Concrete Saw Cut Spacing Calculator
Joint spacing, saw depth and the sawing window from slab thickness.
Inputs
Result
Formula
| Joint spacing | L = k × thickness (k = 30 normal, 24 hot, 28 cold) |
| Saw depth | h = t ÷ 4 to t ÷ 3 |
| Panel area | A ≈ L² |
| Check | thickness ≥ 3 × max aggregate size |
A 150 mm slab with 20 mm aggregate in normal weather: 4.50 m maximum spacing, 38 mm saw depth (up to 50 mm), sawing within 12 hours, and panels of 20.25 m².
FAQ
Why is spacing based on slab thickness?
Crack spacing scales with thickness. A thin slab releases shrinkage stress quickly and tolerates closer joints; a thick slab is more restrained, so joints must be spaced further apart or they fail to control where cracking happens. The 24-30 times thickness rule comes from decades of field observation.
What if the cut is too shallow?
A joint creates a deliberate weak section so the shrinkage crack runs neatly from its base. Below a quarter of the thickness the remaining section is too strong, the crack leaves the joint line and appears elsewhere on the surface — the cut was wasted. Go to a third of the thickness; a few minutes of blade wear beats replacing a slab.
When should I saw?
The criterion is hard enough to cut without raveling: the surface resists a thumbprint while the interior is still green. Typically 6-12 hours after placing at normal temperatures, 4-6 hours in hot windy weather, up to 20 hours when cold. Too early ravels the edges and loosens aggregate; too late means microcracks have already formed.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.