What the standard defines
A group size is a shorthand for a battery’s case: roughly its length, width and height, and the position and polarity of its terminals. It exists so that a vehicle, a boat or a locker designed around one battery can take another. Groups 24, 27 and 31 are the ones that recur on this site, because they are the sizes leisure and motorhome batteries are built to.
What is published is each maker’s own dimensions table. Those tables are what makes a group size usable: they give the millimetres the convention only implies.
What it does not promise
It promises nothing about capacity, chemistry, terminal type or weight. A lithium battery built to drop into a Group 31 tray can hold twice the usable energy of the lead-acid battery it replaces, and weigh half as much.
It is also not a tolerance. Two batteries of the same group can differ by enough millimetres to matter in a tight locker, and terminal height is a common surprise when a lid has to close.
What it works with
Group sizes decide which battery box fits, which hold-down strap reaches, and whether a replacement drops into an existing tray. They are read here alongside the maker’s published dimensions, which take precedence whenever the two disagree.
What people ask before they order
Will any Group 31 battery fit my Group 31 tray?
Usually, but the tolerance is a trade habit rather than a specification, and lithium replacements for a lead-acid group are sometimes a few millimetres different. The dimensions the maker publishes are what decides, and this site reports them.
Does the group size tell me the capacity?
No. It describes a case, not what is inside it. Two batteries of the same group can differ substantially in amp-hours, especially across chemistries.
Last reviewed 16 September 2026