Concrete Slab Calculator
Calculate concrete cubic yards, bags, rebar grid, sub-base, form boards, and total cost for any concrete slab project.
ft
ft
in
Cubic Yards
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80 lb Bags —
60 lb Bags —
Extended More scenarios, charts & detailed breakdown ▾
ft
ft
in
%
Cubic Yards
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80 lb Bags —
60 lb Bags —
Slab Area —
Professional Full parameters & maximum detail ▾
ft
ft
in
in
in
sq ft/roll
ft
Concrete & Sub-base
Concrete (yd³) —
Sub-base (yd³) —
Reinforcement
Rebar Linear Feet —
Wire Mesh Rolls —
Expansion Joint (lf) —
Formwork & Finish
Form Boards (lf) —
Sealer Coverage —
Cost
Total Material Cost —
How to Use This Calculator
- Enter the length and width in feet and thickness in inches.
- Results show cubic yards needed and bag counts for 80 lb and 60 lb bags.
- Use Extended tabs for rebar and cost estimate details.
Formula
Volume (cu ft) = L × W × (Thickness ÷ 12)
Cubic yards = Volume ÷ 27
80 lb bags = ceil(cu yd × 45) | 60 lb bags = ceil(cu yd × 60)
Example
Example: 20 ft × 30 ft × 4 in slab → 200 cu ft → 7.41 yd³ → 334 bags of 80 lb (with 10% buffer).
Frequently Asked Questions
- A slab calculator focuses on slab-specific elements: rebar grid spacing, sub-base depth, form boards around the perimeter, expansion joints, and wire mesh — not just volume.
- For a typical residential slab with 12-inch rebar spacing, divide each dimension by the spacing and add 1. Total bars × their lengths gives you linear feet needed.
- Yes — most slabs need a 4-inch compacted gravel sub-base to provide drainage and stability. Thicker sub-base may be needed on unstable soils.
- Form boards are placed around the slab perimeter. Perimeter = 2 × (Length + Width). 2×6 or 2×8 lumber is standard for 4–6 inch slabs.
- Ready-mix concrete runs $120–$200 per cubic yard. A 400 sq ft (10×40 ft), 4-inch slab needs about 5 yards at roughly $700–$1,000 in material.