Calculator Form
Use this construction tool to compute CBR values, determine the governing penetration, estimate a design value, and produce export-ready summaries.
Example Data Table
This sample shows how the calculator reports two typical subgrade checks using the common default reference loads.
| Condition | Load 2.5 mm (kN) | Load 5.0 mm (kN) | CBR 2.5 mm (%) | CBR 5.0 mm (%) | Governing CBR (%) |
|---|---|---|---|---|---|
| Soaked | 1.10 | 1.52 | 8.25 | 7.59 | 8.25 |
| Unsoaked | 1.55 | 2.10 | 11.62 | 10.49 | 11.62 |
Formula Used
CBR at 2.5 mm = (Test Load at 2.5 mm ÷ Standard Load at 2.5 mm) × 100
CBR at 5.0 mm = (Test Load at 5.0 mm ÷ Standard Load at 5.0 mm) × 100
Governing CBR usually starts with the 2.5 mm value. If the 5.0 mm value is higher, repeat testing is normally checked before adopting it.
Design CBR = Governing CBR ÷ Safety Factor
Estimated Resilient Modulus ≈ Design CBR × Modulus Coefficient
The modulus estimate is a quick screening correlation. Always confirm final pavement design values with your project standard and engineering review.
How to Use This Calculator
- Select the load unit used by your test report.
- Choose soaked or unsoaked sample condition.
- Enter measured loads at 2.5 mm and 5.0 mm penetration.
- Review or edit the standard reference loads if your procedure differs.
- Add moisture, dry density, and swell values when you want richer reporting notes.
- Enter a safety factor and modulus coefficient for conservative design screening.
- Tick the repeat-confirmed box only when your lab has verified a higher 5.0 mm result.
- Press the calculate button to display the result block above the form.
- Use the CSV and PDF buttons to save the summary.
Frequently Asked Questions
1) What is subgrade CBR?
Subgrade CBR is a penetration resistance ratio comparing your soil sample with a standard reference load. Higher values generally indicate stronger pavement support.
2) Which value usually governs, 2.5 mm or 5.0 mm?
Usually the 2.5 mm value governs. If the 5.0 mm value is higher, many procedures ask for repeat testing before adopting that higher number.
3) Why does soaked testing matter?
Soaked testing simulates severe moisture exposure and often controls flexible pavement design. It commonly gives a lower, more conservative strength assessment.
4) Can I change the standard loads?
Yes. The starting values are common defaults, but you can edit them to match your laboratory method, local standard, or client reporting format.
5) What is design CBR?
Design CBR is the governing CBR reduced by a chosen factor for conservative planning. It helps screening, but it does not replace your pavement manual.
6) How reliable is the resilient modulus estimate?
It is an approximation from a CBR correlation. Use it for early comparisons or concept design, then verify final values with the required project method.
7) What inputs should I verify first?
Check load units, penetration readings, moisture condition, surcharge setup, and sample preparation. Small entry mistakes can change the reported strength category.
8) Is this enough for final pavement thickness design?
No. Final thickness also depends on traffic, drainage, climate, layer properties, reliability targets, and the design method adopted on the project.