Soil Porosity Calculator
Leave this at 2.65 unless you have measured it. It is the average density of mineral soil particles.
Optional. Loads the soil USDA maps at a US address to prefill and tailor the result.
The formula, explained
Porosity is the share of soil volume that is pore space, the air and water between the solid particles. You get it from bulk density without any extra measurement: f = 1 − bulk density ÷ particle density[1]. As soil density rises, porosity falls[3]: the same weight of particles packed into less volume leaves less room between them.
When 2.65 does not fit
The 2.65 g/cm³ is the average mineral-particle density, and it is used when particle density is not measured[1]. If you have a measured particle density for your soil, enter it in place of 2.65: for the same bulk density, a lower particle density gives a lower porosity (calculated).
Porosity at the root-restricting densities
NRCS lists the bulk density that restricts roots for each texture[4]. Run through the formula with 2.65, those densities leave about 32.1 to 44.5 percent pore space (calculated)[1]. The coarser the texture, the higher the density it takes to restrict roots, so sandy soils still restrict roots at a lower porosity than clays do.
| Soil texture | Restricts roots above (g/cm³) | Porosity there |
|---|---|---|
| sand, loamy sand | 1.80 | 32.1% |
| sandy loam, loam | 1.80 | 32.1% |
| sandy clay loam, clay loam | 1.75 | 34% |
| silt, silt loam | 1.75 | 34% |
| silt loam, silty clay loam | 1.65 | 37.7% |
| sandy clay, silty clay, clay loam | 1.58 | 40.4% |
| clay (<45% clay) | 1.47 | 44.5% |
Porosity by bulk density
The formula with the default particle density of 2.65 g/cm³ gives these values (calculated)[1]. Around 1.3 g/cm³, a soil sits near the 50% considered ideal[2].
| Bulk density (g/cm³) | Porosity |
|---|---|
| 1.0 | 62.3% |
| 1.1 | 58.5% |
| 1.2 | 54.7% |
| 1.3 | 50.9% |
| 1.4 | 47.2% |
| 1.5 | 43.4% |
| 1.6 | 39.6% |
| 1.7 | 35.8% |
| 1.8 | 32.1% |
How this calculator works
It computes f = 1 − Db ÷ particle density, where Db is your bulk density, then shows it as a percent[1]. The particle density defaults to 2.65 g/cm³, the value used when it is not measured[1]. A bulk density entered in lb/ft³ is divided by 62.428, calculated from NIST’s conversion factor[5]. The verdict calls a result within 2 points of 50% “about ideal”; that band is ours, not a published one. With Use my soil, USDA’s mapped bulk density for your address is used[6].
Frequently asked questions
What is soil porosity?
It is the share of a soil’s volume that is pore space, holding air and water; the rest is solid particles[1].
What is a good porosity for soil?
Around 50% is considered ideal, which leaves room for both air and water[2].
Does porosity tell me how much water soil holds?
Only partly. Pore size matters too: water in the large pores drains away too fast for plants, water in the medium pores is what plants use, and the smallest pores hold water too tightly[7]. The water holding capacity calculator gives the plant-available water.
Why use 2.65 for particle density?
It is the average density of mineral soil particles, used when particle density is not measured directly[1].
Sources
- Oklahoma State University Extension PSS-2287, Basics of Soil Bulk Density
- South Dakota State University Extension, Bulk Density Is an Indicator of Soil Health
- Kansas State University, Soils Laboratory Manual, ch. 8: Soil Density (Moorberg and Crouse)
- USDA NRCS Soil Health Guide, Bulk Density
- NIST, Guide for the Use of the International System of Units (SP 811), Appendix B.8: Factors for Units Listed Alphabetically
- USDA NRCS, Soil Survey Geographic Database (SSURGO), via Soil Data Access
- Kansas State University, Soils Laboratory Manual, ch. 9: Soil and Water Relationships (Moorberg and Crouse)
Marko Visic · MPharm, BSc Physics · Last reviewed November 13, 2026