Subbase Thickness Calculator

Design subbase layers with clear, practical inputs fast. Choose units, run scenarios, compare outcomes easily. Export results to share with crews and clients confidently.

Calculator Inputs

Choose inputs and cost basis in your preferred system.
Sets practical minimum thickness for constructability.
Higher traffic increases required granular thickness.
Lower CBR means weaker soil, so thickness increases.
Water reduces support and increases deformation risk.
Adds thickness buffer where freeze-thaw is common.
Lower quality needs more thickness for the same support.
Compaction below 95% increases required thickness.
Use higher values for uncertainty or poor controls.
Longer life slightly increases thickness in this model.
Choose how you want to supply the project area.
Meters (metric) or feet (imperial).
Meters (metric) or feet (imperial).
m² (metric) or ft² (imperial).
Cost per m³ for placed subbase.
Typical range: 1.6–2.2 t/m³ depending on aggregate.
Used to estimate the number of delivery loads.

Formula Used

This calculator uses a practical, CBR-based thickness model that scales granular thickness with loading, durability, and site risk multipliers:

T = max(Tmin, round5mm((K × TI × L × D × M × C / √CBR) + F) × SF)
  • T = recommended subbase thickness (mm)
  • K = calibration constant (170)
  • TI = traffic index (light→industrial)
  • L = service life multiplier
  • D = drainage multiplier
  • M = material quality multiplier
  • C = compaction multiplier
  • F = frost allowance (mm)
  • SF = safety factor
  • Tmin = minimum thickness by project type (mm)

Note: Field conditions vary. For critical projects, verify with local specifications, geotechnical data, and pavement design standards.

How to Use This Calculator

  1. Select your unit system and project type.
  2. Pick a traffic class that matches expected loading.
  3. Enter the subgrade CBR from tests or geotechnical notes.
  4. Set drainage and frost risk based on site observations.
  5. Choose material quality and expected compaction level.
  6. Provide area using length × width or direct area.
  7. Add unit cost and delivery assumptions if needed.
  8. Press Calculate to view thickness, volume, loads, and cost.
  9. Use CSV or PDF buttons to export your result.

Example Data Table

Scenario Project Traffic CBR (%) Drainage Frost Thickness (mm) Area (m²) Volume (m³)
A Parking Medium 6 Good None 235 300 70.50
B Roadway Heavy 4 Fair Moderate 395 500 197.50
C Industrial Industrial 8 Poor Severe 520 250 130.00

Examples are illustrative. Your result depends on your selected factors.

Related Calculators

Paver Sand Bedding Calculator (depth-based)Paver Edge Restraint Length & Cost CalculatorPaver Sealer Quantity & Cost CalculatorExcavation Hauling Loads Calculator (truck loads)Soil Disposal Fee CalculatorSite Leveling Cost CalculatorCompaction Passes Time & Cost CalculatorPlate Compactor Rental Cost CalculatorGravel Volume Calculator (yards/tons)Gravel Weight Calculator (by material type)

Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.