HowMuchIt / Rebar
How much rebar do I need?
Rebar is laid in a grid, and the plans set the spacing. Type the slab size and that spacing: the calculator counts the bars each way and the total feet.
Work out your rebar grid
The bar number is about the diameter in eighths of an inch. Spacing, concrete cover and laps are set for each job by the plans, an engineer or the local code: this tool only counts the grid you were given.
Sizes and grades
The bar number is about the diameter in eighths of an inch: #3 is 3/8 inch, #4 is 1/2, #5 is 5/8. Grades such as 40 and 60 give the minimum yield strength in thousands of psi. Set the bars on supports, chairs or bolsters, so they sit at the right depth inside the concrete, not on the ground.
What the calculator does not decide
Bar spacing, the concrete cover over the bars and how far bars overlap at a splice are set for each job, by the plans, an engineer or the local code. The calculator only counts the grid you were given.
Questions people ask
How much rebar for a 20 x 20 slab at 12 inches?
21 bars each way, 20 ft long: 840 ft in all, before laps.
What size is #4 rebar?
1/2 inch.
Good to know
- The bar number is about the bar diameter in eighths of an inch: #3 = 3/8 in, #4 = 1/2 in, #5 = 5/8 in (exact for #3 to #8).1,2,3
- Set rebar on bar supports (chairs or bolsters) so it is held at the right depth inside the concrete.4,5
- Bar spacing, concrete cover and lap (overlap) lengths are set for each job: take them from the plans, an engineer or the local code.6,7
- Rebar comes in grades such as 40 and 60; the grade number is the minimum yield strength in thousands of psi (ksi).2,8
How we checked this page
Checked on October 27, 2026. We kept a fact only when two independent publishers agree, and each one is numbered to its sources below. Where sources give different numbers, we keep only what they share. We never write a price. This page was written with an AI assistant and checked against the sources below.
Sources
- Reinforced Concrete Terminology, Concrete Reinforcing Steel Institute (CRSI)
- Steel Rebar, Nucor
- Rebar (unit definition only), Wikipedia
- Placing Bars, Concrete Reinforcing Steel Institute (CRSI)
- Building a Concrete Patio, QUIKRETE
- Splicing Bars, Concrete Reinforcing Steel Institute (CRSI)
- Concrete Calculator (rebar section), Concrete Network
- Rebar Properties, Concrete Reinforcing Steel Institute (CRSI)