Rebar Calculator Formula
Understand the math behind the rebar calculator. Each variable explained with a worked example.
Formulas Used
Bars Along Length
bars_lengthwise = floor(slab_width / spacing_ft) + 1Bars Along Width
bars_widthwise = floor(slab_length / spacing_ft) + 1Total Bars (both directions)
total_bars = (floor(slab_width / spacing_ft) + 1) + (floor(slab_length / spacing_ft) + 1)Total Linear Feet
total_linear_ft = (floor(slab_width / spacing_ft) + 1) * slab_length + (floor(slab_length / spacing_ft) + 1) * slab_widthRebar Sticks to Buy
rebar_sticks = ceil(((floor(slab_width / spacing_ft) + 1) * slab_length + (floor(slab_length / spacing_ft) + 1) * slab_width) / rebar_length)Variables
| Variable | Description | Default |
|---|---|---|
slab_length | Slab Length(feet) | 20 |
slab_width | Slab Width(feet) | 20 |
spacing | Rebar Spacing(inches) | 18 |
rebar_length | Rebar Length(feet) | 20 |
spacing_ft | Derived value= spacing / 12 | calculated |
How It Works
How to Calculate Rebar Needed
Formula
Bars Along Length = floor(Width / Spacing) + 1
Bars Along Width = floor(Length / Spacing) + 1
Rebar is laid in a grid pattern (both directions). Standard rebar comes in 20-foot lengths. Overlap joints should be at least 24 bar diameters (about 18 inches for #4 rebar). Common sizes are #3 (3/8"), #4 (1/2"), and #5 (5/8").
Worked Example
A 20×20 ft slab with rebar on 18-inch centers, using 20 ft rebar.
- 01Spacing in feet = 18 / 12 = 1.5 ft
- 02Bars along length = floor(20 / 1.5) + 1 = 14 bars (each 20 ft long)
- 03Bars along width = floor(20 / 1.5) + 1 = 14 bars (each 20 ft long)
- 04Total bars = 14 + 14 = 28 bars
- 05Total linear feet = 14 × 20 + 14 × 20 = 560 lin ft
- 06Rebar sticks = 560 / 20 = 28 sticks
Frequently Asked Questions
What size rebar for a 4-inch slab?
#3 (3/8 inch) or #4 (1/2 inch) rebar on 18-24 inch centers is standard for 4-inch residential slabs. Some codes allow welded wire mesh (6×6 W1.4/W1.4) instead.
How much do I overlap rebar joints?
The standard splice length is 24 bar diameters minimum. For #4 rebar (1/2 inch), that is 12 inches. For #5 rebar (5/8 inch), that is 15 inches. Many specs call for 18 inches minimum.
Where should rebar be placed in a slab?
Place rebar at 1/3 of the slab thickness from the bottom, supported on chairs (rebar supports). For a 4-inch slab, the rebar should be about 1.5 inches from the bottom.
Learn More
Guide
Foundation Calculation Guide: Footings, Slabs, and Structural Loads
Learn how to calculate foundation requirements including footing sizes, concrete volumes, soil bearing capacity, and structural load distribution for residential buildings.
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