain rating. SW is the standard low-drain version, made for ordinary analogue movements. W is the high-drain version, made for watches with a light, an alarm, a chronograph or other extra load.

The leading digits are rounded, so use them as a guide only. The height digits are the ones that matter when a cell looks right but will not sit properly.

Common silver oxide watch battery sizes

This table covers sizes that come across a repair bench often. Dimensions are approximate. Check the exact figures against the manufacturer's data sheet before you rely on them.

How do you identify an old watch battery?

Work through these steps in order and stop when you have a match.

1. Read the cell. The number is normally stamped on the flat positive side. Tilt it under a loupe, because the print is often faint.
2. Check the case-back and the movement. Many watches have the battery number engraved on the back or printed on the movement or battery clip. A movement holder or data sheet will list it if you know the calibre.
3. Measure the cell. If the print is worn off, measure the diameter and the height with callipers. Match the diameter first, then the height. Two cells can share a diameter and differ by half a millimetre in height, and that is often the difference between a watch that closes and one that does not.
4. Check the drain rating. If the watch has a backlight, alarm or chronograph, choose the W type where the manufacturer offers one. A standard SW cell in a high-drain watch may run flat early.
5. Confirm the chemistry. Silver oxide cells are 1.55 V. Lithium coin cells (the CR range, such as CR2032) are 3 V and are not interchangeable with silver oxide cells.

Silver oxide or alkaline: does the difference matter?

Alkaline button cells (LR codes, sold as AG numbers) are often the same physical size as a silver oxide cell. They are a cheaper cell, but their voltage drops steadily as they discharge, while a silver oxide cell holds a steadier voltage for most of its life. For watches, silver oxide is the usual choice. A same-size alkaline cell may fit, but you should not treat it as a like-for-like replacement in a customer's watch.

Can you substitute one watch battery size for another?

In general, no. Match the number or the IEC code. Two points are worth keeping in mind:

• SW and W cells in the same size are not always interchangeable. The case is the same but the load capability is not. If the watch calls for a high-drain cell, fit a high-drain cell.
• Different brands use different numbering. A cross-reference chart is reliable only when it comes from the manufacturer. Where a chart and the physical cell disagree, trust the measurement.

Which watch battery sizes should you keep in stock?

The right mix depends on your customers, so look at what you have actually replaced over the last few months before you decide. As a starting point, most repair benches go through these often:

• The thin, small-diameter cells for slim ladies' and fashion watches: 321, 364, 377, 379.
• The 9.5 mm family for mid-size analogue watches: 371, 373, 395 and 399.
• The larger cells for bigger cases and some digitals: 390 and 389, 394, 303 and 357.

Keep the high-drain version of any size where you service chronographs or watches with lights. Rotate stock by date so the oldest cells go out first. A small, tidy range of the common numbers, labelled clearly, saves more time at the bench than a large range you cannot find your way around.

If you buy for a shop or a workshop, you can see Renata batteries alongside straps, glasses, movements and other parts in the watch spares catalogue. SM Watch Wholesale sells to the trade only, and trade customers sign in to see prices. For the practical side of getting an order to you, see delivery and collection.

Frequently asked questions

What is the difference between a 377 and an SR626SW?

Nothing. The 377 is the short reference number and SR626SW is the IEC code for the same size and chemistry. Always confirm against the maker's chart when you are cross-referencing between brands.

What does the W in SR927W mean?

It marks a high-drain cell, for watches with lights, alarms or chronograph functions. SW is the standard low-drain version for ordinary analogue watches.

What is the watch battery price?

Prices change, so we do not list them in this guide. SM Watch Wholesale is a trade supplier, and trade customers sign in on the spares catalogue to see prices.

Can I use a lithium CR cell in place of a silver oxide cell?

No. Lithium CR cells are 3 V and silver oxide cells are 1.55 V. Fitting the wrong one can damage the movement.

Does SM Watch Wholesale sell to the public?

No. It supplies jewellers, watch and clock repairers and retailers. You can read more on the About Us page.