What bar spacing do rats need?

Half an inch, or 13mm. The MSPCA publishes 1/2in apart as the figure for rats, and the RSPCA publishes the functional test behind it: the spacing must be small enough to stop a rat poking its head through, because once the head is through the whole body follows. Three-quarter-inch and one-inch bars are both common on cages that rats are routinely housed in, and both are large enough for a young or small rat to get out of.

This is the shortest decision on the whole of rat housing and the one most often got wrong, because it is invisible in a photograph. Two cages shot from six feet away look identical. The difference between them is a gap you could cover with a fingernail, and it decides whether the cage works.

The mechanism, stated precisely

A rat's skull is the widest rigid part of it. Everything behind the skull — ribcage, shoulders, hips — compresses. That is why the RSPCA's test is phrased the way it is, and the phrasing is worth quoting rather than paraphrasing: the bar spacing should be small enough to stop your rats from being able to poke their head through, because once the head is through the whole body will follow.

There is no partial version of this. A rat does not half-escape. It either cannot get its head through the gap, in which case the cage holds it indefinitely, or it can, in which case the cage holds it only until it decides otherwise. Everything else about an enclosure is a matter of degree. This is a switch.

It is also not a fixed property of the animal. A rat that was too big for a one-inch gap at nine months is not necessarily too big for it at two and a half years, when it has lost condition. The same is true after an illness, and it is permanently true of small females and of anything under about four months. An owner who measured once and concluded the cage was fine measured the wrong thing: the cage has to hold the smallest animal that will ever live in it, in the worst condition it will ever be in.

Which cages pass, and which do not

The important row is the Ferret Nation. It is, as far as frame, doors, pans, ramps and floor area go, the same cage as the Critter Nation, and it usually costs less. The entire difference is the bar gap. We cover the two side by side in the Critter Nation comparison and in the full rat cage roundup, and the conclusion is the same in both places: for rats, the cheaper of the two identical cages is the wrong one.

How to measure a gap you are unsure about

1. 1.Measure the gap, not the pitch. Bar pitch is center to center and includes the thickness of the wire. The number that matters is the clear air between two bars. On a cage with 2mm wire, a 15mm pitch is a 13mm gap, and the listing will often quote whichever is more flattering.
2. 2.Measure in several places. Door frames, corner posts and the junction between a cage and its stand frequently have a wider gap than the panels do, and an animal only needs one.
3. 3.Use a coin if you have no gauge. A US dime is 17.9mm across and a penny is 19.05mm. If either passes through the gap flat, the spacing is well over half an inch and the cage fails. This is rough and it is enough to settle the common case.
4. 4.Check horizontal and vertical bars separately. Some cages run tight horizontal bars on the panels and a wider vertical mesh on the doors.
5. 5.When in doubt, assume it fails. The cost of being wrong in one direction is a cage that climbs slightly less well. The cost of being wrong in the other is an animal in the wall cavity.

What to do about a cage you already own

Most people reading this already have the cage. The fix is half-inch galvanized hardware cloth, cut to the panel size and cable-tied over the inside of the bars. It is cheap, it takes an afternoon, it is visibly a bodge, and it works.

Two cautions on the material, both of which this site takes seriously elsewhere. Galvanized wire carries zinc, and zinc is toxic if ingested — the VCA names it specifically in its chinchilla housing guidance as a reason not to use galvanized wire. Rats chew. So this is a stopgap rather than a permanent build, it should be inspected monthly for chewed edges, and it is not the same thing as buying the right cage. Cage materials and toxicity covers galvanizing, powder coating and zinc in full.

The second caution is edges. Hardware cloth cuts leave spurs that will open a foot. Fold or file every cut edge, and run a hand over the whole panel before the animals go back in.

What bar spacing does not decide

It is worth being clear about the limits of this specification, because it is easy to leave a page like this believing a half-inch cage is automatically a good cage. It is not. The Critter Nation single unit has perfect bar spacing and one tier at 65cm of height, which fails the published consensus on both counts. Bar spacing is a veto, not a recommendation: it removes cages from consideration and never puts one on the list by itself.

The other thing it does not decide is climbing. Horizontal bars at half an inch give a rat much less purchase than the wider spacing on a ferret cage, which is a genuine loss for an animal that climbs as much as this one does. The answer is not wider bars. It is rope, hammocks, branches and the vertical space the cage size guide argues for — and that is a furnishing problem with a cheap solution, which an escape is not.