Filtration Size Calculator

Calculator Inputs

Enter soil particle sizes from a gradation curve. Use D15 and D85 for the core checks.

Filter type

Choose based on your detail and submittal requirements.

Criteria set

Switch to explore tradeoffs between retention and flow.

Particle size unit

All results are shown in millimeters.

Soil D15

15% passing size from gradation curve.

Soil D50 (optional)

Median size. If blank, an estimate is used.

Soil D85

85% passing size from gradation curve.

Safety factor (0.60–2.50)

Higher values tighten the retention limit.

Clogging sensitivity

Tighter openings when clogging is a concern.

Target flow (optional)

L/s·m²

Used only to trigger a high-flow caution note.

Reset

Formula Used

The calculator evaluates two common screening checks using soil gradation sizes:

Retention: D15(filter) ≤ k_R × D85(soil) × C / SF
Permeability: D15(filter) ≥ k_P × D15(soil) / C

Here, k_R and k_P come from the chosen criteria set, C is a clogging adjustment, and SF is a safety factor. For geotextiles, O95 is estimated from D85 with a conservative multiplier.

How to Use

Read D15, D50, and D85 from your soil gradation curve.
Select units that match your gradation report.
Choose a criteria set based on risk and hydraulics.
If clogging is a concern, enable tighter openings.
Press Calculate and review the feasible D15 range.
Export the results for your submittal package.

Example Data Table

Sample inputs and typical output ranges for quick reference.

Soil D15 (mm)	Soil D50 (mm)	Soil D85 (mm)	Criteria	Recommended D15 Range (mm)	Selected D15 (mm)	Estimated O95 (mm)
0.12	0.35	0.80	Balanced	0.48 – 3.20	1.24	1.20
0.05	0.18	0.40	Conservative	0.20 – 1.02	0.45	0.41
0.30	0.70	1.50	More permeable	1.50 – 7.50	3.35	2.70

Example outputs are illustrative and may differ with safety factor and clogging settings.

Notes for Construction Use

Use consistent lab methods for soil sizes and percent passing.
High fines content can require additional clogging checks.
Transition layers may be needed when checks conflict.
Confirm geotextile properties using manufacturer certification.
Verify hydraulic gradients, thickness, and drainage paths.

Particle Size Inputs and Gradation Curves

Use reliable sieve and hydrometer results to define D15, D50, and D85. D15 represents the finer fraction governing permeability, while D85 reflects the coarser fraction that can drive soil particle migration. Always confirm units and report method, then use the same dataset for design checks and submittals.

Retention Check and Soil Migration Control

Retention focuses on limiting the filter opening so base soil does not pipe through. The calculator applies a retention limit by relating filter D15 to soil D85 with an adjustable criterion factor and safety factor. Conservative settings are appropriate near structures, steep gradients, or where loss of fines threatens stability.

Permeability Check and Hydraulic Performance

Permeability requires the filter to pass water faster than the protected soil, reducing uplift pressure and preventing pore pressure buildup. The tool provides a minimum D15 based on soil D15 and a clogging adjustment. If the minimum exceeds the maximum, consider a transition layer, a coarser drain, or a different filter class.

Geotextile Versus Granular Selection Notes

Geotextiles offer consistent opening size and quick installation, but they can clog if fines or biological growth are expected. Granular filters can self heal and are tolerant of some sediments, yet they require controlled gradation and placement. Use the estimated O95 and the suggested D15 range to shortlist products or aggregate blends.

Documentation, Testing, and Field Verification

Record input assumptions, the chosen criteria set, and the final design value. For critical work, verify compatibility with project specifications, manufacturer certifications, and laboratory permittivity or gradient ratio testing. During construction, inspect overlap, seams, contamination, and drainage path continuity to ensure the selected filter performs as intended.

When working with broadly graded soils, review uniformity and gap grading because a single Dx value may hide instability. For drains under pavements or walls, document thickness, confinement, and expected head loss so reviewers can trace performance. Keep a QA checklist: verify material certification, avoid segregation, protect the surface from mud, and maintain outlet protection.

FAQs

What does D15 mean in this calculator?

D15 is the particle size where 15% of the soil is finer. It influences permeability and helps set the minimum filter opening required to transmit flow without excessive head loss.

Why is D85 used for retention?

D85 represents the coarser fraction that can mobilize during seepage. Relating filter opening to D85 is a common screening approach to reduce soil migration and piping risk.

What if the recommended D15 range is not feasible?

If the minimum D15 exceeds the maximum, add a transition layer, adjust the criteria, or select a different filter type. You may also need better gradation data or laboratory verification.

How should I choose the safety factor?

Use higher factors where consequences are high, gradients are steep, or uncertainty is large. For routine drainage with good data, values near 1.0 are common, but project specifications should govern.

Is the estimated O95 the same as a product specification?

No. It is an estimate to help shortlist geotextiles. Always use certified manufacturer data for AOS/O95, permittivity, and survivability, and confirm the selection against the project requirements.

Can I use microns or inches for inputs?

Yes. Select the input unit and enter D-values accordingly. The tool converts everything to millimeters internally and reports results in millimeters for consistent comparison.