Calculator
Choose units, set slab type, then enter span, loads, and subgrade support. The form adapts for large, small, and mobile screens.
Example data table
These examples show typical inputs and resulting recommended thickness values.
| Scenario | Type | Length | Width | Traffic | Subgrade | f'c | Recommended thickness |
|---|---|---|---|---|---|---|---|
| Home driveway entry | Driveway | 3.0 m | 3.5 m | Light truck | CBR 6% | 30 MPa | ≈ 130–160 mm |
| Warehouse entry | Industrial | 4.0 m | 4.0 m | Delivery truck | k 50 MN/m³ | 35 MPa | ≈ 180–230 mm |
| Bridge approach zone | Bridge | 5.0 m | 3.7 m | Heavy truck | CBR 4% | 40 MPa | ≈ 230–300 mm |
Actual project details may require reinforcement and joint design beyond this estimator.
Formula used
This calculator sizes thickness using a simplified cracking-based bending stress check on an effective strip width:
- beff = min(W, bcontact + 2h) (load spreads through slab depth)
- q = (P · I · S · γ) / beff (equivalent line load, kN/m)
- M = q · L² / 8 (simply supported slab strip under UDL)
- σ = 6M / (beff · h²) (bending stress)
- h ≥ √(6M / (beff · σallow))
- σallow = fr/k, where fr ≈ 0.62√f'c
Where: P is wheel load, I is impact factor, S is subgrade support factor, γ is safety factor, L is span/length, W is slab width, and h is slab thickness.
How to use this calculator
- Select your unit system and slab type.
- Enter the approach length and usable slab width.
- Choose a traffic class, or switch to a custom wheel load.
- Provide subgrade support using CBR or a k-value.
- Set concrete strength, safety factor, and exposure conditions.
- Press Calculate thickness to view results above the form.
- Download CSV or PDF to share inputs and outputs.
Project notes
Load level and traffic intensity
Approach slabs see concentrated wheel loads at the pavement transition. This tool uses default wheel loads of 10 kN (passenger), 20 kN (light truck), 40 kN (delivery), and 60 kN (heavy). A dynamic impact factor from 1.15 to 1.30 increases demand for braking, bumps, and settlement.
Subgrade support and base response
Support quality controls how much thickness is needed to limit cracking. When CBR is below 3, the calculator applies a 1.35 support multiplier; CBR 3–5 uses 1.20; CBR 5–10 uses 1.10; CBR 10–20 uses 1.00; and stronger soils reduce to 0.95. If you prefer k-value, ranges are treated similarly to reflect weak, moderate, and stiff foundations. Better compaction and a well-graded base typically improve performance.
Concrete strength and allowable stress
Higher compressive strength improves the modulus of rupture, estimated as fr ≈ 0.62√f’c (MPa). The estimator converts that cracking strength into an allowable bending stress using a durability divisor, tightening limits for freeze–thaw exposure. Typical f’c values are 25–35 MPa for general work and 35–45 MPa for harsh exposure.
Thickness sizing and practical rounding
A simplified strip model spreads wheel load across an effective width, then checks bending moment and stress to compute required thickness. Minimum checks are added by slab type, and allowances apply for long spans, void risk near joints, and severe exposure. Final values are rounded up to the next 5 mm and the next ¼ inch for buildable dimensions.
Using outputs for planning
Use the result as a starting thickness for budgeting, grading, and base preparation. Review the “Checks & baselines” panel to see whether the empirical or flexural path governs. Export CSV for tender comparisons and PDF for submittals, then validate reinforcement, joints, drainage, and local requirements with project drawings. For approaches near bridges, consider dowels, tied joints, and erosion control so the slab stays supported. Maintain positive drainage to prevent pumping and settlement over time. Confirm field densities, base thickness, and curing plans before pouring any concrete.
FAQs
1) What thickness is common for a residential driveway approach?
Many residential approaches land around 125–160 mm, depending on subgrade quality and traffic. Use this calculator to check span, wheel load, and exposure, then round up to a buildable thickness.
2) Why does a low CBR increase the recommended thickness?
Low CBR indicates weaker support and higher slab deflection risk. The calculator applies a higher support multiplier to represent poorer foundation response, which increases bending demand and raises the thickness estimate.
3) Can I use k-value instead of CBR?
Yes. Select the k-value method when you have plate test data or a project specification. The tool maps k-value ranges to a comparable support multiplier, so the flexural check stays consistent.
4) Will higher concrete strength always reduce thickness?
Not always. Higher strength raises cracking resistance, but minimum thickness, exposure allowances, void risk, and rounding can still govern. Compare the “Min check,” “Empirical,” and “Flexural” lines to see what controls.
5) Does this calculator design reinforcement and joints?
No. It estimates slab thickness only. Reinforcement, dowels, joint spacing, base layer thickness, and drainage details must be designed to match loading, movement, and local standards for your specific site.
6) How should I use the CSV and PDF exports?
Use CSV for quick comparisons across scenarios and for estimating templates. Use PDF for sharing a single run with stakeholders. Always attach project assumptions, drawings, and acceptance criteria alongside exports.