Rat Per-Diem Cost Calculator

NIH Guide Table 3.3 — Rat Floor Area

The NIH Guide (8th ed., 2011) Table 3.3 specifies minimum floor area requirements for laboratory rats. These values are floor area per individual rat in group housing — not per cage and not per pair.

• Up to 100 g: ≥17 $\text{in}^{2}$ per rat
• 100–200 g: ≥23 $\text{in}^{2}$ per rat
• 200–300 g: ≥29 $\text{in}^{2}$ per rat
• 300–400 g: ≥40 $\text{in}^{2}$ per rat
• 400–500 g: ≥60 $\text{in}^{2}$ per rat
• Over 500 g: ≥70 $\text{in}^{2}$ per rat

Floor area is the binding constraint for group housing, but cage height also matters: the Guide requires a minimum of 7 in (17.8 cm) ceiling clearance for adult rats, measured at the highest point an animal can stand on its hind legs. Most vendor cages exceed this comfortably.

Nursing dam-with-litter combinations require ~124 $\text{in}^{2}$ minimum (independent of dam weight) per the NIH Guide. Single-housed rats are an institutional choice and may require additional enrichment justification at AAALAC-accredited facilities.

Why Rat Housing Is Expensive

Rat per-diem is 2–3× the per-diem of mice at most US academic vivariums. The reasons compound:

A standard rat IVC rack holds ~40 rat cages compared to ~140 mouse cages. The same rack square-footage produces ~3.5× fewer billable cages.

Rats consume ~5–8× the feed and bedding mass of mice per day. Husbandry labor scales with bedding change frequency and weight handled.

Weighing 250–500 g, adult rats fit only 3–5 per medium cage compared to 5 mice in a smaller mouse cage. This dramatically increases cage count for the same animal count.

When planning a rat study, the per-diem cost is usually the dominant variable cost — often more than the drug or test article. Use this calculator at the protocol design stage to choose between rat and mouse models, or to size cohorts that fit a budget.