How to Measure a Soil Core
1. Collect a known volume
Press a rigid ring or core of known inside dimensions into representative, undisturbed soil. Avoid compressing the sample and trim it flush with both ends.
2. Dry and weigh
Transfer the complete sample and dry it to constant mass using the temperature and duration specified by the method you follow. Record the dry soil mass.
3. Calculate consistently
Use the core's actual occupied volume and keep a record of depth, texture, coarse fragments, sampling date and any corrections so repeated results are comparable.
Bulk Density and Porosity Formulas
Dry bulk density (ρb) = oven-dry soil mass ÷ total bulk sample volume
Cylindrical core volume = π × (inside diameter ÷ 2)² × filled length
Total porosity (%) = [1 − (bulk density ÷ particle density)] × 100
Solid fraction (%) = (bulk density ÷ particle density) × 100
Pore volume = total sample volume × porosity fraction
Void ratio = pore volume ÷ solid volume = (particle density ÷ bulk density) − 1
Bulk density uses the entire field volume occupied by the sample, including solids and pores. Particle density describes only the soil solids. Both densities must use the same units in the porosity formula; this calculator converts every input to grams and cubic centimetres first.
Worked Examples
Known-volume sample
A 265 g oven-dry sample occupying 200 cm³ has a bulk density of 1.325 g/cm³. With particle density 2.65 g/cm³, estimated porosity is 50%.
Cylindrical core
A 5 cm inside-diameter core filled to 5 cm has a volume of 98.17 cm³. If the oven-dry mass is 130 g, bulk density is about 1.324 g/cm³ and porosity is about 50.03% at 2.65 g/cm³ particle density.
Equivalent units
A result of 1.325 g/cm³ is also 1.325 Mg/m³, 1,325 kg/m³ and approximately 82.72 lb/ft³.
Interpretation and Important Limits
Do not use a universal “good” or “bad” cutoff. Root restriction and compaction interpretations depend on soil texture, clay mineralogy, organic matter, structure, coarse fragments, crop, depth and water condition. Compare like-for-like samples and use locally appropriate soil survey, laboratory or extension guidance.
Total porosity from density is an indirect estimate. It does not separate macropores from micropores, or air-filled pores from water-filled pores. Shrink–swell soils, fragile cores, roots, cracks, stones, sample loss and compaction during collection can all bias the result.
If coarse fragments are removed before weighing, the volume they occupied cannot remain uncorrected in the denominator. Correction methods vary with the reporting basis and intended use. Follow the relevant laboratory or soil-survey method rather than applying an undocumented adjustment.
The 2.65 g/cm³ default is commonly used for many mineral soils. Organic-rich soils, volcanic materials and soils with unusual mineralogy may have a different particle density. A measured particle density gives a better porosity estimate.
Calculation Basis and References
- USDA NRCS: Soil Quality Indicators — Bulk Density — defines bulk density as dry soil weight divided by total soil volume and describes the 2.65 g/cm³ mineral-particle assumption.
- USDA NRCS National Soil Survey Handbook, Part 618 — defines particle density and documents its use in porosity calculations and the limits of the 2.65 g/cm³ estimate.
- USDA NRCS: Soil Health Assessment — places bulk density and porosity among physical soil-health indicators and cautions against evaluating soil health from a single measure.
Soil Bulk Density and Porosity FAQs
How is soil bulk density calculated?
Divide the oven-dry mass of the soil by the total volume it occupied in the field. The bulk volume includes both soil solids and pore space.
How is total soil porosity estimated from bulk density?
Total porosity percent equals [1 − (bulk density ÷ particle density)] × 100. Both densities must use the same units.
Why is 2.65 g/cm³ used for particle density?
It is a common estimate for many mineral soils, not a universal value. Organic-rich, volcanic and unusual mineral soils can differ, so replace the default with a measured or locally supported particle density when available.
Should I use wet soil mass or oven-dry soil mass?
Use oven-dry mass, normally obtained by drying to constant mass under the selected laboratory method. Wet or air-dry mass includes water and overstates dry bulk density.
Are g/cm³ and Mg/m³ equivalent?
Yes. 1 g/cm³ equals 1 Mg/m³, 1 kg/L and 1,000 kg/m³.
Does a high bulk density always mean soil is compacted?
No single threshold fits every soil. Texture, organic matter, structure, coarse fragments, depth and sampling conditions affect interpretation. Compare representative samples from the same soil and depth with suitable local guidance.
How should coarse fragments be handled?
Do not remove stones from the dry mass while retaining their entire volume in the denominator. A fine-earth result requires a method-specific correction for the volume displaced by fragments. Record the method and follow the protocol for the intended use.
Are my soil measurements stored?
No. All calculation logic runs client-side in your browser, and this calculator does not send or store the values you enter.