Rebar Weight Calculator
Calculate
Units
12–18 in is common for slabs
Usually 20 ft (also 40 and 60 ft)
40 is a common rule of thumb; follow your plans
Rebar for the slab
649 lb
971 ft of #4 bar · buy about 49 × 20 ft bars
| Lengthwise bars | 24 × 19.5 ft |
|---|---|
| Crosswise bars | 20 × 23.5 ft (2 pieces lapped) |
| Grid | 12 in on center, 3 in from the edges |
Runs longer than a 20 ft stock bar are spliced with a 40-diameter lap (20 in), which is included in the length.
Add 5–10% for cutting waste, chairs and bends. Spacing, bar size and cover come from your plans or local code, not from this estimate.
Estimate the rebar for a slab grid, with the bar count, total length, laps where runs exceed a stock bar, number of stock bars to buy and total weight, or the weight of any number of bars, in US bar sizes (#3 to #18) or metric diameters.
Formula
Worked Example
The grid has 24 bars running the 20 ft way (19.5 ft each after cover) and 20 running the 24 ft way. Those are longer than a 20 ft stock bar, so each is two pieces lapped by 40 × ½ in = 20 in. 971 ft × 0.668 lb/ft = 649 lb.
Frequently Asked Questions
How much does rebar weigh per foot?
#3 weighs 0.376 lb/ft, #4 0.668, #5 1.043, #6 1.502 and #8 2.670. The bar number is the diameter in eighths of an inch, so #4 is ½ in. In metric, weight in kg per meter is the diameter in mm squared divided by 162: 12 mm bar weighs 0.888 kg/m.
What rebar spacing is used for a slab?
Residential slabs and driveways commonly use #3 or #4 bars at 12 to 18 in on center each way, with about 1½ to 3 in of concrete cover. Footings and structural slabs follow the engineer's plans; local code sets the minimums.
How long should a rebar lap splice be?
A common rule of thumb is 40 bar diameters (20 in for #4), and 30–60 diameters appear in practice depending on the bar, concrete strength and code. Structural drawings specify the required lap and override any rule of thumb.