Rebar Calculator: Grid, Footing & Steel Weight
Use this rebar calculator to estimate steel reinforcing bar (rebar) requirements for driveways, foundation slabs, stem walls, and spread footings. Calculates bar counts in both grid directions, lap-splice overlaps, standard stick counts (10 ft and 20 ft), and total steel tonnage.
Buy 65 sticks of 20-ft #4 rebar (857 lbs total steel weight) + 1 roll of 16-gauge tie wire.
| Concrete Slab Footprint | 24' × 24' (576 sq ft) |
|---|---|
| Grid Reinforcing Spacing | 12" on center in both directions |
| Grid Runs Required | 25 rows in length + 25 rows in width = 50 runs |
| Lap Splice Allowance | 20 inches overlap at bar joints |
| Total Steel Linear Footage | 1,283 linear ft |
| Rebar Support Chairs (1 per 10 sq ft) | 58 chairs (keeps rebar suspended in slab center) |
| Total Steel Weight | 857 lbs (0.43 US tons) |
| Recommended Stock Purchase | 65 pieces (20 ft lengths) |
How to Measure for Rebar Calculator
- Measure concrete slab length and width in feet, or total footing trench perimeter.
- Select your rebar bar size: #3 (3/8"), #4 (1/2" standard residential), #5 (5/8"), or #6 (3/4").
- Select your grid spacing (standard slab reinforcing is 12", 16", or 18" on-center in both directions).
- Determine your lap-splice length: standard building code requires overlapping bars by at least 40 to 48 bar diameters (e.g. 20" to 24" lap for #4 rebar) whenever joining sticks.
- Deduct concrete side clearance (typically 3" clear cover against earth, or 1-1/2" to 2" inside form boards).
How the Calculation Works
The rebar grid calculator determines bar quantities by dividing each slab dimension by the on-center spacing: `Bars in Length = Ceil((Width ft × 12) / Spacing in) + 1` and `Bars in Width = Ceil((Length ft × 12) / Spacing in) + 1`. Total linear footage combines both directions, adding lap-splice allowances for every joint between 20-foot standard lengths. Total steel weight is computed using ASTM standard unit weights (#4 rebar weighs 0.668 lbs per linear foot).
- Number of rows in each direction = (24 / 1) + 1 = 25 runs in length, 25 runs in width = 50 total continuous 24-foot runs. Each 24-foot run requires two 20-ft sticks overlapped by 20" (1.67 ft): effective run length = 24 + 1.67 = 25.67 ft of steel = two 20-ft bars per run (with offcuts used for perimeter dowels). Total 20-ft sticks = 50 runs × 2 sticks = 100 sticks (or 50 sticks of 20-ft + 50 sticks of 10-ft = 68 sticks of 20-ft equivalent). Total linear feet = 1,284 LF. Steel weight = 1,284 × 0.668 = 858 lbs (0.43 tons). Rebar chairs (1 per 10 sq ft) = 58 chairs.
Rebar Calculator Reference & Specifications
| ASTM Bar Size | Nominal Diameter | Weight per Linear Foot | 40d Lap Splice Length | Common Jobsite Application |
|---|---|---|---|---|
| #3 Rebar | 3/8" (9.525 mm) | 0.376 lbs / ft | 15.0 inches | Swimming pool gunite, light sidewalks, patio slabs |
| #4 Rebar | 1/2" (12.700 mm) | 0.668 lbs / ft | 20.0 inches | Residential garage slabs, driveways, footings, CMU block |
| #5 Rebar | 5/8" (15.875 mm) | 1.043 lbs / ft | 25.0 inches | Commercial slabs, heavy retaining wall stems, grade beams |
| #6 Rebar | 3/4" (19.050 mm) | 1.502 lbs / ft | 30.0 inches | Deep structural bridge piers, foundation tiebacks |
| #7 Rebar | 7/8" (22.225 mm) | 2.044 lbs / ft | 35.0 inches | Heavy civil engineering, highway pavement |
| #8 Rebar | 1" (25.400 mm) | 2.670 lbs / ft | 40.0 inches | High-rise foundation mats & shear wall columns |
Frequently Asked Questions About Rebar Calculator
How far apart should rebar be in a residential concrete driveway?
Most structural concrete guidelines recommend placing #4 (1/2") rebar on a 12-inch or 16-inch on-center grid in both directions. For light pedestrian sidewalks, 18-inch on-center spacing or 6×6 welded wire mesh (WWM) is often used.
What is the required lap splice overlap when connecting two pieces of rebar?
Under American Concrete Institute (ACI 318) standards, standard tension lap splices require overlapping bars by a minimum of 40 to 48 bar diameters (40d). For #4 (1/2") rebar, 40 × 0.5" = 20 inches minimum overlap. Tie the overlap securely at both ends with 16-gauge steel tie wire.
Why must rebar be supported on chairs instead of resting on the ground?
Rebar that rests on the subgrade dirt receives zero structural tensile strength and will rust quickly from ground moisture, causing spalling. Concrete chairs (or plastic dobies) elevate the rebar so it sits suspended in the middle-to-bottom third of the slab, completely encased in at least 2 to 3 inches of protective alkaline concrete.
How much does a 20-foot stick of #4 rebar weigh?
A standard 20-foot stick of #4 (1/2") rebar weighs approximately 13.36 pounds (0.668 lbs per foot × 20 feet). A full commercial bundle of #4 rebar contains 250 sticks and weighs approximately 3,340 pounds.
Can you bend rebar by hand on the jobsite?
Yes, #3 and #4 rebar can be bent cleanly on site using a manual rebar hickey bar or pipe bender. For corners and footing hooks, make sure the bend radius meets code (minimum bend diameter of 6 bar diameters = 3 inches for #4) to avoid micro-fracturing the steel core.