What available water capacity means
Available water capacity (AWC) is the depth of water a soil can hold that plant roots can actually pull back out. It sits between two limits: field capacity, the water content left in a well-drained soil once gravity has finished pulling the excess through, and the permanent wilting point, the point where a plant can no longer extract enough water to avoid lasting damage. Water above field capacity drains away; water below the wilting point stays in the soil but is held too tightly for roots to use. AWC is what's left in between.
Multiplied by a rooting depth, AWC becomes total available water (TAW): the size of the water "reservoir" a crop or landscape can draw on between irrigations or rain. That's the number this calculator produces. It is not the same as how full that reservoir is today — for current soil-water status, use the soil moisture calculator instead.
| Soil texture | AWC range (in/in) | Typical (in/in) |
|---|---|---|
| Sand | 0.01–0.03 | 0.02 |
| Loamy sand | 0.06–0.08 | 0.07 |
| Sandy loam | 0.11–0.13 | 0.12 |
| Loam | 0.16–0.18 | 0.17 |
| Silt loam | 0.19–0.21 | 0.20 |
| Silt | 0.16–0.18 | 0.17 |
| Sandy clay loam | 0.14–0.16 | 0.15 |
| Clay loam | 0.19–0.21 | 0.20 |
| Silty clay loam | 0.19–0.21 | 0.20 |
| Sandy clay | 0.15–0.17 | 0.16 |
| Silty clay | 0.15–0.17 | 0.16 |
| Clay | 0.14–0.16 | 0.15 |
These are the typical values this calculator uses when you pick a texture. If you know your soil's texture but not these numbers, the soil texture calculator can help identify it. For a specific mapped soil series, the soil and water quality calculators hub links to tools that use more site-specific data, including the NRCS Web Soil Survey.
These are ranges, not certainties. Two fields of the same named texture can differ because of structure, compaction and organic matter. Where you have lab or Web Soil Survey data for your specific soil, use it instead of the typical value.
How this calculator works
The calculator finds the available water capacity of each soil layer, converts it to a depth of water using that layer's thickness, then adds the layers together for the whole root zone.
1. AWC per layer
From soil texture
AWC = typical value for the selected texture (in/in)
Uses the typical value from Table 2–1 of the NRCS Irrigation Guide, shown above.
From your own values
AWC = Field capacity − Wilting point
Both entered as volumetric water content, in percent. The result is a ratio (in/in or mm/mm), so it carries over unchanged between unit systems.
2. Available water per layer
Layer available water
Layer AW = AWC × Layer depth
This is the same relationship FAO Irrigation and Drainage Paper 56 uses for total available water: TAW = 1000 × (θFC − θWP) × Zr, with the 1000 factor converting meters of root depth to millimeters of water.
3. Total for the root zone
Total available water (TAW)
TAW = Layer 1 AW + Layer 2 AW + Layer 3 AW
Only layers with a depth entered are included. Weighted-average AWC = TAW ÷ total depth.
Results are shown to two decimal places in US units and one decimal place in metric, matching the precision of the typical AWC values themselves. For a quick single-layer estimate, entering a texture in Layer 1 alone and leaving Layers 2 and 3 blank works fine — the layering is only there for profiles that genuinely change with depth.
When this calculator isn't the right tool
It calculates the size of the water-storage reservoir in your soil. Use a different tool for:
- How full that reservoir is right now. See the soil moisture calculator.
- How much of it you should actually use before stressing a crop. See the management allowed depletion calculator.
- How deep your crop's roots actually reach. See the root zone depth calculator.
- Identifying your soil's texture in the first place. See the soil texture calculator.
- Turning this total into an irrigation schedule. See the soil water balance calculator or the irrigation scheduling calculator.
Available water capacity calculation example
A vegetable bed has 8 inches of silt loam topsoil over 10 inches of loam subsoil, for an 18-inch root zone.
- Layer 1 (silt loam, 8 in): AWC 0.20 in/in × 8 in = 1.60 in.
- Layer 2 (loam, 10 in): AWC 0.17 in/in × 10 in = 1.70 in.
