Spacing is usually set by the plans or by the slab's use: closer for driveways and garages.
How the rebar calculator works
RebarMath lays out a grid. In each direction the number of bars is the clear span (the slab dimension minus edge cover on both sides) divided by the spacing, rounded down, plus one; each bar is as long as the other dimension minus cover. A run longer than a stock bar is made of lapped pieces: each extra piece adds one lap of 40 bar diameters (20 in for #4) by default. The total length plus waste, divided by the stock length, gives the number of bars to order, and weight uses the standard ASTM A615 unit weights. Footings use continuous bars along the length plus optional cross bars.
Rebar sizes, weights and laps
| Bar | Diameter | Weight per ft | 20 ft bar | Lap at 40 db |
|---|---|---|---|---|
| #3 | 0.375 in | 0.376 lb | 7.52 lb | 15 in |
| #4 | 0.5 in | 0.668 lb | 13.36 lb | 20 in |
| #5 | 0.625 in | 1.043 lb | 20.86 lb | 25 in |
| #6 | 0.75 in | 1.502 lb | 30.04 lb | 30 in |
| #7 | 0.875 in | 2.044 lb | 40.88 lb | 35 in |
| #8 | 1 in | 2.67 lb | 53.4 lb | 40 in |
The 40-diameter lap is a common rule of thumb for slabs on grade; structural laps under ACI 318 depend on concrete strength, bar position, coating and cover and can be much longer, and plans or local codes govern. Bar counts here do not optimize cutting, so check your cut list before ordering. Results are estimates for planning, not engineering.
Frequently asked questions
How do I calculate the number of rebar bars?
(Span − 2 × cover) ÷ spacing, rounded down, + 1. For 20 ft with 3 in cover at 12 in: (240 − 6) ÷ 12 = 19.5, so 19 + 1 = 20 bars.
What is the maximum rebar spacing in a slab?
Commonly no more than 18 in or three times the slab thickness, whichever is less, under ACI 318 for slabs.
Is 12 inch rebar spacing standard?
12 in on center each way is a common choice for residential slabs and driveways, but use the plans if there are any.
Is RebarMath free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.