How much concrete do I need?

Formula

L × W × T ÷ 27feet in, cubic yards out

Coverage at 4″

81 ft² per yd³before waste

Coverage at 6″

54 ft² per yd³before waste

Waste factor

+10%assumption, not a standard

Short version

• Length × width × thickness, all in feet, divided by 27 gives cubic yards.
• Convert inches to feet first: 4 inches is 0.333 ft, 6 inches is 0.5 ft.
• One cubic yard covers 81 ft² at 4″, 65 ft² at 5″, 54 ft² at 6″, and 41 ft² at 8″.
• Add about 10% on top — that is an estimating assumption, not a published standard.
• Order by the cubic yard from a ready-mix supplier; bags only make sense for small pours.

How do you calculate concrete in cubic yards?

Concrete volume is calculated in feet and then converted, because a cubic yard is 27 cubic feet. Take the length in feet, multiply by the width in feet, multiply by the thickness in feet, and divide the result by 27.

The step that trips people up is thickness, which is almost always given in inches. Divide the inches by 12 first: a 4-inch slab is 0.333 ft, a 5-inch slab is 0.417 ft, a 6-inch slab is 0.5 ft.

A worked example: a 24 ft by 24 ft garage floor at 5 inches thick is 24 × 24 × 0.417 = 240.2 ft³. Divided by 27, that is 8.9 cubic yards before waste, and about 9.8 cubic yards with a 10% allowance.

For an odd-shaped pour, split the area into rectangles, calculate each one separately, and add the volumes. Footings, thickened edges, and steps are separate volumes — measure and add them rather than folding them into the slab number.

How much area does a cubic yard of concrete cover?

A cubic yard covers 81 square feet at 4 inches thick, and less as the slab gets thicker. The table below shows coverage per cubic yard and the finished volume for two common slab sizes.

Volumes in the table are theoretical — they do not include the waste allowance.

How much extra concrete should you order?

Order about 10% more than the calculated volume. That figure is an estimating convention rather than a published requirement, and it exists because the concrete in the ground almost never matches the concrete on paper.

NRMCA’s guidance on yield explains why the gap appears. A difference of just 1/8 inch in the thickness of a 4-inch slab represents about 3% of the volume — a full cubic yard on a 32-yard order. Forms deflect under the pressure of fresh concrete, subgrade dips swallow material, and spreading forms add width.

Hardened concrete also occupies slightly less space than the fresh mix. NRMCA notes the in-place volume can be roughly 2% below the fresh volume once air content drops and the paste settles and shrinks.

Ready-mixed concrete is sold by the volume of fresh concrete discharged from the truck, measured under ASTM C94 using the density of the fresh mix. Running short mid-pour is far more expensive than a little leftover, because a cold joint forms where the first load has already set.

How many bags of concrete equal a cubic yard?

Bagged concrete mix is the wrong tool for anything larger than a few square feet, and the bag count explains why. Manufacturers publish a yield per bag on the data sheet — commonly around 0.60 ft³ for an 80 lb bag, 0.45 ft³ for a 60 lb bag, and 0.30 ft³ for a 40 lb bag — but formulations differ, so check the specific product.

At those published yields, a cubic yard takes roughly 45 of the 80 lb bags, 60 of the 60 lb bags, or 90 of the 40 lb bags. Mixing that many by hand is a long day, and consistency between batches suffers.

Divide your calculated cubic feet by the bag yield printed on the bag to get the count, then add the same 10% cushion. Bags are the right answer for post holes, small pads, and repairs; anything approaching a cubic yard is cheaper and more uniform from a truck.

What do you tell the ready-mix supplier?

Give the supplier the volume in cubic yards plus the performance you need, not a recipe. NRMCA’s ordering guidance is explicit that mixture proportions are the producer’s job — your job is stating the requirements.

Have these ready when you call: total cubic yards, what the concrete is for, the specified compressive strength, whether the placement is exposed to freezing or deicing chemicals, the slump you need for placing and finishing, and the delivery date, time, and access at the site.

For most flatwork the slump ordered is 3 to 5 inches. Nominal maximum coarse aggregate size is usually 3/4 or 1 inch, and it has to be small enough to flow through the thinnest part of the section and between any reinforcement.

Truck capacity, minimum load size, and short-load fees are set by individual suppliers, with no national standard, so ask when you book. Also ask about placement rate — how fast your crew can take concrete determines how many trucks are scheduled.

Common mistakes

• Forgetting to convert inches to feet — using 4 instead of 0.333 for thickness overstates the order by a factor of 12.
• Ordering the exact calculated volume — a slightly deep subgrade or a spreading form leaves you short, and a cold joint forms while you wait for more concrete.
• Measuring the forms instead of the excavation — the subgrade under the forms is rarely flat, and every low spot pulls extra concrete.
• Estimating footings and steps as part of the slab — those volumes are separate and are the most common source of an under-order.
• Buying bags for a slab-sized pour — 45 bags per cubic yard means uneven batches, a long mixing day, and usually a higher total cost.