- Total available water: 1.60 + 1.70 = about 3.30 in over an 18 in root zone.
- Weighted-average AWC: 3.30 ÷ 18 = 0.18 in/in.
The same calculation in metric
The same profile in millimeters: 200 mm of silt loam over 250 mm of loam, a 450 mm root zone.
- Layer 1 (silt loam, 200 mm): 0.20 × 200 = 40.0 mm.
- Layer 2 (loam, 250 mm): 0.17 × 250 = 42.5 mm.
- Total available water: 40.0 + 42.5 = about 82.5 mm over a 450 mm root zone.
- Weighted-average AWC: 82.5 ÷ 450 = 0.18 (unchanged, since AWC is a dimensionless ratio).
Because AWC is expressed per unit of soil depth, the weighted average comes out identical in both unit systems — only the depths and volumes look different.
Measuring and using AWC
Finding field capacity and wilting point
The most accurate route is the USDA NRCS Web Soil Survey, which reports available water capacity for the specific mapped soil series on a property, already broken out by layer. A soil testing lab can measure field capacity and wilting point directly with a pressure plate: roughly one-tenth bar tension for field capacity in sandy soils, one-third bar in finer-textured soils, and 15 bar tension for the wilting point. Where neither is available, the texture-based typical values in this calculator are a reasonable starting point, particularly for medium-textured soils where the ranges in the table above are narrow.
Applying the result
Total available water is the ceiling, not a target. Most crops show water stress well before the soil actually reaches the wilting point, so irrigation scheduling typically works from a management allowed depletion (MAD): a percentage of TAW that's considered safe to use up before refilling. The management allowed depletion calculator turns this page's total into that usable slice. From there, the soil water balance calculator tracks how depletion changes day to day with rain, irrigation and crop water use.
Common mistakes
- Using surface texture for the whole profile. Subsoil often differs from topsoil; a two- or three-layer entry catches that, a single-texture guess doesn't.
- Ignoring restrictive layers. Bedrock, a hardpan or a shallow water table can cut the effective rooting depth well below what a plant's species would normally reach. Measure to the restriction, not past it.
- Skipping rock content and salinity. Coarse fragments and salts both reduce AWC below the typical texture values (see the factors below). Where either is significant, treat this calculator's result as an upper estimate.
- Confusing AWC with current soil moisture. AWC doesn't change day to day; how full the reservoir is right now does. Don't use a dry-season reading as if it were the soil's storage capacity.
What changes available water capacity
Texture sets the starting point, but several other soil properties push the real value up or down from the typical figures in the table above.
| Factor | Effect on AWC | What to adjust |
|---|---|---|
| Rock fragments | More coarse fragments leave less soil volume to hold water, lowering AWC. | Use lab or Web Soil Survey data for rocky soils; typical texture values will overstate storage. |
| Salinity | Salts make soil water harder for roots to extract, acting like reduced AWC. | Check the sodium adsorption ratio calculator and consider a leaching program if salinity is high. |
| Organic matter | Higher organic matter generally improves structure and increases AWC, especially in sandy soils. | Favor the higher end of a texture's typical range in well-managed, high-organic-matter soils. |
| Soil structure | Granular, blocky and prismatic structure hold water more favorably than massive or platy structure. | No calculator adjustment; note it when comparing a lab value with the texture-based typical. |
| Compaction | Denser soil has less pore space, which can lower AWC well below the typical value for that texture. | Use measured values where compaction is suspected, particularly under wheel tracks or turf traffic areas. |
| Depth to a restrictive layer | Bedrock, hardpan or a high water table caps how deep roots — and this calculator's layers — should go. | Stop entering layers at the restriction, not at the crop's typical maximum root depth. |
Depth within the soil profile matters too: the NRCS Irrigation Guide notes that deeper soils tend to be more consolidated, with less structure and organic matter, which is part of why a subsoil layer commonly has a different AWC than the topsoil above it — another reason to enter them as separate layers rather than one blended guess.
Frequently asked questions
What is available water capacity (AWC)?
