Slab size
Thickness units
Waste
Order
- Cubic yards
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- 60 lb bags
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- 80 lb bags
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Enter length, width, and thickness.
This is an estimate, not structural engineering, not a mix design, and not a price quote. It does not choose thickness, strength (PSI), or reinforcement. Confirm volume and mix with your ready-mix plant or inspector. Check the yield printed on the bag.
How the volume is counted
Slab volume is length × width × thickness. Thickness in inches is divided by 12 to get feet, then divided by 27 to get cubic yards. Waste is extra material — 10% is a common pad for spillage and uneven subgrade.
Bag counts always round up. Typical yields used here: 80 lb → 0.60 cubic feet, 60 lb → 0.45 cubic feet. The bag in your hand may differ.
Example: 10 ft × 10 ft × 4 in with 10% waste is 1.36 yd³ — 62 bags of 80 lb, or 82 bags of 60 lb.
FAQ
How is a slab measured in this calculator?
Length × width × thickness. Plan dimensions are in feet (or meters). Thickness is in inches (or centimeters) and is divided by 12 to get feet. That cubic-foot volume is divided by 27 to get cubic yards. You type the thickness; this page does not pick it.
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab at 4 inches is 33.33 cubic feet, or 1.23 cubic yards before waste. At the default 10% waste that is 1.36 yards — 62 bags of 80 lb, or 82 bags of 60 lb. Change thickness in the form. This tool does not tell you how thick a patio or driveway should be.
Should I order cubic yards or bags?
Yards are what you read to a ready-mix plant. Bags are what you mix on the job. This page computes both from the same slab volume. 80 lb bags assume a typical 0.60 cubic foot yield; 60 lb bags assume 0.45. Counts always round up. Read the yield on the bag you actually buy.
How many 80 lb bags are in a cubic yard? What about 60 lb?
About 45 bags of 80 lb (27 ÷ 0.60). About 60 bags of 60 lb (27 ÷ 0.45). A leftover fraction is still another bag. Those yields are typical, not a product SKU. Confirm on the bag.
What does the waste percent do?
It pads the volume for spillage and an uneven subgrade. Default is 10%. Use 5% if the grade is tight; 15% if the forms are rough. Waste is extra material, not advice on how thick to pour.
Is this structural engineering?
No. Quantity only. It does not choose thickness, strength (PSI), rebar, mesh, or mix design. Do not size a footing or a load-bearing slab with this page. That is a builder, inspector, or engineer.
Do I still confirm the order with the plant?
Yes. Copy the order line and read it to the ready-mix plant. They confirm volume, mix, and a minimum load. This is not a price quote. Under about 1 cubic yard, bags are usually simpler than a truck; over 1 yard, ready-mix is usually less mixing. That cutoff is a rule of thumb — ask the plant.