I recently got three new baby male rats, and I needed a new cage for them. I knew that Martin's rat cages are well regarded, so I did some research on them. I solicited recommendations from a pet rat owner list (the "Ratlist"), and performed a number of calculations and comparisons of my own.

I calculated the amount of floor space in each cage by adding together the areas of the floor and shelves (not ramps). This measurement meant more to me than the cubic dimensions and volume of a cage, as my rats use the floor and shelves far more than they use the walls. I then calculated the price per square inch to get an idea of how much floor space I was getting for my money. I considered only the powder coated version of each cage.

In general, the consensus of the many replies sent to me was to get an R-680 or larger cage. The four favorite cages were the R-680 and R-685, and the tall R-690 and R-695. These would all be large enough for three boys. The smaller R-670 also got positive reviews, as did the larger R-699 Ruud cage. Here's the breakdown:

Cage number	Cage name	Scale Diagram (1/8 inch = 2 in)	Dimensions (W x D x H)	Shelf arrangement	Floor space (in2)	Price (powder coat)	Price/in2	Special features	Summary
R-670	The Rat Cabin		24 x 14 x 24	Two 1/3 shelves	560 in2	$56	$0.10 per in2	-	Several people own and like this cage, and find it quite roomy enough for 2-3 males, especially if the rats get a great deal of free range time outside the cage. It has room for hammocks, tunnels, a wheel, and toys, and it fits easily in a bathtub for cleaning. Others consider it a bit on the small side and prefer something larger. It is the cheapest cage on this list, but at $0.10 per square inch, its floor space is actually the most expensive.
R-680	The Rat Lodge		30 x 18 x 24	Two 1/3 shelves	900 in2	$74.75	$0.08 per in2	Large door	This cage has larger dimensions than the R-670, but the same two-shelf layout. The added width and depth allow for over 60% more floor space than the R-670 (900 vs. 560 in2). People also like the R-680 because of its large front door, which lets you remove large cage furniture like the Wobust Wodent wheel without dismantling the cage or the wheel. The R-685 and the R-690 series cages have smaller doors that do not let you do this.
R-685	The Rat Retreat		30 x 18 x 24	One full second floor	1080 in2	$90	$0.08 per in2	Flip-top lid	The R-685 has the same external dimensions and cubic volume as the R-680, but it has a different internal layout: it has a full second floor instead of two shelves. Because of this different internal layout, it has 20% more floor space than the R-680: 1080 in2 vs. 900 in2 , and is priced accordingly. Its flip-top lid allows easy access to the second floor.
R-690	The Rat Tower		24 x 14 x 36	One full second floor Two 1/3 shelves	896 in2	$76.75	$0.09 per in2	-	The R-690 has the same small tray size as the R-670, but is a foot taller and has an additional full second floor. Because of this additional floor, it has 50% more floor space than the R-670. In fact, it has almost exactly the same floor space as the R-680. It has 20% less floor space than the R-685. Its special advantage is that because of its smaller tray size, it fits into a bathtub, making cleaning easier than the R-680 series or the R-695. Its smaller size also makes it easier to handle than the R-695.
R-695	The Rat Skyscraper		30 x 18 x 36	One full second floor Two 1/3 shelves	1440 in2	$99.50	$0.07 per in2	-	The R-695 is a tall cage with the same larger tray size as the R-680 series, but it has the full second floor and two 1/3 shelf layout of the R-690. The result is more floor space: 1440 in2 , which is 60% more floor space than the R-690, and 33% more than the R-685. This cage is the largest of the regular rat cages made by Martin's, and at $0.07 per square inch it provides a lot of cage for the price. This is probably more space than necessary for three males, but its large size lets you (a) add more rats in the future, and (b) give the residents enough room to play even without a lot of free range time. But because of its size it is a bit difficult to handle, which makes cleaning more difficult.
R-699	The Ruud Cage		30 x 18 x 60	Two full additional floors Four 1/3 shelves	2340 in2	$170	$0.07 per in2	-	The R-699 is a tower formed by placing an R-680 on top of an R-695, and replacing the upper cage's tray with a full floor. The result is two full additional floors and four 1/3 shelves, for an inner floor space of 2340 in2 . This is a huge cage, standing 5 feet tall. To make it a bit more manageable, owners suggest attaching the top and bottom cages with temporary clips so the cage can be broken down during cleaning. A cage of this suze is luxurious overkill for three males. It is much loved by rats, but its enormous size and weight make it quite difficult to move, handle and clean.

Cages in order of least to greatest floor space:

R-670	R-690	R-680	R-685	R-695	R-699
560 in2	896 in2	900 in2	1080 in2	1440 in2	2340 in2

 

For those who are interested in what I ended up choosing, here was my thought process. I was looking for a cage for three male rats. For the rats' sake, I wanted the cage to be spacious, with lots of floor space. My rats don't get hours of free range time every day, so I wanted a cage they could romp around in. For my sake, I wanted a cage that had good door access, was relatively easy to handle and clean, and offered good bang for the buck. I have room for a large cage so size wasn't a constraint for me.

I chipped away at the above list. It was a tough decision, but I could only buy one cage! I eliminated the R-670 as a bit small for a full-time cage, and crossed off the R-699 as being just too large to handle (alas!). Between the R-680 and R-685, I liked the R-685's greater internal floor space and flip-top lid more than I liked the R-680's big door. Between the R-690 and R-695, I liked the greater width, depth and floor space and gentler ramp slope of the R-695, despite its increased bulk.

This narrowed the field to two cages: the R-695 and the R-685 -- the two cages with the most floor space after the Ruud cage. I loved the design of the R-685: its efficient use of internal space and its flip top lid make it a good all-around cage, and a nice compromise between the rats' need to have a lot of space and the human's need to have a cage that is easy to handle, access, and clean. But I was attracted to the larger size of the R-695, as my preference in cages has always been "bigger is better" (up to a point!). It was a tough call. I ended up asking Martin's if they could customize the R-695 with a flip-top lid, and they could ($10 extra). This should increase access nicely. I also discovered the extra handles, which would increase the ease of handling of such a large cage. So I ended up ordering the R-695, with a flip-top lid and two extra handles for easier access and handling.

The Martin's R-965 I ordered arrived in a big, flat cardboard box...	...which was stuffed with shredded newspaper:
The cage was shipped flat, along with the tray and other accessories:	When spread out, the parts looked like this:
Assembly didn't take very long -- maybe thirty minutes. We used cable ties instead of the metal c-clips so we could break the cage back down if we needed to some day:	Here is the assembled cage!
The rats explore their new cage for the first time:	With a nestbox to hide in, and hammocks to snuggle in, the rats settle in to their new home: