The best way to measure cage volume is by using water rather than performing a calculation using the length, width and height. There are several reasons for this:

• Often cage corners are rounded – if you use the standard L x W x H calculation, your calculated value will probably be larger than the actual cage volume.
• Often the sides of the cage are not perpendicular to the floor. This allows cages to be nested while in storage or during changes, but will probably cause error in a volume calculation. If you measure the top of the cage for use in your calculations, most likely your result will be a significantly higher volume than the actual cage. If you use the bottom dimensions of the cage in your calculation, most likely you will underestimate the actual cage volume.
• Many times a cage manufacturer will list specifications for their cages. It is rare to see a volume measurement in the specifications, but very common to see L x W x H values. These values are often produced to allow calculations for space requirements either for the animals or for warehousing of materials. Without knowing which dimensions the published values represent, there can be significant error in the calculation.
• Use of the correct value. To get an accurate volume measurement, you should use internal dimensions (ID) rather than outer dimensions (OD). Surprisingly, this difference in wall thickness can have a fairly large effect on calculation error.

Having done many of these calculations and comparisons, it is not unusual to find a discrepancy of 10 – 20% volume variation between the two methods outlined below. The water weight calculation is certainly more accurate. That being said, to get a rough idea of the cage volume, doing a quick L x W x H calculation is acceptable and easy.

Calculate the Volume of a Cage by Weighing Water

To get a more accurate measurement of cage volume and avoid all of the nuances outlined above, just use water! Since water has a density very close to 1 g / 1 mL, you can measure the weight of the water required to fill the box to get the volume. For example – if the weight of the water required to fill the cage is 5,000 grams, then the volume of the cage is very close to 5,000 mL or 5 liters. To perform this measurement:

1. Identify a scale with a capacity greater than the expected volume of the cage. Mouse cages will typically require up to 20 pound capacity and rat cages may go to 50 pounds or more once water is added. Most shipping scales are able to work in this range.
2. Place the cage that is being measured on the scale and either record the weight or tare the scale. The weight of the cage itself should not be counted in the final calculation.
3. Fill the cage with water until it is to the brim. Since the density of 1 g/mL is based on pure water, if you have the ability to use reverse osmosed (RO), deionized or distilled water that would be best. Even very poor tap water though should not cause more than a 2% error in the calculation (the density of sea water is ~1.025 g/mL so hopefully your tap water has less minerals dissolved in it than that!). Temperature can also have an effect on the density of water, but the effect is so small that it can be ignored if you are doing the test at room temperature.
4. Record the weight of the water filled cage.
5. Enter your data in the table below to get the volume in liters.

Calculate the Volume of a Cage by Measuring

While this calculator has limited accuracy for the reasons noted above (that cages are rarely true rectangular prisms), it can still be used to get a rough estimate of the volume of your cages. For the best results, attempt to measure the internal dimensions. Enter your measurements in the table below to calculate volume.