About

Reinforced concrete combines the high compressive strength of concrete with the high tensile strength of steel rebar. For slab-on-grade foundations, driveways, or patios, a grid of rebar is essential to control cracking and add structural integrity.

This calculator performs a dual function: it estimates the concrete volume required for the pour and simultaneously calculates the total linear footage and weight of the steel reinforcement bars needed based on your specified grid spacing (e.g., 16 inches On Center). This ensures you order enough steel to tie the complete grid without multiple trips to the supplier.

Formulas

Rebar is calculated by determining the number of bars running Length-wise and Width-wise based on spacing.

Rows = LengthSpacing + 1

Cols = WidthSpacing + 1

Total Weight:

W_total = (Rows × Width + Cols × Length) × Weight_{per_ft}

Reference Data

Rebar Size	Diameter (inches)	Weight (lb/ft)	Metric Mass (kg/m)
#3	0.375 (3/8")	0.376	0.560
#4	0.500 (1/2")	0.668	0.994
#5	0.625 (5/8")	1.043	1.552
#6	0.750 (3/4")	1.502	2.235
#8	1.000 (1")	2.670	3.973

Frequently Asked Questions

OC stands for 'On Center'. It means the measurement is taken from the center of one bar to the center of the next bar. Common spacings are 12", 16", 18", or 24".

Yes. When rebar lengths aren't long enough to span the full slab, they must be overlapped (spliced). A common rule is to overlap by 40 times the bar diameter (e.g., ~20 inches for #4 rebar). This calculator gives net length; you should add ~10% for laps.

#3 (3/8 inch) or #4 (1/2 inch) rebar is standard for residential driveways, typically placed in a 12-inch to 18-inch grid pattern.