Feature
Group 27 Battery Dimensions — the Exact Numbers, and What Actually Fits
Gene Okafor · · 2 min
Group numbers are the sizing system nobody explains at the parts counter. They come from BCI — the Battery Council International — and they define the physical box: length, width, height, and which corner the posts sit on. Nothing about a group number promises capacity or cold-cranking amps; two group 27 batteries can differ by hundreds of CCA.
Group 27 dimensions
| Length | Width | Height | |
|---|---|---|---|
| Inches | 12 1/16 | 6 13/16 | 8 7/8 |
| Millimeters | 306 | 173 | 225 |
Height is measured to the top of the post on standard-post designs. If your tray sits under a low hood brace or a trunk floor panel, measure to the underside of whatever closes above the battery — a quarter inch matters.
The variants: 27, 27F, 27M
- 27 — the standard box, positive post on the left when the posts face you.
- 27F — the same box with the polarity reversed (“F” layouts flip the posts). Common in Ford, Toyota, and Honda applications. A plain 27 in a 27F tray leaves the cables reaching the wrong way.
- 27M — the marine version: same footprint, usually dual terminals (SAE posts plus threaded studs) and a deeper-cycle plate design.
What group 27 is actually used for
The 27 box is on the large side for a passenger car. You’ll meet it most in:
- RVs and travel trailers — as a deep-cycle house battery, where the extra volume buys amp-hours (typically 85–105 Ah).
- Boats — the 27M is one of the two default marine sizes alongside 24M.
- Trucks, vans, and diesels — where cold-start current and reserve capacity justify the footprint. Typical CCA for a group 27 starting battery runs roughly 700–900.
Group 27 vs. its neighbors
| Group | L × W × H (in) | Rough capacity |
|---|---|---|
| 24 | 10 1/4 × 6 13/16 × 8 7/8 | 70–85 Ah |
| 27 | 12 1/16 × 6 13/16 × 8 7/8 | 85–105 Ah |
| 31 | 13 × 6 13/16 × 9 7/16 | 95–125 Ah |
Width is identical across all three; the box only grows in length (and, for 31, height). That’s why an RV tray built for a 27 will often take a 24 with a spacer but will never close over a 31.
Before you buy
- Measure the tray, not the old battery — previous owners upsize too.
- Check the post layout against your cable reach; that’s the 27-vs-27F trap.
- Match the duty: starting, deep-cycle, or dual-purpose. The box says nothing about which one you’re holding — the label’s CCA and reserve capacity numbers do.