Concrete Slab Calculator
Use this concrete slab calculator to find the exact volume of concrete needed for any rectangular slab — patios, garage floors, sidewalks, shed pads, and pool surrounds. Enter length, width, and thickness to get cubic yards for ready-mix truck ordering, the number of 60lb and 80lb pre-mixed bags, and a side-by-side cost comparison. The calculator follows ACI 318 standards with a built-in waste factor you can adjust from 5% to 15% depending on the complexity of your pour. Residential patios and sidewalks use a standard 4-inch thickness; garage floors require a minimum 4 inches (6 inches if you park a truck or RV). For slabs over 1 cubic yard, ready-mix almost always beats bagging — mixing more than 50 bags by hand or in a rented mixer is punishing work and usually costs more than a ready-mix delivery. Use this calculator to compare both options before calling your concrete supplier.
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Top PickQuikrete 60lb Concrete Mix — Home Depot~$4.98/bag* Affiliate links — we may earn a commission at no cost to you.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab at 4 inches thick?
A 10×10 slab at 4 inches thick is 1.23 cubic yards. With a 10% waste factor you need approximately 74 bags of 60lb mix (0.45 cu ft each) or 56 bags of 80lb mix (0.60 cu ft each). Budget about $370 for 60lb bags or $360 for 80lb bags at current prices.
Should I order ready-mix or use bags?
For slabs over 1 cubic yard (roughly 5×5 ft at 4 inches thick), ready-mix concrete is more economical and far less labor-intensive. Ready-mix typically costs $125–$150 per cubic yard delivered. Bags make sense for small pours under 0.5 cubic yards, or when a truck can't access the site.
What is the standard thickness for a concrete patio slab?
The standard residential patio slab is 4 inches thick — this meets most building codes for pedestrian traffic and is strong enough for outdoor furniture and grills. Use 6 inches if the slab will support a vehicle, hot tub, or heavy above-ground pool. Driveways should always be at least 6 inches.
How much does a cubic yard of concrete cover at 4 inches thick?
One cubic yard of concrete covers 81 square feet at 4 inches thick (27 cu ft ÷ 0.333 ft depth). At 6 inches thick, one yard covers 54 sq ft. At 3 inches (not code for most slabs), it covers 108 sq ft. Always add 10% for waste and irregular forms.