Gravimetric vs. volumetric soil moisture
Soil moisture can be described two ways, and this calculator gives you both. Gravimetric water content is a weight ratio: the mass of water in a sample divided by the mass of the same sample once it's oven-dried. It's the number a lab produces directly from a wet-weight/dry-weight measurement, and it's expressed as grams of water per gram of dry soil, or as a percentage.
Volumetric water content converts that weight ratio into a fraction of the sample's total volume, using the soil's bulk density. According to USDA NRCS, water content in grams per cubic centimeter equals the gravimetric water content times the bulk density, because one gram of water occupies exactly one cubic centimeter. Volumetric water content is the more directly useful of the two for irrigation, because it shares the same basis as rainfall and application depths, and it's what lets you convert a lab result into an actual depth of water.
| Measure | What it is | Typically used for |
|---|---|---|
| Gravimetric water content | Water mass ÷ dry soil mass | The direct lab result; comparing samples by weight. |
| Volumetric water content | Gravimetric content × (bulk density ÷ water density) | Comparing to field capacity/wilting point; converting to a depth. |
Both numbers describe the same sample; volumetric content is always the gravimetric content scaled by how dense the dry soil is. A sandy soil and a clay soil holding the same gravimetric water content will have different volumetric water contents, because they pack a different mass of solids into the same volume. That's why bulk density is required for this calculator, not optional. For the amount of water your soil can hold between saturation and permanent wilting, rather than what it holds right now, see the available water capacity calculator.
How this soil moisture calculator works
The calculator works out how much water your sample lost when it dried, expresses that as a fraction of the dry soil's weight, then rescales it into a fraction of the sample's volume using your bulk density. Multiplying that fraction by the depth your sample represents gives an equivalent depth of water.
1. Water mass and dry soil mass
Water mass
Water mass = Wet weight − Dry weight
If you used the same container for both weighings, its weight cancels out here and doesn't need to be known for this step.
Dry soil mass
Dry soil mass = Dry weight − Container weight
Container weight is 0 if your weights are already net soil weight.
2. Gravimetric water content
Gravimetric water content
Gravimetric water content = Water mass ÷ Dry soil mass
Shown as a percentage. This is USDA NRCS's standard soil water content formula.
3. Volumetric water content
Volumetric water content
Volumetric water content = Gravimetric water content × (Bulk density ÷ Density of water)
Density of water is 1 g/cm³ (62.43 lb/ft³). This works because one gram of water occupies one cubic centimeter, so the mass ratio converts directly into a volume ratio.
4. Equivalent depth of water
Depth of water
Depth of water = Volumetric water content × Sample depth
USDA NRCS applies this the same way to convert water content per cubic centimeter into inches of water per foot of soil depth.
5. Solids, water and air (the composition bar)
Porosity
Porosity = 1 − (Bulk density ÷ Particle density)
Particle density is assumed to be 2.65 g/cm³ (165.4 lb/ft³), the typical value for mineral soil quartz and silicate particles, unless you've measured your own.
The air-filled share is porosity minus the volumetric water content. If your calculated volumetric water content comes out higher than the estimated porosity, the calculator flags it: that combination isn't physically possible, so double-check the weights, container weight and bulk density.
Typical bulk density by soil texture
| Soil texture | Ideal for plant growth | Restricts root growth |
|---|---|---|
| Sand, loamy sand | under 1.60 g/cm³ | over 1.80 g/cm³ |
| Sandy loam, loam | under 1.40 g/cm³ | over 1.80 g/cm³ |
| Silt, silt loam | under 1.40 g/cm³ | over 1.75 g/cm³ |
| Sandy clay loam, clay loam | under 1.40 g/cm³ | over 1.75 g/cm³ |
| Sandy clay, silty clay | under 1.10 g/cm³ | over 1.58 g/cm³ |
| Clay (over 45% clay) | under 1.10 g/cm³ | over 1.47 g/cm³ |
These are ranges for typical, undisturbed mineral soil, not a value to type in without measuring: two soils of the same texture can differ enough to change your result noticeably. If you don't know your texture, the soil texture calculator estimates it from a sand/silt/clay breakdown or a simple field test.
