Rebar Calculator

How to use the rebar calculator

1. Enter the length and width of the slab (or of the wall elevation you are reinforcing).
2. Set the spacing of the bars that run each way, center to center, in inches. Eighteen inches is a common residential slab grid; your drawings control.
3. Pick the bar size. #4 (½ inch) is the usual slab bar; #3, #5 and heavier sizes are included with their published weights.
4. Optional planning lap. If a run is longer than 20 ft, the calculator can add a lap at each splice. Leave it at 0 if you only want raw grid length. The lap is an allowance, not a Code-compliant splice length.

Grid count and bar weights

Bars running the length: floor(width ÷ spacing) + 1. Bars running the width: floor(length ÷ spacing) + 1. Raw length is then (bars along × length) + (bars across × width). When a single run is longer than one 20 ft stick and you enter a lap, each such run gets (ceil(run ÷ 20) − 1) laps.

Unit weights follow ASTM A615 / the CRSI Manual of Standard Practice: #3 is 0.376 lb/ft, #4 is 0.668, #5 is 1.043, #6 is 1.502, #7 is 2.044, #8 is 2.670. Sticks = total length including laps, divided by 20, rounded up. For the cost of a slab that already includes a rebar add-on, see the slab cost calculator or the driveway cost calculator.

A worked example: 20 × 20 ft slab, #4 at 18 inches

Width 20 ft ÷ 1.5 ft + 1 = 14 bars running the length, and 14 running the width. Raw length = 14 × 20 + 14 × 20 = 560 ft. Weight = 560 × 0.668 = 374.08 lb. That is exactly 28 sticks of 20 ft, with no laps. A 24 × 20 ft slab at the same spacing, with a 24 inch planning lap, adds 2 ft on each of the 14 long bars (each 24 ft run needs one splice).

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Bars along = floor(width ÷ spacing) + 1; bars across = floor(length ÷ spacing) + 1.
• Unit weights are ASTM A615 / CRSI: #4 is 0.668 lb/ft, and so on.
• Stock length is 20 ft. A planning lap is added only when a run exceeds 20 ft and a lap is entered. It is not a design splice.
• This is a material takeoff, not a structural design.

Sources

1. ASTM International, ASTM A615/A615M, Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement. The specification behind the standard US bar sizes and unit weights used here.
2. Concrete Reinforcing Steel Institute, Reinforcing Bar Properties. Published lb/ft schedule: #3 0.376, #4 0.668, #5 1.043, #6 1.502, #7 2.044, #8 2.670.

Last reviewed November 12, 2026. Product figures vary by brand; the label on what you buy wins.