Reinforcement Steel Calculator

Estimate the exact amount of rebar needed for your concrete slab or foundation. Calculate total linear feet, weight, and the number of standard 20-foot pieces required.

Slab length feet Please enter a positive number

Slab width feet Please enter a positive number

Grid spacing inches Standard OC spacing is 16" or 18" Please enter a valid spacing

Rebar size Determines the total weight of the steel

Overlap & Waste buffer %

Accounts for splices, overlaps, and offcuts Please enter a valid percentage

Calculation Results

Rebar to order

— pieces (20-foot)

Includes your requested waste and overlap buffer.

How to use this rebar calculator

Enter slab dimensions

Input the total length and width of the concrete pad or foundation in feet. The tool assumes a standard grid layout across this area.

Set your grid spacing

Enter the on-center (OC) spacing between each bar. 16 inches or 18 inches are the most common spacings for residential driveways and patios.

Select the rebar size

Choose the thickness of your reinforcement steel. #4 rebar (1/2 inch) is standard for most residential concrete flatwork. This determines the total weight of your order.

Review the material takeoff

The calculator instantly tells you how many standard 20-foot pieces to order, the total linear footage, and the estimated delivery weight.

Rebar Grid Linear Footage (10% Waste Factor)

Slab Length	Slab Width	Rebar Spacing	Total Linear Feet
10 ft	10 ft	12 inches	243 LF
10 ft	10 ft	16 inches	188 LF
10 ft	10 ft	24 inches	132 LF
20 ft	20 ft	12 inches	925 LF
20 ft	20 ft	16 inches	704 LF
20 ft	20 ft	24 inches	485 LF
30 ft	30 ft	12 inches	2,047 LF
30 ft	30 ft	16 inches	1,552 LF
30 ft	30 ft	24 inches	1,056 LF
50 ft	50 ft	12 inches	5,610 LF
50 ft	50 ft	16 inches	4,235 LF
50 ft	50 ft	24 inches	2,861 LF

Frequently asked questions

How much rebar do I need for a 20x20 slab?

For a 20x20 foot slab with standard 16-inch grid spacing, you will need a base of 640 linear feet of rebar. When you add a standard 10% overlap and waste buffer, the total comes to roughly 704 linear feet, which requires exactly 36 standard 20-foot pieces.

What does the rebar size number mean?

Rebar sizes are based on eighths of an inch. Therefore, #3 rebar is 3/8 inch in diameter, #4 rebar is 4/8 (or 1/2) inch, and #5 rebar is 5/8 inch. #4 is the most common size used in residential concrete slabs and driveways.

Why do I need a waste and overlap buffer?

Rebar must be spliced together when a slab is longer than a single 20-foot piece of steel. Building codes generally require splices to overlap by at least 40 times the diameter of the bar (e.g., 20 inches for #4 rebar). A 10% waste buffer accounts for these mandatory structural overlaps and corner cuts.

How far apart should rebar be spaced in a driveway?

For most residential driveways, rebar is laid out in a grid pattern spaced either 16 inches or 18 inches apart. The exact spacing required will depend on your local building codes, soil conditions, and the intended vehicle loads.

About this calculator

This reinforcement steel calculator is built for concrete contractors and DIYers to generate accurate rebar takeoffs for flatwork. By calculating the exact grid layout based on your slab's length and width, the tool guarantees you order the precise number of 20-foot sticks required for the job.

The mathematical formulas used in this estimator are:

Rows Needed = (Width ÷ (Spacing ÷ 12)) + 1
Columns Needed = (Length ÷ (Spacing ÷ 12)) + 1

Base Linear Feet = (Rows × Length) + (Columns × Width)
Total Linear Feet = Base Linear Feet × (1 + (Waste % ÷ 100))

Pieces (20ft) = Total Linear Feet ÷ 20

Note on Edge Clearance: In real-world application, the rebar grid sits roughly 2 to 3 inches inside the edge of the concrete forms to prevent the steel from rusting. This minor reduction in width and length is generally offset by the 10% waste buffer, meaning the overall material order remains highly accurate.