How Much Concrete Do I Need? A Complete Guide
By MasonryCalc Team ·
How Much Concrete Do I Need?
Whether you’re pouring a patio, laying a garage floor, or setting fence posts, knowing how much concrete to order is critical. Order too little and the job stops. Order too much and you’re paying for wasted material.
The good news: the calculation is simple, and our Concrete Slab Calculator does it for you instantly.
The Basic Concrete Formula
For any rectangular pour:
Volume (cubic feet) = Length (ft) × Width (ft) × Thickness (ft)
To convert to cubic yards (the unit concrete is sold in), divide by 27:
Cubic Yards = Cubic Feet ÷ 27
Worked Example: 12 × 10 ft Patio at 4 Inches Thick
- Convert thickness: 4 inches ÷ 12 = 0.333 ft
- Volume: 12 × 10 × 0.333 = 40 cubic feet
- Cubic yards: 40 ÷ 27 = 1.48 cubic yards
- Add 10% waste: 1.63 cubic yards to order
How Many Bags Do I Need?
If you’re mixing by hand rather than ordering ready-mix:
| Bag Size | Yield per Bag | Bags for 1 yd³ |
|---|---|---|
| 40 lb | 0.30 cu ft | ~90 bags |
| 60 lb | 0.45 cu ft | ~60 bags |
| 80 lb | 0.60 cu ft | ~45 bags |
For pours over 1 cubic yard, ready-mix concrete is almost always cheaper and ensures consistent quality.
Common Concrete Depths by Application
- Footpaths and patios: 3–4 inches
- Residential driveways: 4 inches
- Garage floors: 4–6 inches
- Heavy vehicle areas: 6+ inches
- Fence post footings: Depth = 1/3 post height
Tips for Ordering Concrete
- Always add 10% to your calculated volume for waste.
- Order ready-mix for anything over 1 cubic yard.
- Have everything ready before the truck arrives — you have 90 minutes from batching.
- Check the mix design — 3,000 psi (20 MPa) for patios, 4,000 psi (25 MPa) for driveways.
- Confirm access — ready-mix trucks are large and need a clear path to the pour site.
Use Our Free Calculator
Skip the manual maths — our Concrete Slab Calculator handles imperial and metric, shows bag counts for 40, 60 and 80 lb bags, and highlights the +10% waste recommendation.
Pouring a floor slab? For floor screed (cement + sand, no aggregate) see our screed calculator.
Frequently Asked Questions
How many 80lb bags of concrete are in a cubic yard?
45 bags of 80lb premixed concrete make one cubic yard. Each bag yields approximately 0.60 cubic feet and there are 27 cubic feet in a cubic yard.
How much concrete do I need for a 10×10 slab?
A 10×10 foot slab at 4 inches thick needs 1.23 cubic yards or 56 bags of 80lb premixed concrete.
Should I use bags or ready-mix concrete?
For jobs under 1 cubic yard (about 45 bags of 80lb), bags are practical. For larger pours ready-mix is usually cheaper, faster and more consistent.