When this calculator isn't the right tool
It's built to convert one sample's own wet and dry weight into a moisture reading. Use a different tool for:
- How much water your soil can hold between field capacity and wilting point, rather than what it holds right now. See the available water capacity calculator.
- Deciding when to irrigate based on how much of that available water has been used up. See the management allowed depletion calculator.
- Measuring bulk density itself from a core sample's dry weight and volume, if you don't have one yet.
- Tracking soil moisture over days or weeks from weather and irrigation data instead of a physical sample. See the soil water balance calculator.
Soil moisture calculation example
A field-moist sample weighs 150 g. After drying at 105°C to a constant weight, it weighs 130 g. The sample was weighed in a small tin that weighs 10 g empty, the soil's bulk density is 81.2 lb/ft³ (about 1.30 g/cm³), and the sample represents the top 6 in of soil.
- Water mass: 150 g − 130 g = 20 g.
- Dry soil mass: 130 g − 10 g = 120 g.
- Gravimetric water content: 20 ÷ 120 = 0.1667, or 16.7%.
- Volumetric water content: 0.1667 × (81.2 ÷ 62.43) = 0.1667 × 1.3007 = 0.2168, or about 21.7%.
- Depth of water: 0.2168 × 6 in = 1.30 in over the sampled 6 in.
The same calculation in metric
A 200 g field-moist sample, from the same 20 g tin, weighs 170 g after oven-drying, with a bulk density of 1.4 g/cm³ measured on a 15 cm sample depth:
- Water mass: 200 − 170 = 30 g.
- Dry soil mass: 170 − 20 = 150 g.
- Gravimetric water content: 30 ÷ 150 = 0.20, or 20%.
- Volumetric water content: 0.20 × (1.4 ÷ 1) = 0.28, or 28%.
- Depth of water: 0.28 × 15 cm = 4.2 cm over the sampled 15 cm.
Notice the second sample has a higher gravimetric water content (20% vs. 16.7%) but its volumetric water content is closer to the first once the denser bulk density (1.4 vs. 1.30 g/cm³) is factored in. Comparing gravimetric numbers alone across soils of different density can be misleading, which is exactly why the volumetric figure exists.
Taking a reliable soil sample
The arithmetic in this calculator is exact; the result is only as good as the sample and weighings that go into it. A few practices make the biggest difference.
Weigh the wet sample immediately
Water is lost to evaporation from the moment a sample is exposed to air, especially from a small, high-surface-area sample. Weigh it in a sealed container or bag as soon as you can after collecting it, and note that weight before you do anything else.
Dry it fully, not just partly
USDA NRCS's standard reference procedure oven-dries the sample until its weight stops changing between checks, commonly around 105°C (221°F) for 24 hours or more, since drying time depends on sample size and soil type. For a quicker field check, a microwave on medium power in 4-minute cycles works too; reweigh after each cycle and stop once the weight holds steady.
Keep the container weight consistent
If you weigh the sample in a can, tin or bag, use the exact same container for the dry weighing, and enter its empty weight in the container field. Mixing up containers, or forgetting to account for one, is one of the most common sources of error in this calculation.
Match the bulk density to the same layer
Bulk density should come from the same depth interval as your moisture sample, ideally from a core taken alongside it. Bulk density generally increases with depth, since subsurface soil has less organic matter and aggregation, so a topsoil bulk density used with a subsoil moisture sample will skew the volumetric result.
Sample depth vs. root zone depth
The depth you enter here should match the layer your sample was pulled from, not necessarily the full root zone. To estimate an appropriate root zone depth for a crop or turf area, use the root zone depth calculator, then run this calculator once per layer if the root zone spans more than one soil layer, and add the resulting depths together.
