How Much Concrete Do I Need? Formulas, Examples, and a Free Calculator

Whether you're pricing a 20×20 garage slab or pouring footings for a deck, the first question is always the same: how many cubic yards do I need to order?

Get it wrong and you're either paying short-load fees for an emergency half-yard, or watching the truck driver dump your leftover concrete in a pile you didn't budget for. Neither is a good look in front of a customer.

This guide covers the core formula, worked examples for the most common pour types, waste factor rules of thumb, and a quick-reference table you can screenshot for the truck.

The Core Formula

Every concrete volume calculation comes back to the same equation:

That's it. Length times width gives you square footage. Multiply by thickness (in feet) and you get cubic feet. Divide by 27 because there are 27 cubic feet in one cubic yard.

The tricky part isn't the math — it's knowing what to plug in, how much waste to add, and how to handle shapes that aren't perfect rectangles.

Common Slab Calculations

Standard Driveway (20 × 20 at 4 inches)

Example: 2-car driveway
Length: 20 ft  |  Width: 20 ft  |  Thickness: 4 in (0.333 ft)

Garage Floor (24 × 30 at 4 inches)

Example: standard 2-car garage
Length: 24 ft  |  Width: 30 ft  |  Thickness: 4 in (0.333 ft)

Sidewalk (4 × 50 at 4 inches)

Example: 50-foot walkway
Length: 50 ft  |  Width: 4 ft  |  Thickness: 4 in (0.333 ft)

Footings and Walls

Footings and walls use the same core formula, but you're usually working with depth instead of thickness, and the dimensions are less forgiving because structural requirements are fixed.

Continuous Footing (12 in wide × 12 in deep × 60 ft long)

Example: garage perimeter footing
Length: 60 ft  |  Width: 1 ft (12 in)  |  Depth: 1 ft (12 in)

Pier Footings (Round, 12 in diameter × 36 in deep)

Volume = π × (radius²) × depth

For a 12" diameter pier: π × (0.5²) × 3 = 2.36 cubic feet per pier
2.36 ÷ 27 = 0.087 CY per pier
For a deck with 8 piers: 0.087 × 8 = 0.70 CY. Add 20% waste for round forms (concrete doesn't cooperate with Sonotubes as nicely as you'd like) and you're at about 0.84 CY — order 1 yard.

Quick Reference Table

Here's a table you can screenshot and keep on your phone. It shows cubic yards needed for common slab sizes at 4-inch thickness, before waste factor.

Remember: add 10% for waste on clean flatwork, 15% for footings, and 20% for pier footings or complex shapes.

What About Irregular Shapes?

Not every pour is a rectangle. Curved driveways, L-shaped patios, and angled walkways are common. For these, you have a few options:

• Break it into rectangles. An L-shaped patio is just two rectangles joined together. Calculate each one separately, then add the volumes. This works for most residential jobs.
• Use the triangle formula for angled sections. Area = ½ × base × height. Then multiply by thickness and divide by 27 like any other pour.
• Use a digital takeoff tool for curves. If you're tracing curved driveways, round patios, or free-form pool decks, manual math gets messy fast. This is where drawing the shape on screen and letting the software calculate the area pays for itself — especially when you're doing it five times a week on different bids.

The Waste Factor: How Much Extra to Order

The waste factor is the gap between the math and reality. Your subgrade isn't laser-flat. Your forms bow out slightly under pressure. The truck driver spills a little during the pour. The finishers pull a little more than expected on the edges.

Here's what experienced contractors typically add:

What About Cost?

Ready-mix pricing changes constantly and varies by region, PSI rating, delivery distance, and order size. Rather than listing numbers that'll be outdated in six months, here's what to know: always call your local batch plant for a current per-yard quote, ask about their short-load minimum (usually 8–10 CY — ordering less means a surcharge), and check if Saturday or early-morning delivery costs extra. Get quotes from at least two suppliers.

For small jobs under 1–2 cubic yards, price out bagged concrete too. An 80-pound bag of Quikrete yields about 0.6 cubic feet, so one cubic yard takes roughly 45 bags. Depending on your local ready-mix minimums and short-load fees, bags can be comparable or cheaper on small pours — and you don't have to wait for a truck.

Common Mistakes That Cost You Money

• Forgetting the thickness conversion. Four inches is 0.333 feet, not 4 feet and not 0.4 feet. This single mistake can throw your order off by 20%. If you catch it after the truck shows up, it's already too late.
• Measuring the planned area, not the actual area. The blueprint says 20×20 but the forms are set at 20'3" × 20'6" because the grade required a slight adjustment. Always measure the forms, not the plan.
• Skipping the waste factor. "The math says 5 yards, so I'll order 5 yards" is how you end up calling the batch plant mid-pour begging for an emergency load. Build in the 10% from the start.
• Not accounting for thickened edges. Many driveways and garage slabs have a thickened edge (sometimes called a turndown or monolithic footing) around the perimeter. A slab that's 4 inches in the field but 12 inches at the edges uses significantly more concrete than the flat-slab calculation suggests. Calculate the perimeter beam separately and add it.
• Ordering way too much for small jobs. For anything under 2 cubic yards, do the math on bags versus ready-mix. Between short-load fees and minimum order requirements, bagged concrete is sometimes the smarter call on small pours.