Zerdly

Road Crossfall Calculator

Solve road crossfall (camber) from any two of road width, crossfall percentage and the fall across the carriageway. Used by road designers, surveyors and pavement crews setting up levels and string lines.

100% FreeRuns in your browserBasic
How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter exactly two of: road width B (m), crossfall (%), and fall across the road (mm).
  2. The calculator solves the third value and shows the crossfall as both a percentage and a 1-in-N ratio.
  3. Leave the value you want to find blank — the tool detects which two you provided.

How it works

Crossfall is the transverse slope of the road surface used to shed water. It is defined as fall divided by width: crossfall(%) = fall(mm) / (B(m) × 1000) × 100, which simplifies to fall / (10·B).

Rearranging gives fall = crossfall% × B × 10 and B = fall / (10·crossfall%). The crossfall is also expressed as a ratio 1 in N, where N = 100 / crossfall%.

Worked example

7 m carriageway at 3% crossfall. Fall = 3 × 7 × 10 = 210 mm across the road. As a ratio, N = 100 / 3 = 33.3, i.e. about 1 in 33 — within the usual 2.5–3% range for a sealed road.

Common mistakes

  • Entering the fall in metres instead of millimetres — a 3% crossfall over 7 m is 210 mm, not 0.21.
  • Providing all three values or only one — the tool needs exactly two to solve the third.
  • Confusing crossfall (transverse, across the road) with longitudinal grade (along the road).

Frequently asked questions

What is a normal road crossfall?

Sealed carriageways are typically 2.5–3% (about 1 in 40 to 1 in 33). Unsealed roads often use a steeper 4–6% to shed water; specific values come from the relevant road-design standard.

What does '1 in 33' crossfall mean?

The surface drops 1 unit vertically for every 33 units horizontally, which equals a 3% crossfall (N = 100 / 3 ≈ 33).

Can I find the road width from the fall and crossfall?

Yes. Enter the fall (mm) and crossfall (%) and leave width blank; the tool returns B = fall / (10·crossfall%).

Is crossfall the same as camber?

Camber usually refers to a crowned road that falls both ways from a central high point; crossfall is the one-way transverse slope. The percentage-to-fall arithmetic is identical for each side.

Related tools

Explore more in Civil Construction, Building Materials & Trades.

Tip: Enter any known values to calculate the remaining results.

All calculations run in your browser. Your inputs are never saved or transmitted.