Reinforcement
Number of bars at a given spacing
- where
- length available for bars (mm)
- length across the bars (m)
- end cover to the first bar (mm)
- number of bars
- maximum bar spacing (mm)
- actual spacing (mm)
Explain this equation
Works out how many bars are needed at a given maximum spacing across a slab or wall, and the actual spacing they end up at.
The first and last bars sit in from each end by the cover, leaving L_n between them. Dividing by the maximum spacing and rounding up gives the number of gaps; there is one more bar than gaps. The actual spacing is the length shared equally between those gaps, which is never more than the maximum.
Fence-post counting: bars = gaps + 1.
Sense check: 5 m with 50 mm covers at 200 mm maximum gives 26 bars at about 196 mm.
Watch out: do the count separately for each direction and each layer of reinforcement.
Works out how many bars are needed to cover a width at no more than a given spacing, and the actual spacing they end up at.
Enter your values to see the result.
Good to know
The first and last bars sit at the cover distance from each end, and the gaps between them are rounded up so no gap exceeds the specified spacing; that is why one is added to the number of gaps. Run it twice for a slab: once for bars in each direction.
Common mistake
Dividing the length by the spacing and stopping there, which is one bar short (the fence-post error).
Limits
Evenly spaced straight bars. Openings, trimming bars and extra bars at edges are separate.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.