Slab foundation calculator
Concrete volume and reinforcing mesh for a raft slab.
Fill in the fields and the result will appear here automatically.
Counts rectangular slab volume and geometric reinforcement for two assumed meshes. Alongside concrete volume it shows spacing, steel length and mass at a fixed density of 7850 kg/m³. Rows follow floor(side/spacing) + 1, which does not always place a row at the far edge. Reserve affects concrete only; reinforcement arrangement, cover and laps require a separate design.
How it works
Formula and logic
Rectangular slab: concrete V = L·W·t; reserve w from 0 to 50% applies only to concrete. Two meshes are assumed: nL = floor(W/s) + 1, nW = floor(L/s) + 1; rebar length R = 2(nL·L + nW·W). Spacing counts use the shortest decimal representation of entered numbers. Mass = R·π·d²·7850/4,000,000 for d in mm; 7850 kg/m³ is a fixed model density. Cover, an extra far-edge row, laps, anchorage and structural capacity are not included.
Example
A 10 by 8 slab, 0.3 m thick with 200 mm mesh of 12 mm bar, needs 25.2 m³ of concrete and 1,452.46 kg of steel.
Fields and units
- Slab length — m
- Slab width — m
- Slab thickness — m
- Mesh spacing — m
- Rebar diameter — mm
- Allowance — %
How to use
- — Enter the length, width and thickness of the slab.
- — Enter design mesh spacing in metres: 200 mm is entered as 0.2 m.
- — Enter the rebar diameter in millimetres.
- — Set concrete reserve only; count extra reinforcement separately.
Method and limitations
- Calculation method
- Formula and logic
- Limitation
- This is a geometric quantity calculation within the described model. It does not establish load capacity, safety or project compliance; check design parameters separately.
FAQ
Why plus one bar?
The model places the first row at 0, then s, 2s through floor(side/s)·s. Ten intervals therefore give eleven rows. If the side is not a multiple of spacing, the last row does not reach the opposite boundary; an extra row and cover must be determined separately from the drawing.
Are two layers always right?
No. Two layers are a fixed counting assumption, each with two directions. Required reinforcement depends on slab analysis, loads and support conditions. The quantity does not approve that arrangement.
Where does 0.888 kg per metre come from?
From the geometry: a 12 mm circle is 113.1 mm², and steel at 7,850 kg/m³ gives 0.888 kg for every metre of it. It is arithmetic, not a table lookup.
Is the overlap of bars included?
No. Mesh lengths follow the sides without laps, anchorage or cutting allowances. The reserve field changes concrete only, not steel length or mass. Determine added lengths from the design and count them separately.
Why a separate calculator if the general concrete one gives volume?
Besides concrete volume, this calculator counts mesh in two layers and two directions. Rows equal floor(side/spacing)+1, starting with a row at 0. This does not add a bar at each edge; cover, edge rows and laps must be determined separately from the design.