Rebar Spacing Calculator — Reinforcement Grid Layout
Plan your slab rebar grid layout, including bar counts, total length, 20ft pieces, rebar ties, and estimated weight.
How to Use This Calculator
- Enter your slab length and width in feet.
- Select the rebar size and spacing on center.
- Adjust the overlap length if needed (24" is a common default).
- Click "Calculate" to see bar counts, 20ft pieces, tie count, and weight.
How the Calculation Works
length_in = length_ft × 12
width_in = width_ft × 12
bars_lengthwise = ceil(width_in / spacing_in) + 1
bars_widthwise = ceil(length_in / spacing_in) + 1
total_length_ft = (bars_lengthwise × length_ft) + (bars_widthwise × width_ft)
total_pieces_20ft = ceil(total_length_ft / (20 - overlap_ft)) The calculator converts slab dimensions to inches to determine how many bars fit at the chosen spacing in each direction. It then converts back to feet for total rebar length and divides by usable length per 20ft bar after accounting for overlap (lap splice).
Variables:
- length_ft: Slab length in feet
- width_ft: Slab width in feet
- spacing_in: Rebar spacing on center in inches
- overlap_ft: Overlap (lap splice) length in feet
Pro Tips
- Always follow your engineer’s design and local building code for rebar size and spacing.
- Use rebar chairs or supports to keep the steel in the correct position within the slab.
- Overlap (lap splice) lengths often depend on bar size and design loads—24" is a common minimum but may need to be longer.
- Tie intersections enough to hold the grid together, but don’t overtighten and kink the bars.
Recommended Rebar Tools
Rebar Tie Wire
Wire for tying rebar intersections together.
View on Amazon →Rebar Tying Tool
Speeds up tying rebar on large projects.
View on Amazon →Rebar Chairs / Spacers
Support rebar at the correct height in the slab.
View on Amazon →Rebar Cutter / Bender
Cut and bend rebar safely and accurately.
View on Amazon →Frequently Asked Questions
How far apart should rebar be in a concrete slab?
Common spacing is 12"–24" on center, but the correct spacing depends on slab thickness, loads, and local code. Always follow an engineer’s design.
How much should rebar overlap (lap splice)?
24" is a common minimum lap length for small residential projects, but proper lap splice length depends on bar size, concrete strength, and code requirements.
Do I always need rebar in a concrete slab?
Light slabs sometimes use welded wire mesh instead of rebar, but slabs that support vehicles, walls, or heavy loads typically need rebar. Check local code and an engineer’s recommendations.
How many rebar ties do I need?
A simple rule of thumb is about 4 ties per intersection. This calculator uses that estimate to give you an approximate tie count.