We use 7 inch deep bus tubs for breeding and birthing. we put 1.3 to 1.4 in breeder tubs, and .1 to .2 females in birthing tubs. The neat part is that you can farm out babies to other moms because they don't care. Mom will feed every one in the tub. This helps if you have one litter of 4 or 5 and then one mom blows out 13. You can give some to the mom with few to help mom #2 out. Or you feed some off from one mom leaving one or two left. Take the one or two and give them to another mom. Then the first mom is free to enter the cycle again. FYI We use the large mortar tubs for wieners, and the small ones for hold backs and resting moms. I do not use the tubs from Lowes but Home Depot or Tractor Supply. They are heavier built.