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Group 21 battery size

A group 21 car battery is at most 8.19 inches long, 6.81 wide and 8.74 tall (208 x 173 x 222 mm), with top posts.

Length
8.19 in (208 mm)
Width
6.81 in (173 mm)
Height
8.74 in (222 mm)
Terminals
top posts
Group 21, drawn to scale, in inches (the same scale on every group page)
Front8.19 in8.74 inTop8.19 in6.81 in

Same box, other group numbers

  • Group 21R: 8.19 x 6.81 x 8.74 in, different terminal layout (BCI figure 11, not 10).

Same box does not mean same battery: if the terminals are on the other side, the cables may not reach.

Close in size

Groups within 10 mm (0.4 in) on every side:

  • Group 121R: 8.19 x 6.97 x 8.46 in, different terminal layout (BCI figure 11H, not 10).

Before you buy a group 21 battery

  • Read the group number on the label of the battery in your car, or in the owner's manual: it should say 21.
  • A group with an "R" after the number has its terminals reversed: do not swap one for the other without checking where your cables reach.
  • Choose cold cranking amps (CCA) at least as high as the original, especially for cold winters.
  • Measure your battery tray and the room above it, then compare with the size printed for the exact battery you pick, not only with the group number.
  • Group 21 has top posts: the new battery must match, and the hold-down must fit the box.
  • Most stores take the old battery back for recycling, often against a core charge.

Good to know about group 21

  • Groups 21 and 21R have the same maximum case, 208 x 173 x 222 mm; only the terminal layout differs (BCI figure 10 for 21, figure 11 for 21R).1,2
  • The R in 21R means the terminals are reversed compared with a group 21 battery.2,3
  • Group 21 and 21R batteries have their terminals on top.2,4

How we checked this page

Checked on October 16, 2026. We kept a fact only when two independent sources agree, and each one is numbered to its sources below. We never write a price: the listings come straight from eBay and show what sellers ask. This page was written with an AI assistant and checked against the sources below.

Sources

  1. BCI group numbers, dimensional specifications and ratings, Battery Council International
  2. Battery BCI group numbers, dimensional specifications, polarity and terminals (bulletin 5300270), Cummins Inc.
  3. What the BCI group size number on your car battery means (Viraaj Bhatnagar, 2026-09-06), Jalopnik
  4. Battery, top terminal, BCI group 21R, Lordco Auto Parts

Group 21 batteries listed on eBay

Dimensions are the BCI maximums for the case, terminals excluded. Sources: Battery Council International, "BCI Group Numbers, Dimensional Specifications and Ratings" (batterycouncil.org), read October 15, 2026; Jalopnik, Viraaj Bhatnagar, "What the BCI group size number on your car battery means" (jalopnik.com), September 6, 2026; Battery Mart, "Group 51R batteries" (batterymart.com), read October 16, 2026.