Available water capacity is the depth of water a soil can hold that's actually usable by plant roots: the difference between field capacity (fully drained, wetted soil) and the permanent wilting point (the point where roots can no longer pull out any more water). It's usually expressed as inches of water per inch of soil, so it can be multiplied by any root zone depth. The USDA NRCS Irrigation Guide lists typical AWC by soil texture, from about 0.02 in/in for sand up to about 0.20 in/in for silt loam and clay loam.
How is available water capacity different from soil moisture right now?
AWC is the size of the reservoir, not how full it currently is. It tells you the maximum water your root zone could store between a soaking rain and the point plants wilt, based on soil texture and depth. How much of that capacity is actually filled on a given day depends on recent rainfall, irrigation and crop water use, which the soil moisture calculator estimates separately.
See the soil moisture calculator.
Where can I find my soil's field capacity and wilting point?
The USDA NRCS Web Soil Survey lists estimated available water capacity for the specific soil series mapped on your property, layer by layer. A soil testing lab can measure field capacity and wilting point directly, usually with a pressure plate test: about one-third bar tension for field capacity in most soils (one-tenth bar for sandy soils), and 15 bar tension for the wilting point. Without either, the texture-based typical values in this calculator are a reasonable starting point.
Why does available water capacity depend on soil texture?
Texture controls pore size, and pore size controls how tightly a soil holds water against gravity and root suction. Sandy soils have large pores that drain quickly, so they hold little usable water. Medium-textured soils like loam and silt loam have the best balance of pore sizes and hold the most available water. Very fine clay soils hold a lot of total water, but much of it is bound so tightly that roots can't extract it, so their available water capacity drops again.
What is the difference between available water capacity and readily available water?
Available water capacity, or total available water, is the full reservoir: everything between field capacity and the permanent wilting point. In practice, most crops start showing water stress well before the soil reaches wilting point, so growers only plan to use a fraction of that total, commonly 30 to 50 percent, called the management allowed depletion (MAD). The management allowed depletion calculator applies that percentage to the total this calculator gives you.
Does rock content or salinity change available water capacity?
Yes, both reduce it. Rock fragments take up space that would otherwise hold water, so the NRCS Irrigation Guide's correction tables lower AWC as coarse fragment content rises. Salts in the soil solution make water harder for roots to extract even when it's physically present, which has a similar effect to lowering AWC. Neither adjustment is built into this calculator, so treat its result as an upper estimate for rocky or saline soils.
How deep should I measure, and why does the calculator ask for layers?
Measure to your crop's effective root depth, or to a restrictive layer such as bedrock, a hardpan or a water table if that's shallower. Real soil profiles change texture with depth, so a topsoil and a subsoil layer often hold very different amounts of water per inch. Entering each layer separately and adding the results gives a more accurate total than applying one texture to the whole root zone.
Can I use this for container gardening or raised beds?
Yes, if you know or can estimate the growing medium's field capacity and wilting point, or its closest soil texture equivalent. Potting mixes and amended raised-bed soil often hold more available water than field soil of a similar texture because of added organic matter, so a lab test or manufacturer data will be more accurate than the general texture values here.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Formulas, unit conversions and both worked examples verified by hand against the NRCS Table 2–1 typical values.
- Layer-summing method cross-checked against the FAO–56 total available water equation and a published NRCS multi-layer example.
- AWC values quoted only where they appear in the cited NRCS table; no invented ranges.
Sources
- USDA Natural Resources Conservation Service. National Engineering Handbook, Part 652, Irrigation Guide, Chapter 2 – Soils (Table 2–1, Available Water Capacity by Texture). September 1997.
- Food and Agriculture Organization of the United Nations. Crop Evapotranspiration – Guidelines for Computing Crop Water Requirements (FAO Irrigation and Drainage Paper 56), Chapter 8, Equation 82.
- USDA Natural Resources Conservation Service. Available Water Storage (Web Soil Survey map unit description).
- University of Wisconsin–Madison Division of Extension. Irrigation Scheduling.