Work out road pavement thickness instantly with clear inputs, formula shown and shareable results.
Flexible pavement thickness grows with the logarithm of design traffic and falls as subgrade strength rises. This chart approximation uses a 400 mm reference for 10 million standard axles on a CBR 5 subgrade and scales by (5/CBR)^0.4. The granular thickness left after the surfacing is split roughly 45 percent base and 55 percent sub-base.
Total thickness
T mm = 200 x (log10 msa + 1) x (5 / CBR)^0.4
Layer split
Granular = T - surfacing; base = 45 percent of granular, sub-base the remainder
Preliminary estimate only. Pavement design must follow the design guide adopted by the road authority and be verified by a qualified highway engineer.
T mm = 200 x (log10 msa + 1) x (5 / CBR)^0.4. Flexible pavement thickness grows with the logarithm of design traffic and falls as subgrade strength rises.
The pavement has to spread the wheel load until the stress reaching the subgrade is tolerable. Halving CBR raises the required thickness by roughly a third, which is why subgrade improvement often pays for itself.
This calculator takes 3 inputs: Subgrade CBR, Design traffic, Bituminous surfacing thickness. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.