Concrete Calculator
This concrete calculator estimates how much ready-mix or bagged concrete you need for a slab, sidewalk, pad or footing. It accounts for a waste percentage and converts the total into the three most common bag sizes so you can buy the right amount the first time.
Your answer
- 80 lb Bags
- 62
- 60 lb Bags
- 82
- 40 lb Bags
- 123
How it works
Formula
Cubic Feet = L × W × (Thickness ÷ 12) × (1 + Waste%) • Cubic Yards = Cu Ft ÷ 27 • Bags = ceil(Cu Ft ÷ yield) (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³)
Worked example
A 10 × 10 ft pad at 4 in with 10% waste = 33.33 ft³ × 1.10 = 36.67 ft³ → 1.36 yd³, or 62 × 80 lb bags.
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How to estimate concrete for a slab
Concrete volume is length × width × thickness, converted to cubic yards. Because forms are never perfectly plumb and subgrade is never perfectly flat, add a waste factor of 10% (15% for irregular shapes, curved forms or slopes). Ready-mix suppliers deliver in half-yard increments and charge a short-load fee below 3–5 yards, so it is usually worth rounding up rather than short-pouring and adding a cold joint later.
Slab thickness by application
Sidewalks and light patios: 4 inches over 4 inches of compacted gravel. Residential driveways: 4 inches (5 inches in northern climates or for RVs). Garage floors: 4 inches with wire mesh or fiber. Shed and workshop floors: 4–5 inches. Heavy-truck aprons or shop floors: 6 inches with rebar. Footings for a garden wall: 8 inches wide × 12 inches deep. Structural footings: sized per local code.
Bags vs ready-mix — which is cheaper?
One cubic yard of concrete equals about 45 × 80-lb bags, 60 × 60-lb bags, or 90 × 40-lb bags. In dollar terms, bagged concrete costs roughly $5–$7 per cubic foot ($135–$190 per yard) plus your time to mix. Ready-mix delivered runs $140–$180 per yard with a short-load fee (typically $70–$150) for orders under 3–5 yards. The break-even is usually between 1 and 2 cubic yards — smaller than that, bags win; bigger than that, ready-mix wins on cost, speed and consistency.
Worked example
A 20 ft × 24 ft garage floor at 4 inches with 10% waste: 20 × 24 × (4 ÷ 12) = 160 ft³. Add 10% waste → 176 ft³. Divide by 27 → 6.52 cubic yards. Order 6.5 or 7 yards ready-mix at 3000 PSI, with wire mesh laid at mid-slab. In bags that would be about 293 × 80-lb bags — well past the point where the truck is cheaper.
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