How Much Concrete Do I Need? A Simple Formula

How Much Concrete Do I Need? A Simple Formula

To figure out how much concrete you need, multiply length by width by thickness in feet, then divide by 27 to get cubic yards. That one formula covers slabs, footings, and pads. Add 5 to 10 percent for waste and you have your order.

Concrete is sold and priced by volume, not by area, so every estimate comes back to cubic yards. The mistake I see most often is people measuring square footage and stopping there. Square footage tells you nothing until you factor in how thick the pour is. A 4 inch slab and a 6 inch slab over the same footprint are two very different orders, and the plant needs yards.

The concrete calculator formula

Here’s the whole thing:

cubic yards = (length_ft × width_ft × thickness_ft) ÷ 27

The number 27 is there because a cubic yard is 3 feet by 3 feet by 3 feet, which is 27 cubic feet. Get all three of your measurements into feet first, and the math falls out clean.

The part people fumble is thickness, because slabs are specced in inches. Convert inches to feet by dividing by 12:

• 4 inches ÷ 12 = 0.33 ft
• 6 inches ÷ 12 = 0.50 ft
• 8 inches ÷ 12 = 0.67 ft

How to figure concrete for a slab, worked example

Say you’re pouring a 10 ft by 12 ft patio at 4 inches thick:

• Thickness: 4 ÷ 12 = 0.33 ft
• Volume: 10 × 12 × 0.33 = 40 cubic feet
• Cubic yards: 40 ÷ 27 = 1.48 cubic yards
• With 10 percent waste: about 1.63 cubic yards, so order 1.75

Always round up to the nearest quarter yard when you’re ordering ready-mix. The truck can’t come back cheaply for a wheelbarrow’s worth, and being short stops the whole pour while it sets up unevenly.

Common slab quantities at a glance

This table saves you the arithmetic for standard slab sizes at 4 inches thick, before waste. It’s a quick sanity check against whatever number your measurements give you.

Slab size	Area (sq ft)	Cubic yards at 4 in
8 x 8	64	0.79
10 x 10	100	1.23
10 x 12	120	1.48
12 x 12	144	1.78
12 x 20	240	2.96
20 x 20	400	4.94

For a full driveway or anything with an odd shape, break it into rectangles, calculate each, and add them up. The concrete slab calculator handles that for you and lets you set the thickness.

Don’t forget the extras

The raw formula gives you the ideal volume. Real jobs need a cushion:

• Waste: 5 to 10 percent for spillage and mix stuck in the truck.
• Subgrade dips: A low spot in the base quietly swallows concrete. Compact and level first.
• Thickened edges: Slabs are often poured thicker at the perimeter for strength, which adds volume the flat calculation misses.
• Footings and piers: Calculate these as separate elements and add them to the slab total.

To convert your final volume into a ready-mix order, or to check yards against square footage, use the concrete yardage calculator. If you’re doing a small pour with bagged mix instead, the concrete bag calculator turns your cubic yards into a 40, 60, or 80 lb bag count.

Calculate concrete needed without the math

If you don’t want to run the numbers by hand every time, that’s fine. The formula is worth understanding so you can spot a bad estimate, but for day-to-day work most people just enter dimensions and read off the answer. Our companion guide on how to calculate concrete yards goes deeper on odd shapes like circles, footings, and stairs.

The main thing is to never order off square footage alone. Volume is what shows up on the truck, and volume is what you pay for.

FAQ

What is the formula for how much concrete I need?

Multiply length by width by thickness, all in feet, then divide by 27 for cubic yards. For a slab measured in inches, divide the thickness by 12 first to convert it to feet.

How do I figure concrete for a slab in inches?

Convert the thickness from inches to feet by dividing by 12, then multiply by the length and width in feet and divide by 27. A 4 inch slab uses 0.33 ft as its thickness value.

How much concrete do I need for a 10x10 slab?

A 10 by 10 slab at 4 inches thick needs about 1.23 cubic yards before waste. Round up to roughly 1.5 yards when ordering to cover spillage and an uneven subgrade.

Why do I divide by 27?

Because a cubic yard is 3 feet on every side, which equals 27 cubic feet. Dividing your cubic feet by 27 converts the volume into the cubic yards that ready-mix plants sell.

How much extra concrete should I order?

Add 5 to 10 percent to your calculated volume. That covers spillage, mix left in the truck, and low spots in the base, and it keeps you from stopping the pour to mix more.

Do I measure concrete by square feet or cubic yards?

Cubic yards. Square footage ignores thickness, and thickness is what turns an area into a volume. The plant fills the truck by yard, so your estimate has to be in yards too.

Ready to size your pour? Enter your dimensions in the concrete slab calculator and get cubic yards, bags, and a waste allowance instantly.