Rebar Calculator
Estimate rebar bars and total length for a slab grid.
Center-to-center spacing of the grid.
Results update as you type.
How it works
A rebar calculator estimates how many reinforcing bars and how much total bar length a concrete slab needs, based on the slab's dimensions and the spacing of the reinforcing grid. Reinforcement is laid as a grid — one set of bars running one way and another set running across it — so the tool counts the bars needed in each direction and adds up their combined length in feet. For each direction it divides the slab dimension by the bar spacing, takes the whole number of gaps, and adds one so that a bar sits at both edges of the grid rather than leaving the perimeter unreinforced. Getting reinforcement right matters because steel gives concrete its tensile strength: concrete is strong in compression but cracks easily under bending and tension, and a correctly spaced grid holds those cracks tight and keeps the slab acting as one piece. This calculator is a clear planning estimate that assumes a simple rectangular slab and a uniform grid; it does not add lap-splice overlaps where bars join, edge cover setbacks, or bends, so treat the total length as a baseline and add a margin. Builders, DIYers and estimators use it to price steel and plan a layout before ordering. Always follow your local building code and any engineer's drawing for the required bar size, grade and spacing.
Bars each way = ⌊(span in inches) ÷ spacing⌋ + 1; total length = (bars widthwise × slab width) + (bars lengthwise × slab length)
A reinforcing grid runs bars in both directions across the slab. The number of bars laid across the width is found by dividing the slab length in inches by the spacing, taking the whole number, and adding one so both edges carry a bar. The same logic applied the other way gives the bars laid along the length. Each set of bars spans the opposite dimension, so the widthwise bars are as long as the slab width and the lengthwise bars are as long as the slab length. Multiplying each count by its bar length and summing the two gives the total lineal feet of rebar to order.
Source:Standard construction estimation (grid layout method)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter slab length, slab width and bar spacing.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Total bars updates instantly as you type — no button to press.
Frequently asked questions
What rebar spacing should I use for a slab?
Residential slabs commonly use a grid spaced 12 to 18 inches on center, but the right spacing depends on the slab's thickness, the loads it carries and the bar size, and is set by local code or an engineer. Closer spacing means more steel and a stronger, crack-resistant slab. Always follow the structural drawing or building department requirement for your project rather than guessing at a spacing.
Does this include overlap for splicing bars?
No. The total length is a clean grid estimate that does not add lap splices — the overlap where two bars are tied together end to end, typically around 40 bar diameters. On any slab larger than a single bar length you will need these overlaps, so add a margin of roughly 10 to 15 percent to the calculated total to cover splices, plus a little for waste and offcuts when you order.
How much extra rebar should I order?
Add around 10 to 15 percent to the calculated length to cover lap splices where bars join, offcuts, and the occasional damaged bar. Steel is ordered in fixed stock lengths, so you will also round up to whole bars. Ordering a small surplus is cheaper than pausing a pour to fetch more steel, and leftover bar is easy to store and reuse on future jobs.
Why add one bar to each direction?
Dividing the slab dimension by the spacing counts the gaps between bars, not the bars themselves. A row with four gaps has five bars, so the formula adds one to place a bar at both edges of the grid. Skipping that plus-one would leave the slab perimeter without reinforcement and undercount the steel. This fencepost logic is standard whenever you count evenly spaced items across a length.
Does bar spacing affect slab strength?
Yes, strongly. Closer bar spacing puts more steel into the slab, which resists tension and bending better and holds any cracks that form tighter and finer. Wider spacing uses less steel but gives less crack control and load capacity. The correct spacing balances strength against cost for the expected loads, and should come from your building code or a structural engineer rather than being chosen for material savings alone.