Rebar Weight and Spacing Calculator (Two-Way Slab)

Construction
All tools
Report
-
Total bars needed
-
Total length incl. waste (ft)
-
Total weight (lb)
-
Total weight (tons)

Rebar Weight and Spacing Calculation

Generated by calculatgeek.com Rebar Weight and Spacing Calculator

Diagram of a two way concrete slab rebar grid showing bars running parallel to the length spaced across the width and bars parallel to the width spaced across the length
A two-way slab needs rebar running in both directions. Total steel weight is the sum of the bars parallel to the length and the bars parallel to the width.

Reinforcing bar, rebar, is priced and ordered by weight, but laid out on the job by spacing. This calculator bridges the two: enter a slab size, a bar size, and a spacing, and it works out how many bars are needed in each direction, the total length of bar including a waste and lap allowance, how many stock lengths to order, and the total installed weight.

Rebar unit weight

US customary rebar sizes are numbered so that the bar number roughly equals its diameter in eighths of an inch, a number 5 bar is close to 5/8 inch in diameter. Nominal weight per foot follows a simple relationship to diameter:

Weight (lb/ft) = 2.670 x diameter^2, diameter in inches

This reproduces the standard ASTM rebar weight table closely, for example about 0.376 lb/ft for a #3 bar, 1.043 lb/ft for a #5 bar, and 2.670 lb/ft for a #8 bar exactly, since an #8 bar has a 1 inch diameter.

Bar count and total length

For a rectangular slab, bars usually run in two directions. The number of bars needed in each direction depends on the spacing across the perpendicular dimension:

  • Bars parallel to the slab length are spaced across the width: count = floor(width in inches / spacing) + 1.
  • Bars parallel to the slab width are spaced across the length: count = floor(length in inches / spacing) + 1.

Multiplying each bar count by its length and summing both directions gives the total linear footage of bar needed before waste. Adding a waste and lap allowance, typically 5 to 10 percent to cover cut offs and overlaps at splices, gives the practical order quantity, which the calculator then converts into total weight and number of full stock lengths to purchase.

Worked example

For a 20 ft by 15 ft slab reinforced with #5 bar at 12 inch spacing both ways, using 20 ft stock lengths and a 5 percent waste allowance, the layout needs 16 bars parallel to the length and 21 bars parallel to the width, 37 bars in total, for about 667 linear feet of bar after waste. At 1.043 lb/ft for a #5 bar, that comes to roughly 695 lb of steel, or about 34 stock lengths of 20 ft bar.

Rebar Weight and Spacing Calculator

Related Calculator and Reference

For earthwork volumes on the same project, see the Excavation Cut and Fill Calculator. Formula reference: ASTM A615 standard reinforcing bar sizes and weights.