How to use this calculator
- Enter the mat size. Use the overall rectangular length and width of the slab, wall panel or footing area.
- Set edge cover and spacing. Enter the planned edge inset and target bar spacing from the drawing or design.
- Choose secondary bars. Use same spacing, custom spacing or none depending on the takeoff you need.
- Enter stock assumptions. Set the bar weight per length, stock stick length and waste allowance.
- Read the takeoff. Use bar counts, actual spacing, total length, sticks and estimated weight for planning.
How it works
This rebar spacing calculator treats the job as a rectangular takeoff. It first subtracts the entered edge cover from both sides:
clear_width = W - 2 x cover
clear_length = L - 2 x cover
Primary bars run along the clear length and are spaced across the clear width. The count rounds up so the actual spacing is not wider than the target spacing:
primary_bars = ceil(clear_width / target_spacing) + 1
actual_spacing = clear_width / (primary_bars - 1)
When secondary bars are enabled, the same rule is applied in the other direction. Total bar length is multiplied by the waste allowance, then divided by stock stick length and rounded up.
Worked example
Verified against the live calculator
A 12 ft by 10 ft rectangular mat with
3 in edge cover and 12 in two-way spacing has
11 primary bars and 13 secondary bars. The
straight bar length is 250 ft; with a 10%
allowance that becomes 275 ft, or 14
stock sticks at 20 ft each.
Frequently asked questions
How do you calculate rebar spacing and count?
Subtract edge cover from both sides to get the clear layout width, divide by the target spacing, round up, then add one edge bar: bars = ceil(clear width / spacing) + 1. Actual spacing is clear width divided by bars minus one.
Does this choose the required rebar spacing?
No. It uses the spacing you enter and turns it into a material takeoff. Required reinforcement, slab thickness, retaining-wall steel, concrete cover, bar size and inspection requirements still come from the adopted code, drawings or engineer.
What rebar weight should I enter?
Use the supplier or bar-schedule unit weight for the bar size you are buying. The default is a common #4-style weight, but the calculator is more accurate when you enter the actual bar weight per foot or metre.
Does the waste allowance calculate lap splice length?
No. It is only an ordering allowance for offcuts and trimming. Use the rebar lap splice length calculator when you have project-entered development values; hooks, dowels and final detailing still come from the drawings, code or engineer.
Method & assumptions
- Rectangular layout only. Irregular mats, openings, edge thickenings, hooks, dowels, chairs and bent bars need a bar schedule.
- The calculator uses the target spacing you enter; it does not choose code-required reinforcement or verify a structural design.
- Actual spacing is rounded tighter than or equal to the target spacing by using
ceil(clear span / spacing) + 1. - Estimated weight uses the entered bar weight per length. Supplier unit weights and actual bar sizes control material orders.
- Waste allowance is an ordering allowance only. It is not a lap-splice, development-length, hook, cover or retaining-wall design calculation.
- Use rebar lap splice length, deck footing concrete and wood beam span tools for adjacent construction planning checks.