Rebar Calculator for a Slab (Bars, Sticks, Lap Splices & Weight)
How much rebar a rectangular slab grid needs: bar count each way, lap splices, total linear feet, 20 ft sticks to order and steel weight from ASTM A615 bar weights.
Formula
Bars each way = ⌊(clear span ÷ spacing)⌋ + 1, where clear span = side × 12 − 2 × edge clearance. A run longer than one stick needs ⌈(run − lap) ÷ (stick − lap)⌉ pieces, adding one lap per joint; lap = bar diameters × bar diameter. Linear ft = (bars × run + splices × lap) ÷ 12; sticks = ⌈linear ft × (1 + allowance) ÷ stick length⌉; weight = linear ft × ASTM A615 lb/ft.
Worked example
Inputs: Slab length = 30 ft; Slab width = 24 ft; Bar spacing on center (from your plans) = 12 in; Edge clearance each side = 3 in; Bar size = 4; Stick length = 20 ft; Lap splice length in bar diameters (from your plans or engineer) = 40; Cut-off allowance = 5 %.
| Bars running lengthwise | 24 |
|---|---|
| Bars running widthwise | 30 |
| Lap splices | 54 |
| Lap length in | 20 |
| Total linear ft | 1,503 |
| Sticks to order | 79 |
| Steel weight lb | 1,004 |
| Tie intersections | 720 |
FAQ
How much does #4 rebar weigh per foot?
0.668 lb per foot (ASTM A615 nominal), so a 20 ft stick of #4 weighs about 13.4 lb. #3 is 0.376 lb/ft and #5 is 1.043 lb/ft.
How many sticks of rebar for a 24 × 30 slab at 12 in on center?
With 3 in edge clearance, #4 bars, 20 ft sticks and 40 bar-diameter laps, that is 24 bars one way and 30 the other, 54 lap splices, about 1,503 linear ft, and 79 sticks with a 5% allowance, about 1,004 lb of steel.
What lap length should I use?
Use the lap on your plans or from your engineer; it depends on bar size, concrete strength and code. The 40 bar-diameter default (20 in for #4) is only an editable placeholder.
Does this cover footings or walls?
No. It is for a single flat mat in a rectangular slab. For a two-layer mat, run it twice; for footings, count bars from your footing detail.
Sources
- ASTM A615 Table 1 nominal bar weights and diameters (#3 0.376, #4 0.668, #5 1.043, #6 1.502 lb/ft), as reproduced in the Re-Steel Supply conversion table
- Geometry (volume/area formulas); no external constant needed
Related
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Last reviewed 2026-10-14.