Concrete Calculator
How much concrete to order — yards for a plant, bags for a small pad. Click any i for detail.
Slab size
Length (ft)
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Length
Long side of the rectangle in feet. |
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Width (ft)
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Width
Short side of the rectangle in feet. |
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Thickness (in)
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Thickness
Typical patio 4 in; driveway often 4–6 in. |
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Waste (%)
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Waste
Over-excavation and pump leftover. 10% is a common adder. |
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Bag volume (cu ft)
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Bag
80-lb bags are ~0.60 ft³; 60-lb ~0.45; 40-lb ~0.30. |
How to use this calculator
- Enter length and width in feet.
- Set thickness in inches.
- Add waste if the plant or bags should cover extras.
- Compare yards vs bags.
Results explained
Net yards = L × W × (thickness÷12) ÷ 27. The headline includes waste. Bags divide that volume by the bag size you entered.
Quick reference: bags vs yards
Ready-mix is usually cheaper above about 1 yard once short-load fees are counted.
| Item | Detail |
|---|---|
| 80-lb bag | ~0.60 cubic feet |
| 1 cubic yard | 27 cubic feet ≈ 45 eighty-pound bags |
| 4 in slab, 100 sq ft | ≈ 1.23 yd³ net |
| Typical plant increment | ¼ or ½ yard |
For dollars see the concrete slab / patio / driveway cost calculators.
How the estimate is built
cu ft = L × W × (in / 12). cu yd = cu ft ÷ 27. Bags = cu yd × 27 ÷ bag ft³.
Example scenario
A 10×10 ft × 4 in slab is 33.3 ft³ = 1.23 yd³ net, about 1.36 yd³ with 10% waste.
FAQ
Is this a concrete quote?
No — quantity only. Delivery, pump, rebar, and finishing are separate.
Circular pad?
Use πr² × thickness, or split into rectangles. This page is rectangular.
Why not the cost calculator?
Cost pages model labor midpoints. This page answers how many yards or bags to buy.