The average bathtub is designed to fit one average-size person comfortably with enough room to recline. Bathtub capacity is calculated as if no one is bathing, so the actual amount of water a bath can hold with a bather is less than the given figures. Knowing a tub's capacity is important when calculating your monthly water bill or considering the environmental impact of your family's water usage.

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The average bathtub holds between 25 and 45 gallons of water.

Standard Tub Capacity

The capacity of a standard tub is directly related to its size. According to Alliance For Water Efficiency, a standard non-jetted bathtub holds between 25 and 45 gallons of water approximately. The average rectangular bathtub is about 30 inches wide and 5 feet long, according to Plumbing-Basics. They come in many different shapes, however, such as circular or rounded models, and this may affect capacity.

Water Usage

Water usage varies significantly depending on how high you fill the tub. If you fill it halfway, you're using around 20 gallons. If you fill it up to or exceeding the overflow valve, you're using between 40 and 50 gallons. Compare this to a shower, which uses about 20 gallons in five minutes. The duration of a bath, on the other hand, does not change the amount of water used. Whirlpool tubs are less efficient, using approximately 80 gallons of water per bath.

Saving Water

You can save water each time you bathe by following a few simple water-saving tips. The EPA recommends plugging the drain immediately and adjusting the water temperature gradually as you fill the bathtub. If you plan to bathe for 15 minutes or more, taking a bath may be preferable to taking a shower. However, a short shower is the best option for saving water, particularly if you have a showerhead with 2.0 to 3.5 GPM (gallons per minute).

Reusing Greywater

If you're concerned about the amount of water you're wasting by taking a bath, consider reusing it for irrigation or other purposes. Reused bath and shower water, often referred to as greywater, is safe to use on lawns, trees and food crops. New Mexico State University states that you should apply greywater directly to soil and avoid contact with the above-ground portion of the plants. Consult a licensed plumber or water treatment specialist before installing a greywater irrigation system.