Rebar Calculator
Estimate rebar quantity, length, and weight for concrete reinforcement
Length of the concrete area
Width of the concrete area
On-center spacing
#3=3/8", #4=1/2"
Bars (Length Direction)
—
bars
Bars (Width Direction)
—
bars
Total Rebar Length
—
ft
Total Weight
—
lbs
Estimated Cost
—
USD
Disclaimer: Results are estimates based on standard formulas. Not a substitute for professional advice from a licensed contractor, engineer, or architect. Verify all calculations before making purchasing or construction decisions.
How to Use This Calculator
Proper rebar reinforcement prevents cracking and extends the life of your concrete. Our calculator helps you determine the number of bars, total length, and weight needed.
- Measure the length and width of your concrete area
- Choose rebar size based on slab thickness
- Select spacing (18" standard for residential)
- Add 10% for overlaps and waste at joints
Expert Insights
Smart analysis for your Concrete & Masonry project
Slab Thickness Guide
4" for patios, 5-6" for driveways, 6-8" for heavy vehicle loads. Our calculator references your dimensions against ACI 301-20 standards for optimal thickness recommendations.
Bag vs Ready-Mix Analysis
For pours under 1 cubic yard, 80lb bags cost less. Over 3 yards, ready-mix saves 30-50%. Our calculator factors your volume against regional pricing.
Curing & Strength Timeline
Concrete reaches 50% strength in 3 days, 70% in 7 days, and full cure in 28 days. Cold weather? Our calculator flags when heated enclosures are needed.
Calculation Formula
Bars per direction = (Dimension / Spacing) + 1
Total length = Sum of all bar lengths
Weight = Total length x lbs/ft (by bar size)
Concrete Bag Size Comparison
| Bag Size | Bags per Cu Yd | Coverage at 4" | Best For |
|---|---|---|---|
| 40 lb | ~90 bags | ~0.30 sq ft/bag | Small repairs, fence posts |
| 50 lb | ~72 bags | ~0.37 sq ft/bag | Steps, small pads |
| 60 lb | ~60 bags | ~0.45 sq ft/bag | Patios, walkways |
| 80 lb | ~45 bags | ~0.60 sq ft/bag | Driveways, large slabs |
| Ready-Mix | 1 truck load | ~81 sq ft/yard | Projects over 1 cu yd |
Authority References
Our calculations are based on guidelines and standards from these authoritative sources:
- American Concrete Institute (ACI) — ACI 301-20 — Specifications for Structural Concrete. Learn more
- Portland Cement Association (PCA) — Concrete Slab Thickness Guidelines. Learn more
- Quikrete — Concrete Calculator & Mix Guidelines. Learn more
- NRMCA — Concrete in Practice (CIP) Series. Learn more
Frequently Asked Questions
What size rebar do I need for a concrete slab?
#3 or #4 rebar is standard for residential slabs (4-6" thick). Use #4 for driveways and #5 for commercial.
How far apart should rebar be spaced?
Standard spacing is 18" on center for residential slabs and 12" for driveways. Always overlap bars 15-20x the bar diameter at joints.
Do I need rebar in my concrete slab?
Rebar is recommended for slabs 5" or thicker, driveways, and any slab bearing heavy loads. For 4" patio slabs, wire mesh may be sufficient.