Battery sizes follow specific naming codes that tell you the physical dimensions, shape, and sometimes the chemistry of the battery. Once you know the pattern, you can decode nearly any battery label you encounter, whether it’s a AA from a kitchen drawer or a CR2032 from the back of a remote.
Standard Household Batteries: AA, AAA, C, and D
The letter-based names for everyday batteries don’t encode their dimensions the way other systems do. Instead, the letters represent standardized sizes you simply need to recognize. Here are the common ones with their actual measurements:
- AAA: 10.35 mm diameter × 44.3 mm tall, 1.5V
- AA: 14.5 mm diameter × 50.5 mm tall, 1.5V
- C: 25.5 mm diameter × 49.5 mm tall, 1.5V
- D: 32.5 mm diameter × 61 mm tall, 1.5V
- 9V: 26.6 × 17.3 mm rectangular block, 45.4 mm tall
All of these are 1.5V (except the 9V block), so the size difference isn’t about voltage. Larger batteries hold more energy and last longer in high-drain devices like flashlights or portable radios. A AA and a D battery power the same 1.5V circuit, but the D cell will do it for much longer.
Coin Cell Codes: What CR2032 Actually Means
Coin cell batteries use a code that’s surprisingly logical once you crack it. Take CR2032 as an example:
- C: lithium manganese dioxide chemistry
- R: round shape
- 20: 20 mm diameter
- 32: 3.2 mm thick
The first two digits are always the diameter in millimeters. The last two digits are the thickness, with an implied decimal point. So a CR2025 is the same 20 mm diameter but only 2.5 mm thick. A CR1632 is 16 mm across and 3.2 mm thick. This pattern works for every CR battery you’ll find in remotes, key fobs, kitchen scales, and small electronics.
Because a CR2032 and CR2025 share the same diameter, a CR2025 can physically fit in a CR2032 slot, but it’s slightly thinner and holds less energy. Some devices won’t make reliable contact with the thinner cell, so always match the full number when replacing.
Lithium-Ion Cylindrical Batteries: 18650, 21700, and Others
The rechargeable lithium-ion cells used in laptops, flashlights, power tools, and electric vehicles follow the same dimension-based logic as coin cells. The code is five digits: the first two are the diameter, the next two are the length, and the final digit indicates shape (0 means cylindrical).
An 18650 cell is 18 mm in diameter and 65 mm long. A 21700 is 21 mm in diameter and 70 mm long. The 21700 is physically larger, which means it holds more energy per cell. That’s why newer electric vehicles and high-performance flashlights have shifted toward 21700 cells. If you see a 26650 or a 14500, the same rule applies: 26 mm × 65 mm, and 14 mm × 50 mm respectively. The 14500, notably, is the same external size as a standard AA battery.
Watch Batteries and Silver Oxide Codes
Watch batteries are notoriously confusing because every manufacturer uses a different numbering system. A single battery might be labeled SR626SW by one brand, 377 by Energizer, D377 by Duracell, and V377 by Renata. These are all the same cell.
The SR code follows a readable pattern. “SR” indicates silver oxide chemistry. The digits after that encode the dimensions, similar to coin cells: SR626SW is 6.8 mm in diameter and 2.6 mm thick (roughly). The “SW” at the end means it’s designed for low-drain devices like standard watches, while a “W” suffix (like SR626W) signals a high-drain version for watches with backlights or alarms.
If you’re replacing a watch battery, the easiest approach is to match the exact code printed on the old cell, or look up a cross-reference chart to find the equivalent from whatever brand your local store carries. The three-digit numbers (like 377 or 364) don’t encode dimensions at all. They’re just catalog numbers assigned by manufacturers.
Hearing Aid Battery Color Codes
Hearing aid batteries use a standardized color system on their packaging tabs, which makes identification simple even for people with limited vision. Four sizes cover nearly all hearing aids:
- Size 10: yellow tab (smallest)
- Size 312: brown tab
- Size 13: orange tab
- Size 675: blue tab (largest)
These are zinc-air batteries with a nominal voltage of about 1.4V. The tab seals the air holes. Once you peel the tab, air enters the cell and activates it, so don’t remove the tab until you’re ready to use the battery.
Car Battery Group Sizes
Automotive batteries don’t use dimension codes. Instead, they follow BCI (Battery Council International) group sizes, which are standardized designations like Group 24, Group 35, Group 48, or Group 65. Each group number specifies four things: the physical dimensions (within 2 mm tolerance), the terminal position, the voltage, and the chemistry. This ensures the battery fits your vehicle’s tray and the cables reach the correct terminals.
You’ll find the group size printed on your current battery’s label or listed in your vehicle’s owner’s manual. Getting the right group size matters more than capacity ratings, because a battery that doesn’t physically fit the tray or has reversed terminal positions won’t work regardless of how powerful it is. Performance within a group size varies by brand, and is measured in cold cranking amps (how well it starts your engine in freezing weather) or amp hours (total energy storage).
Reading Capacity: mAh and Wh
Beyond physical size, batteries list capacity in milliamp-hours (mAh) or watt-hours (Wh). These tell you how long the battery will last, not how big it is physically.
mAh measures how much current the battery can deliver over time. A 2,000 mAh battery can theoretically supply 2,000 milliamps for one hour, or 500 milliamps for four hours. This is the number you’ll see on rechargeable AAs, phone batteries, and power banks. A higher mAh means longer runtime, assuming the voltage is the same.
The catch is that mAh alone doesn’t tell you total energy, because it ignores voltage. A power bank labeled 10,000 mAh at 3.7V actually stores 37 Wh of energy. Wh (watt-hours) is the more complete measurement because it accounts for both current and voltage. You can calculate it yourself: multiply the voltage by the amp-hour rating. A 1.2V rechargeable AA rated at 2,000 mAh stores 2.4 Wh, while a 3.7V phone battery at the same 2,000 mAh stores 7.4 Wh, more than three times the energy.
Voltage Tells You the Chemistry
The voltage printed on a battery is a reliable clue to what’s inside it. Standard alkaline and zinc-carbon disposables run at 1.5V. Rechargeable NiMH cells (the most common rechargeable AAs and AAAs) run at 1.2V, which is why some older devices behave slightly differently with rechargeables. Lithium iron disulfide disposables, like Energizer Ultimate Lithium, have a nominal voltage of 1.8V but are marketed as 1.5V replacements. Silver oxide button cells sit at 1.55V with very stable output, which is why watches prefer them.
Lithium-ion rechargeable cells operate at 3.2 to 3.7V per cell. CR-type coin cells (lithium manganese dioxide) are 3V. If you see a rechargeable AA labeled 1.5V lithium, it contains a small voltage converter inside that steps down from the lithium cell’s native voltage to match what your device expects.