What changes the result
Once the arithmetic is right, most differences between a calculated result and what you expect trace back to one of these.
| Factor | Effect on the result | What to check |
|---|---|---|
| Incomplete drying | Leaves residual water in the "dry" weight, understating both water contents. | Re-check weight after another drying cycle; it should be unchanged. |
| Evaporation before weighing | Understates the wet weight and every result derived from it. | Weigh the wet sample in a sealed container as soon as possible. |
| Container mismatch | A missed or wrong container weight throws off the dry soil mass and gravimetric content. | Use the same container for both weighings; enter its weight once. |
| Bulk density from the wrong layer or texture | Volumetric content and depth of water scale directly with bulk density. | Use bulk density measured from the same depth as this sample. |
| Sample not representative | Rocks, roots or a disturbed sample skew both weight and volume. | Avoid coarse fragments and take multiple samples where soil varies. |
| Compaction | Raises bulk density, which raises volumetric water content for the same gravimetric reading. | Compare your bulk density to the texture table above. |
If your sample sits in unusually wet, recently irrigated or waterlogged soil, check the water-filled share against the composition bar's air segment: very little remaining air-filled space alongside a high volumetric reading is consistent with a saturated or nearly saturated sample, not a data-entry error.
Frequently asked questions
What's the difference between gravimetric and volumetric soil moisture?
Gravimetric water content is the weight of water in a sample divided by the weight of the same sample once it's oven-dried, expressed as grams of water per gram of dry soil. Volumetric water content converts that into a fraction of the sample's total volume, using the soil's bulk density. Volumetric water content is the one used to convert moisture into a depth of water, since it shares the same basis as rainfall and irrigation depths.
How do I get the wet and dry weights this calculator needs?
Weigh a fresh, field-moist soil sample right after collecting it, then dry it in an oven at about 105°C (221°F) until the weight stops changing between checks, usually 24 hours or more, and weigh it again. If you used a can or bag for both weighings, enter its empty weight in the container field so it's subtracted correctly.
What bulk density should I use if I haven't measured mine?
It's best to measure it, since bulk density varies a lot even within one soil texture. USDA NRCS guidance lists ideal bulk densities of under 1.60 g/cm³ for sandy soils, under 1.40 g/cm³ for silty soils and under 1.10 g/cm³ for clayey soils, with higher values indicating compaction. Use the low end of the range for your texture only as a rough placeholder, and measure your own with a core sample when you can.
Why does the calculator need the sample depth?
Volumetric water content is a fraction of soil volume, and multiplying it by the depth the sample represents turns that fraction into an actual depth of water, the same units used for rainfall and irrigation. If your sample came from the top 6 inches (15 cm), enter that depth; for a full root zone, add up the depths of each layer you sampled.
Can I dry the sample in a microwave instead of an oven?
Yes, USDA NRCS field guidance for educators uses a microwave on medium power in 4-minute cycles, reweighing after each cycle until the weight stops changing. It's a practical field method, though a standard drying oven at a set temperature gives more consistent results for lab or research work.
Does the container or bag weight actually matter?
Only for the dry soil mass, not for the water mass. If you use the same container for both the wet and dry weighings, its weight cancels out when you subtract dry from wet to find the water mass, but you still need to subtract it once to get the dry soil weight the gravimetric ratio is based on.
How accurate is this method compared to other soil moisture tools?
The oven-dry (gravimetric) method is the reference method most other soil moisture tools, including capacitance and TDR probes, are calibrated against. Its main sources of error are an unrepresentative sample, incomplete drying, or a bulk density that doesn't match the sampled soil, not the arithmetic itself.
What does the water-filled, air-filled and solids breakdown assume?
It assumes a typical mineral soil particle density of 2.65 grams per cubic centimeter, used to estimate total pore space (porosity) from your bulk density. Soils high in organic matter have a lower particle density, so the air-filled share shown for those soils will be a slight underestimate.
For the total water your soil can store, see the available water capacity calculator.
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 USDA NRCS's gravimetric method.
- Bulk density ranges checked against the NRCS sources below.
- The composition bar's porosity assumption and its 2.65 g/cm³ particle density constant are stated explicitly, not hidden in the math.
Sources
- USDA Natural Resources Conservation Service. Soil Health – Bulk Density/Moisture/Aeration (Guides for Educators).
- USDA Natural Resources Conservation Service. Bulk Density – Soil Health Guide.
- USDA Natural Resources Conservation Service. Soil Quality Indicators: Bulk Density.
- Oklahoma State University Extension. Basics of Soil Bulk Density.
- South Dakota State University Extension. Bulk Density is an Indicator of Soil Health.