Soil Water Holding Capacity Calculator

What available water capacity means

Available water capacity is the water held between field capacity and the wilting point that plants can actually take up, given per inch of soil. Add it up over the root zone for the total a plant can draw on^[1]. For how we read USDA’s values, see our methodology.

Field capacity is the water a soil still holds once the excess has drained away, which USDA records as the water content at one-third bar. The wilting point is the water held at 15 bar, too tightly for roots to pull out^[1][2]. To see where a sample you dug today sits between the two, use the soil moisture content calculator.

Water held per foot by texture

Here is the median plant-available water USDA records for each texture class, as inches of water per foot of soil^[2]. Coarse soils hold the least; medium and silty soils hold the most. If you don’t know your texture, the soil texture triangle finds it from sand, silt and clay percentages.

When to water

The total is a reserve that plants draw down between rain or watering. USDA’s irrigation handbook says most crops should be irrigated before more than about half of the available moisture in the root zone has been used, and that each irrigation should apply enough to bring the root zone back to field capacity^[3]. For the top foot of loam, which holds about 1.92 inches^[2], that means watering by the time about 0.96 inches is gone (calculated).

Measure it at home: the cup test

Kansas State University’s soils lab manual measures water holding with plastic cups^[4]. A home version:

1. Punch small holes in the bottom of a cup and line it with a piece of paper towel so soil can’t fall through.
2. Add a weighed amount of dry soil.
3. Stand the cup in a pan of water so the soil wets from the bottom up until it’s saturated.
4. Lift it out, let it drain for a day, and weigh it: the soil is now near field capacity.
5. Dry the soil and weigh it again. The weight it lost is the water it held^[4].

Divide the water by the dry soil’s weight for a percentage. The soil moisture content calculator does that math and shows how to dry a sample in a microwave.

Peat, vermiculite and perlite

In potting mixes, vermiculite holds large amounts of water and nutrients, perlite doesn’t absorb water and is added for drainage, and peat is added to raise the mix’s water-holding capacity or lower its weight^[5]. That advice is for container mixes.

In the ground, adding organic matter raises how much water soil holds, most of all in sandy or coarse soil^[6]. Our guides to improving garden soil and improving sandy soil cover how much to add.

How this calculator works

Available water is the available water capacity times the depth, summed by layer^[1]. The same arithmetic turns any water content into a depth of water: Kansas State’s example is a 25 cm layer at 20% water by volume, which holds 5 cm of water^[4]. The median available water capacity for each texture class comes from USDA’s texture dataset^[2]. With “Use my soil”, the sum runs layer by layer over the depth you set, and the whole profile’s water-storage class is USDA’s own rating^[1].

Gallons use 623.4 US gallons per inch of water over 1,000 sq ft, calculated from NIST’s conversion factors for cubic inches and gallons^[7].

Limits: a texture median describes the class across the country, not your garden. “Use my soil” reads the layers USDA recorded for the main soil mapped at your point, and USDA maps soils at scales from 1:12,000 to 1:63,360, so the result describes the mapped soil rather than a test of your yard^[2]. Set the depth to how deep your plants’ roots reach.

Frequently asked questions