Concrete Slab Calculator
Enter the length, width and thickness of the slab to get the volume in cubic yards to order, the equivalent number of 40, 60 and 80 lb bags, the weight of the pour and an optional cost. The waste allowance is applied on top of the form volume, the way ready-mix is actually ordered.
Concrete to order
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Results are estimates for planning purposes. Verify with your supplier, engineer, or a qualified professional before ordering material or performing work.
Bag yields and bags per cubic yard
| Bag size | Yield per bag | Bags per cubic yard | Mixing water (starting) |
|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | 3 pt |
| 50 lb | 0.375 ft³ | 72 | 3.5 pt |
| 60 lb | 0.45 ft³ | 60 | 4 pt |
| 80 lb | 0.60 ft³ | 45 | 6 pt |
| 90 lb | 0.675 ft³ | 40 | 7 pt |
Yields and starting water: QUIKRETE Concrete Mix No. 1101 data sheet. Sakrete's High-Strength Concrete Mix publishes the same 0.30 ft³ (40 lb) and 0.45 ft³ (60 lb) yields. Yield varies with the mix — check the bag you are buying.
Before you order
- Order more than the form volume. ASTM C94, quoted in NRMCA TIP 8, lists the reasons the truck comes up short against a paper calculation: waste, spillage, loss of air content, settlement, form deflection, over-excavation of the subgrade. The waste field above is that allowance.
- Bagged mix is for 2 in and thicker. QUIKRETE Concrete Mix is designed for pouring concrete 2 in (50 mm) thick or more.
- Weight. Normalweight concrete runs 140 to 150 lb/ft³ (NRMCA CIP 36) — roughly 3,780 to 4,050 lb per cubic yard. This calculator uses 145 lb/ft³.
- Control joints. QUIKRETE's slab procedure: cut 1 in (25 mm) deep control joints every 6 ft to 8 ft (1.8 m to 2.4 m).
- Not a structural design. Thickness, reinforcement and subgrade come from your plans or an engineer — this tool only converts the dimensions you enter into volume.
How it's calculated
- Volume
V (ft³) = length (ft) × width (ft) × thickness (in) / 12 → yd³ = ft³ / 27- Volume to order
V order = V × (1 + waste % / 100)ASTM C94, quoted in NRMCA TIP 8, lists why the calculated form volume is never enough: waste, spillage, loss of air content, settlement, form deflection, over-excavation of the subgrade.- Weight
weight (lb) = V (ft³) × 145 lb/ft³Normalweight concrete has a density of 140 to 150 lb/ft³ (2,240 to 2,400 kg/m³); 145 is the midpoint used here.- Bags
bags = ceiling(V order (ft³) / yield per bag), yield = 0.30 ft³ (40 lb), 0.45 ft³ (60 lb), 0.60 ft³ (80 lb)Published yields for QUIKRETE Concrete Mix No. 1101. Sakrete's High-Strength Concrete Mix lists the same 0.30 and 0.45 ft³ yields. Check the bag you buy — yield varies by product.
Frequently asked questions
20 ft x 10 ft x 4 in is 66.7 cubic feet, or 2.47 cubic yards of form volume. With a 10% waste allowance you would order about 2.72 cubic yards.
45. An 80 lb bag of QUIKRETE Concrete Mix yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet, so 27 / 0.60 = 45 bags. It takes 60 bags of 60 lb or 90 bags of 40 lb.
Normalweight concrete has a density of 140 to 150 lb per cubic foot (NRMCA CIP 36), so a cubic yard weighs roughly 3,780 to 4,050 lb. This calculator uses 145 lb per cubic foot.
Order more than the volume you calculate from the forms. ASTM C94, quoted in NRMCA TIP 8, points to waste, spillage, loss of air content, settlement, deflection of the forms and over-excavation of the subgrade. A 5 to 10% allowance is common on small pours; your supplier may also have a minimum load charge.
QUIKRETE Concrete Mix is designed for pouring concrete 2 in (50 mm) thick or more. Thinner repairs need a sand mix or a resurfacer, not a concrete mix.
QUIKRETE's slab procedure is to cut control joints 1 in (25 mm) deep every 6 to 8 ft (1.8 to 2.4 m) with a grooving tool, once the concrete has stiffened slightly.
No. It converts the dimensions you enter into volume, bags and weight. Thickness, reinforcement, subgrade preparation and the mix design come from your plans, your supplier or an engineer.
Sources & references
- QUIKRETE Concrete Mix No. 1101 — product data sheet (yield per bag, mixing water, 4,000 psi at 28 days, 2 in minimum thickness)
- Sakrete High-Strength Concrete Mix — technical data sheet (yield per bag)
- NRMCA CIP 36 — Structural Lightweight Concrete (normalweight density 140–150 lb/ft³)
- NRMCA TIP 8 — Concrete Yield (why to order more than the calculated volume, per ASTM C94)
Content checked against these sources — last reviewed August 27, 2026.