Bar weights and quantities
Number of bars at given centres across a width
- where
- net width between the outer bars (mm)
- width to be covered (m)
- cover or end distance at each edge (mm)
- bars per layer
- maximum bar spacing (mm)
- actual spacing (mm)
- total bars
- number of layers or faces
Explain this equation
Works out how many bars fit across a slab, wall or base at no more than the specified centres. Use it for take-offs and for setting out bars before tying.
Ln is the width left once the cover or end distance c is taken off at both edges: the first and last bars sit on those lines. s is the maximum spacing from the drawing. nb is the bars in one layer, sa the spacing they actually end up at, and k the number of layers or faces.
The width Ln divided by s gives the number of gaps. It is rounded up (the ⌈ ⌉ brackets) so no gap is wider than specified, and one is added because a row always has one more bar than gaps. The 1000 converts metres to millimetres.
Sense check: 4.8 m at 200 mm centres with 50 mm cover gives 25 bars at 195.8 mm. Halving the spacing roughly doubles the bars.
Watch out: run it separately for each direction and each mat, and add trimmers at openings.
Works out how many bars are needed across a slab, wall or base at no more than the specified centres, and the spacing they actually end up at.
Enter your values to see the result.
Good to know
The first and last bars sit at the cover (or the end distance on the drawing) from each edge. The number of gaps is rounded up so no gap is wider than the specified centres, and one is added because there is always one more bar than gaps. Run it once for each direction, and set layers to 2 for top and bottom mats or both faces of a wall.
Common mistake
Dividing the width by the spacing and stopping there, which leaves you one bar short on every run (the fence-post error).
Limits
Evenly spaced parallel bars. Trimmers at openings, extra edge bars and lapped bar runs are added separately.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Always work to the engineer’s drawings and bending schedules, BS 8666, BS EN 1992, BS EN 1090-2, BS 5531/the BCSA National Structural Steelwork Specification and manufacturers’ data. Lifting must be planned under LOLER by a competent person.