Rebar and wire mesh calculator

Bars each way = the slab inside its edge clearance ÷ the spacing, rounded up, plus one: a 20 × 12 ft slab with #4 bars every 18 in, 3 in clear of the edges, takes 9 bars along it and 14 across, or 25 bars of 20 ft with 5% extra.

Your measurements

Example

Use the unit shown for each box, or type another one: 12 ft 6 in, 150 in, 3.8 m and 600 mm all work, as do the marks on a tape measure.

Reinforcement Slab length (ft) Slab width (ft) Bar size Bar spacing, each way Center to center, from your plan or the engineer’s detail. Clear distance from the slab edge How far the outer bars stop short of the forms. Your plan gives it.

Details Stock, laps and your price

Bar length you buy Lap where two bars join Your plan or engineer sets the lap; it is often written as a number of bar diameters. Extra (%) Your price per bar or sheet ($)

Example: replace these measurements

Your reinforcement list

25 #4 bars, 20 ft

240 sq ft measured

Your measured layout

The slab from above, to scale, with the bars inside the edge clearance. Past 40 bars a side, every other bar is drawn; the list counts them all.

Your shopping list

25 #4 bars, 20 ft

Quantities and package details

Bars along the length: 9 bars, 19.5 ft each
Bars across the width: 14 bars, 11.5 ft each
Length of bar in the grid: 336.5 ft
Weight of the bars: 334 lb
Crossings to tie: 126

Set the bars on supports so they sit inside the slab, not on the ground. Bend or cut short bars at the edges as your plan shows.

The calculation

Along the length: 138 in across ÷ 18 in spacing, rounded up, + 1 = 9 bars
Across the width: 234 in ÷ 18 in, rounded up, + 1 = 14 bars
Stock bars: 23 of 20 ft; with 5% extra, rounded up = 25
Weight: 25 × 20 ft × 0.668 lb a foot (ASTM A615 #4) = 334 lb

Common questions

How much rebar do I need for a slab?

Divide the slab’s width inside the edge clearance by the bar spacing, round up and add one: those are the bars that run along the slab. Do the same with the length for the bars across it. A 20 × 12 ft slab with bars every 18 in, 3 in clear of the edges, takes 9 bars along and 14 across, 23 bars of 20 ft before extra.

How much does rebar weigh?

By its size, under ASTM A615: a #3 bar weighs 0.376 lb a foot, a #4 bar 0.668 lb and a #5 bar 1.043 lb. A 20 ft #4 bar is about 13.4 lb, so 25 of them are about 334 lb.

How far should rebar overlap where two bars join?

Your plan or engineer sets the lap, often as a number of bar diameters. At 40 diameters a #4 bar laps 20 in and a #5 bar 25 in. Every lap uses extra bar, so the list adds it to the bars you buy.

How many sheets of wire mesh does a slab take?

Sheets overlap where they meet, so each one covers less than its size. With 5 × 10 ft sheets and 8 in overlaps, a 20 × 12 ft slab with 3 in clear at the edges takes 3 sheets along and 3 across: 9 sheets, or 10 with 5% extra.

How to plan the quantity

For a rebar grid, each direction gets one bar for every spacing across the slab, plus one: bars run the slab’s length less the edge clearance at both ends. A bar longer than the stock you buy is made of several pieces lapped where they meet, and every lap uses extra bar.

For mesh, sheets are laid in rows and columns with an overlap at every seam, so each sheet covers less than its size. The count rounds up in both directions; the cut pieces at the edges come from whole sheets.

Tie points are the crossings in a rebar grid. Many crews tie every other crossing; the list gives the full count so the wire and the time can be judged.

Before you order

This counts material for the spacing and size you enter. Whether a slab needs reinforcement, and which, is a design question for your plan, the local code or an engineer.
Bars and mesh need support at the right height in the slab, on chairs or concrete blocks: the support count depends on the supports you use.

Before ordering, read the guide: How much concrete do I need? Yards, bags and when to order a truck. Or choose another calculator.