Concrete Calculator: How Much Concrete Do I Need for Slabs, Footings & Posts?
Calculate cubic yards and bags of premix concrete needed for patios, driveways, and post holes with mix ratios, waste margins, and cost estimation.
Concrete Calculator: How Much Concrete Do I Need for Slabs, Footings & Posts?
Whether you are pouring a backyard patio slab, pouring deck footings, or setting fence posts, answering the question "how much concrete do i need" accurately prevents costly cold joints from running short or wasting money on surplus ready-mix deliveries.
This guide explains the cubic yard mathematical formulas, premix bag yields (60 lb and 80 lb bags), standard waste allowances, and connects to the CalculatorAll Concrete Calculator.
The Master Concrete Cubic Yard Formula
Because 1 cubic yard equals exactly 27 cubic feet (3 × 3 × 3 ft):
Cubic Feet = Length (ft) × Width (ft) × [Thickness (inches) / 12]
Cubic Yards = Cubic Feet / 27
Total Order Volume = Cubic Yards × 1.10 (Adding a 10% safety waste allowance)
Worked Example: 10 ft × 15 ft Patio Slab (4 inches deep)
Suppose you are pouring a 10 ft by 15 ft concrete slab for an outdoor dining patio at standard 4-inch residential thickness:
- Calculate Thickness in Feet:
4 inches / 12 = 0.3333 ft. - Calculate Cubic Feet:
10 ft × 15 ft × 0.3333 ft = 50.0 cubic feet. - Convert to Cubic Yards:
50.0 / 27 = 1.852 cubic yards. - Add 10% Waste Margin:
1.852 × 1.10 = 2.04 cubic yards(Order 2.0 to 2.25 yards).
How Many Bags of Concrete Do I Need?
If you are mixing bags by hand or with an electric drum mixer:
- 80 lb Bag Yield: ~0.60 cubic feet (requires 45 bags per cubic yard).
- 60 lb Bag Yield: ~0.45 cubic feet (requires 60 bags per cubic yard).
- 50 lb Bag Yield: ~0.375 cubic feet (requires 72 bags per cubic yard).
For the 10 × 15 ft patio (50 cu ft total):
- Using 80 lb bags:
50 / 0.60 = 84 bags(+ 8 contingency bags = 92 bags). - Using 60 lb bags:
50 / 0.45 = 111 bags(+ 11 contingency bags = 122 bags).
Bags vs. Ready-Mix Truck Delivery: When to Switch
- Under 1.0 Cubic Yard (under 45 bags): Bagged premix (Quikrete, Sakrete) is cost-effective and manageable for 1 to 2 people.
- Over 1.5 to 2.0+ Cubic Yards (over 90 bags): Ready-mix truck delivery is strongly recommended to avoid physical exhaustion, ensure uniform structural hydration, and prevent structural cold seams.
Related Construction Calculators
- Concrete Calculator — Calculate cubic yards and bag requirements.
- Drywall Calculator — Calculate sheets and joint compound.
- Flooring Calculator — Calculate square footage and tile/hardwood box counts.
Try the numbers with our calculator
Use your own assumptions instead of relying on a generic example.
Calculate Concrete Volume & Bags
