Concrete Calculator
Cubic yards of concrete needed for a slab, plus bags required by size.
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About the Concrete Calculator
Calculates how much concrete a slab needs — in cubic yards (how ready-mix is sold) and in premixed bags of 3 common sizes — from the slab's length, width, and thickness, with an adjustable waste/overage buffer.
- Enter the slab's length and width in feet.
- Enter the thickness in inches — typically 4 in for patios/sidewalks, 5-6 in for driveways.
- Adjust the waste/overage buffer for spillage, uneven subgrade, or form leakage.
- Read the cubic yards to order and the equivalent bag counts.
A 10ft × 10ft slab at 4 inches thick with a 10% waste buffer: raw volume of 33.3 cubic ft, 36.7 cubic ft with the buffer — about 1.36 cubic yards, or 123 bags at 40 lb, 82 bags at 60 lb, or 62 bags at 80 lb.
| Bag Size | Cured Volume Yield |
|---|---|
| 40 lb | 0.30 ft³ |
| 60 lb | 0.45 ft³ |
| 80 lb | 0.60 ft³ |
Bag yields are Quikrete's published cubic-feet-per-bag figures for standard Concrete Mix — other brands run very close but always check your specific product's label. For pours beyond roughly 1 cubic yard, ready-mix delivery is usually more practical and cost-effective than bags. This calculator only computes concrete volume — it doesn't size rebar, gravel base, or formwork, and doesn't check local building code requirements for slab thickness.
- • A 10% waste buffer is a common default for spillage and uneven subgrade — increase it for a more irregular or sloped site.
- • For anything beyond roughly 1 cubic yard, ready-mix delivery is usually cheaper and far less labor-intensive than mixing dozens of bags by hand.
- • This tool doesn't check your local building code's minimum slab thickness for your application (driveway vs. patio vs. sidewalk) — verify that separately.
- Quikrete — Concrete Mix product data sheet (cubic feet yield per bag size) — accessed 2026-08-30