Concrete Calculator for Slabs, Footings & Pre-Mix Bags
🛒 Recommended Materials & Supplies
Compare pricing and in-stock inventory for your project:
Sakrete High-Strength Concrete Mix (80 lb)
Ideal for 4-inch patio slabs, foundation footings, and structural posts.
Electric Portable Cement Mixer (3.5 Cu Ft)
Saves hours of physical mixing for 20+ bags of dry concrete.
Magnesium Bull Float & Concrete Finishing Kit
Essential for smoothing and floating slabs before final broom finish.
How This Estimator Works
This concrete calculator accurately computes the exact volume of ready-mix concrete or pre-blended dry bags needed for any residential or commercial project. Whether you are pouring a patio slab, driveway, sidewalk, foundation footing, or round post hole, the tool handles both rectangular dimensions (Length × Width × Thickness) and cylindrical forms (Diameter × Depth). It provides simultaneous conversions in cubic yards (standard in the United States and Canada) and cubic meters (standard in the UK, Australia, and New Zealand), and automatically calculates how many 80 lb or 60 lb bags of Sakrete or Quikrete you need. A built-in 5% to 15% waste allowance ensures you never run short mid-pour.
Formulas & Calculation Principles
Rectangular Slab Volume (cu yd) = [Length (ft) × Width (ft) × (Thickness (in) / 12)] ÷ 27 × (1 + Waste % / 100)
Round Hole / Column Volume (cu yd) = [π × (Radius (in) / 12)² × Depth (ft)] ÷ 27 × (1 + Waste % / 100)
80 lb Bags Needed = Total Volume (cu ft) ÷ 0.60 cu ft
60 lb Bags Needed = Total Volume (cu ft) ÷ 0.45 cu ft
25 kg Metric Bags Needed = Total Volume (cu m) ÷ 0.012 cu m
Step-by-Step Worked Example
Planning a 10 ft by 10 ft backyard patio slab that is 4 inches thick with a standard 10% waste buffer:
1. Base volume: 10 ft × 10 ft × (4 ÷ 12) ft = 33.33 cubic feet.
2. In cubic yards: 33.33 ÷ 27 = 1.23 cubic yards.
3. Adding 10% waste buffer: 1.23 × 1.10 = 1.36 cubic yards (36.67 cu ft).
4. Bag requirements:
- 80 lb bags (0.60 cu ft yield): 36.67 ÷ 0.60 = 61.1 -> Buy 62 bags.
- 60 lb bags (0.45 cu ft yield): 36.67 ÷ 0.45 = 81.5 -> Buy 82 bags.
Frequently Asked Questions (FAQ)
How many 80-pound bags of concrete are in a cubic yard?
This comes directly from the bag yield: 27 cubic feet per cubic yard divided by 0.60 cubic feet per bag equals exactly 45 bags of 80 lb mix (27 ÷ 0.60 = 45 bags).
How many 60-pound bags of concrete make a cubic yard?
This comes directly from the bag yield: 27 cubic feet per cubic yard divided by 0.45 cubic feet per bag equals 60 bags of 60 lb mix (27 ÷ 0.45 = 60 bags).
How thick should a concrete patio or sidewalk be?
Four inches (4") is typical for residential patios and walkways. For heavy vehicle driveways or RV pads, 5 to 6 inches with steel rebar reinforcement is recommended.
What waste percentage should I use for concrete?
Use 10% for flat ground slabs with rigid forms. Use 15% for rough ground, footings, or post holes with uneven earth walls.
When is it cheaper to order a ready-mix delivery truck versus mixing bags?
If your project requires more than 1.5 to 2 cubic yards (roughly 70 to 90 bags of 80 lb mix), ordering a ready-mix concrete truck is typically cheaper, significantly faster, and saves immense physical labor.
What are the regional differences between US, UK, and Australia?
The US and Canada measure bulk pours in cubic yards and bagged mix in 60 lb / 80 lb bags. The UK and Australia measure volume in cubic meters (m³) and bag dry concrete in 20 kg or 25 kg bags.
How long does concrete need to cure before walking on it?
You can walk on fresh concrete after 24 to 48 hours. Wait at least 7 days before driving light passenger vehicles, and allow a full 28 days to reach maximum design compressive strength (typically 3,500 to 4,000 PSI).
Technical & Trade Disclaimer
Material quantities and volume estimates provided by this calculator are for planning and estimation purposes. Always consult local building codes, structural engineering drawings, or professional trade contractors before placing bulk orders.