Car Battery Group Sizes Explained
Reviewed by GarageDex editorial
A BCI 'group size' (like 35, 24F, or 51R) is a standard that defines a battery's length, width, and height, its terminal type, and which side the terminals sit on - so a battery in the right group will physically fit your tray and connect correctly.
What a group size actually controls
Battery Council International (BCI) standardizes battery cases so manufacturers, automakers, and stores all speak the same language. A group size pins down three things: dimensions (will it fit the tray and hold-down), terminal type (top post vs side terminal), and terminal position (which side the + and − posts are on). Get those right and the battery fits; get them wrong and it either won't sit in the tray or the cables won't reach.
There are only ~30 common sizes
Despite thousands of car models, most US passenger vehicles use one of about thirty common groups - 35, 24F, 47 (H5), 48 (H6), 51R, 65, 94R (H7), and a handful of others. That's why a small, accurate lookup can cover almost every car on the road.
Group size vs CCA vs Ah
Group size is the shape. Cold cranking amps (CCA) is the starting power, and amp-hours (Ah) is the reserve capacity. You match the group so it fits, then meet or exceed the CCA your car needs.
How to find yours
The fastest way is the label on your current battery, or your owner's manual. Or just look up your year, make, and model - we list the group, CCA, and dimensions for each.
Frequently asked
Does a higher group number mean a better battery?
No. The group number is about physical size and terminal layout, not quality or power. A Group 24F isn't 'better' than a Group 35 - it's just a different shape. Power is measured separately in CCA and Ah.
What does the letter after the number mean (like 24F or 51R)?
Letters note a variation of the base group. 'R' usually means reversed terminal positions; 'F' denotes a specific front-terminal variant. A 51 and a 51R are the same size but the positive and negative posts are swapped - using the wrong one can leave the cables too short.
Can I use a different group size than my car came with?
Sometimes, if it fits the tray, the terminals line up, and it meets the CCA rating. Many cars accept an alternate group. But don't assume - check your tray clearance and terminal position first.
Need your car's exact spec? Look up your battery size →