Plan the reinforcement grid for a concrete slab. Enter the slab size, bar spacing and edge clearance to get the number of bars in each direction, the total length of rebar, how many stock lengths to buy, and the approximate weight.
This is a material estimate only. The bar size and spacing a slab actually needs depend on its loads and on local building codes.
Formula
Bars each way = ⌊(Side − 2 × Clearance) ÷ Spacing⌋ + 1
Each bar runs the length of the slab minus the clearance at both ends. Total length = bars lengthwise × inner length + bars widthwise × inner width.
Worked example
A 20 × 20 ft slab with 3-inch clearance has a 19.5 ft inner square. At 12-inch spacing that is 20 bars each way, each 19.5 ft long: 780 ft of rebar, or 39 bars of 20 ft. In #4 bar (0.668 lb/ft) that weighs about 521 lb.
Frequently asked questions
What is the typical rebar spacing for a slab?
Residential slabs and driveways commonly use #3 or #4 bar at 12–18 inches (30–45 cm) on centre in both directions, but follow your engineer’s or local code requirements.
How much should rebar overlap?
Lap splices are often around 40 times the bar diameter — about 20 inches for #4 bar — but the required length depends on the design code.
Last updated · Calculations checked by automated tests· Editorial policy