How Much Concrete Do I Need? How to Calculate Your Order
Ordering concrete isn't like buying lumber, where you can make a second trip for extra boards. Once you've committed to a delivery, you need the right amount on the truck — too little and your pour stops short, too much and you're paying for material that can't be held on-site.
The calculation is simpler than most people expect. Here's the formula for the most common project shapes, a worked example for a standard slab, and the most important adjustments to make before placing your order.
The Core Formula: Length × Width × Depth ÷ 27
Concrete is sold by the cubic yard. One cubic yard equals 27 cubic feet. The formula for any rectangular slab, footing, or wall is:
(Length ft × Width ft × Depth ft) ÷ 27 = Cubic Yards
The step most people miss: convert all dimensions to feet before calculating. Depth is almost always measured in inches on-site, so divide by 12 before plugging it in. A 4-inch slab is 0.333 feet. A 6-inch slab is 0.5 feet.
Worked example — a 20×30-foot patio at 4 inches deep:
(20 × 30 × 0.333) ÷ 27 = 7.4 cubic yards
Add 10% overage for waste, uneven subgrade, and measurement variance:
7.4 × 1.10 = 8.1 cubic yards → order 8.5 or 9
NRMCA TIP 8 on Concrete Yield confirms that customers should order more than the calculated volume to account for waste, spillage, and over-excavation — the concrete producer cannot guarantee exact yield due to normal material variability. Ordering 5–10% over your calculated volume on any job is standard practice.
Calculating Concrete for Footings and Columns
Footings and columns follow the same cubic footage logic, just with different shapes.
- Rectangular footings: Use the same length × width × depth formula. For a continuous perimeter footing, calculate each wall run separately and add the totals.
- Round columns or tube footings: Use the cylinder volume formula — π × radius² × height — then divide by 27. For a 12-inch diameter sonotube footing 4 feet deep:
- π × (0.5)² × 4 = 3.14 cubic feet = 0.116 cubic yards per footing. For 10 footings of this size: approximately 1.2 cubic yards, plus 10% overage.
- Steps: Calculate each tread-and-riser as a separate rectangular block (tread depth × riser height × width), sum the volumes, then divide by 27.
When to Add Extra Concrete and When Not To...
The 10% overage rule applies to most residential flatwork. For specific job conditions, adjust up or down:
- Add more than 10% when: your subgrade has significant depth variation, your forms follow a curved or irregular shape, the project spans multiple separate pours from one order, or hot conditions will compress finishing time and increase the chance of placement errors.
- Stay closer to 5% when: you're pouring a straightforward rectangular slab on flat, consistent subgrade, and your forms are rigid and true with verified measurements.
NRMCA CIP 31 on Ordering Ready Mixed Concrete notes that concrete normally sets and hardens within two to eight hours after mixing — it cannot be stored on-site for a future pour. Excess material from a delivery that cannot be placed must be handled promptly. The time limit for discharge is set by the producer and stated on the delivery ticket; confirm this window with your supplier before ordering so you understand how much flexibility you have on-site.
How Slab Thickness Changes Your Order Fast
Thickness is the variable with the biggest effect on your total, and the one where project requirements vary most. Confirm the appropriate thickness for your application before calculating.
Industry guidelines, as summarized in Concrete Network's driveway and slab guide drawing on Portland Cement Association recommendations, establish these general benchmarks for residential work: 4 inches for sidewalks and patios not subject to vehicle loads, 4–5 inches for standard residential driveways, and 5–6 inches for driveways bearing trucks or RVs. Structural footings are governed by local building code — confirm with your building department before pouring.
The difference between a 4-inch and 5-inch slab on a 20×40-foot driveway is roughly 2.5 additional cubic yards — a meaningful cost and strength difference if you're under-building for your load.
Frequently Asked Questions About How Much Concrete Is Needed?
How do I calculate cubic yards of concrete for a slab? Multiply length (ft) × width (ft) × thickness (ft), then divide by 27. Convert thickness from inches to feet first by dividing by 12. Add 10% for overage.
What is the minimum order for ready mix concrete delivery? Minimums vary by supplier and logistics. Contact our team to confirm current minimums for your project.
Can I use the same formula for footings and columns? Rectangular footings use length × width × depth. Round tube footings use π × radius² × height. Divide all results by 27 to get cubic yards.
What happens if I order too much concrete? Ready mix cannot be stored on-site for a future pour — it sets and hardens within hours of mixing. The discharge time limit is set by the producer and stated on your delivery ticket. It's better to order slightly more than less, since a second delivery is expensive and disruptive.
Place a Smarter Concrete Order with American Rock & Rent
Once you have your cubic yard total, visit our ready mix concrete page to see available mix types, or call us today and our team will cross-check your calculation before your order is placed. We supply Fresno homeowners and contractors with 3/4 aggregate, 3/8 aggregate, slurry, bagged, and Portland Cement mixes — and we'd rather catch a measurement error over the phone than after the truck leaves.