How much concrete do I need?

Published July 21, 2026.

Start by figuring the volume in cubic feet, then convert. The short version: measure length, width, and thickness in feet, multiply them together to get cubic feet, then divide the cubic feet by 27 to get cubic yards (27 is the number of cubic feet in a cubic yard). If a measurement is in inches, divide it by 12 first to get feet, so 4 inches becomes 0.33 feet. For bags instead of a bulk order, divide your cubic feet by the yield printed on the bag, which is about 0.6 cubic feet for an 80 lb bag. Order about 5 to 10 percent extra to cover spillage and uneven subgrade.

The formula, step by step

Here is the same method written out so you can do it on paper.

Measure length, width, and thickness. Use feet for all three. For a rectangle that is straightforward. For an odd shape, break it into rectangles, figure each one, and add them up.
Convert thickness to feet. Slab thickness is usually given in inches, so divide by 12. Four inches is 0.33 feet, 5 inches is 0.42 feet, and 6 inches is 0.5 feet.
Multiply to get cubic feet. Length times width times thickness, all in feet, gives the volume in cubic feet. The Concrete Network writes the same step as length times width times thickness.
Convert to what you are buying. Divide cubic feet by 27 for cubic yards on a ready-mix order. For bags, divide cubic feet by the bag’s yield: 0.6 for an 80 lb bag, 0.45 for a 60 lb bag, or 0.30 for a 40 lb bag.
Add a little extra. Beds settle, forms are never perfectly square, and some mix is lost to the wheelbarrow. Add roughly 5 to 10 percent, then round up to the next whole yard or bag.

A worked example: a patio that is 12 feet by 10 feet at 4 inches thick. That is 12 times 10, or 120 square feet, times 0.33 feet of thickness, which comes to about 40 cubic feet. Divide by 27 and you get about 1.5 cubic yards. With 10 percent added, you would order about 1.75 cubic yards. In bags, 40 divided by 0.6 is about 67 eighty-pound bags, which is a lot to mix by hand and a clear sign to order ready-mix instead.

Bags per cubic yard

The yield per bag is the number that drives a bag order. These figures come from the Sakrete High-Strength Concrete Mix data sheet. Yields can vary a little by product and brand, so check the bag you buy.

Bag size	Yield per bag	Bags per cubic yard
40 lb	0.30 cubic feet	About 90
60 lb	0.45 cubic feet	About 60
80 lb	0.60 cubic feet	About 45

The bags-per-yard column is 27 cubic feet divided by each bag’s yield. You can skip the fractions with our concrete calculator, which takes your dimensions and returns both the cubic yards and the bag count. It is worth checking your actual bag too, since yields can vary a little by product and brand.

Bags or ready-mix

For a small job, bags are the sensible choice. A short walkway, a small patio, or setting a few posts lets you buy the exact number of bags and mix them as you go, with no leftover pile in the driveway. The Concrete Network points to bags for exactly these smaller pours: walkways, small patios, and footings.

For a larger pour, ready-mix delivered by the cubic yard saves the labor of mixing bag after bag. The Concrete Network recommends ordering by the yard for bigger jobs like driveways. A rough line to draw is one cubic yard, which is about 45 of the 80 lb bags. Once your estimate climbs near or past a yard, delivery usually wins on both time and consistency, since every batch comes from the same truck.

How thick to pour

Thickness is half of the volume calculation, so settle it before you buy. For most home flatwork, 4 inches is the standard minimum. Go thicker only where the load calls for it.

Project	Recommended thickness
Walkway or sidewalk	4 inches
Patio	4 inches (5 to 6 for a hot tub or outdoor kitchen)
Driveway, passenger cars	4 inches
Driveway, trucks or RV	5 to 6 inches

The Concrete Network recommends at least 4 inches for a patio and at least 4 inches for a driveway that carries standard cars, stepping up to 5 or 6 inches for heavier loads. That extra inch is not only insurance: going from 4 inches to 5 inches can raise the slab’s load-carrying capacity by nearly 50 percent. Under the slab, plan for several inches of compacted gravel as well, and remember to dig deep enough for both the gravel and the concrete, as the Sakrete flatwork instructions note.

A few things that throw off the estimate

Forgetting to convert inches to feet. Thickness is almost always given in inches. Divide by 12 before you multiply, or your cubic-foot number will be off by a wide margin.
Ordering the exact amount. Never order right to the number. The Concrete Network suggests building in a margin, and adding about 10 percent covers spillage and small variations in slab depth.
Pouring too thin. A slab under 4 inches is more likely to crack under load. Hold the 4-inch minimum for anything that carries foot traffic or vehicles.
Ignoring the subgrade. An uneven or soft base means some spots take more concrete than planned. Grade and compact first so your thickness stays consistent across the whole pour.

For the exact amount for your slab, including odd shapes and your chosen thickness, run the numbers through our concrete calculator. Concrete work is structural, so for a driveway, a foundation, or anything load-bearing, confirm the thickness and any reinforcement with a licensed pro and your local building codes.

Sources

Sakrete · High-Strength Concrete Mix, Technical Data Sheet. sakrete.com
The Concrete Network · Concrete Calculator, How Much Do I Need? concretenetwork.com/concrete/howmuch/calculator
The Concrete Network · How Much Concrete Do I Need for a Patio? concretenetwork.com/concrete/howmuch/for-a-patio
The Concrete Network · How Thick Should a Concrete Driveway Be? concretenetwork.com/concrete/concrete_driveways/thickness

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Frequently asked questions

How many 80 lb bags of concrete make a cubic yard?

About 45 bags. A cubic yard holds 27 cubic feet, and one 80 lb bag of concrete mix yields about 0.6 cubic feet per the Sakrete data sheet, so 27 divided by 0.6 is roughly 45. In the smaller sizes it takes about 60 of the 60 lb bags or about 90 of the 40 lb bags to reach a full yard. Once your project needs close to a yard, mixing that many bags by hand stops being practical and ready-mix delivery is usually the better call.

How thick should a concrete slab be?

For most residential flatwork, 4 inches is the standard minimum. The Concrete Network recommends at least 4 inches for patios, walkways, and driveways that carry passenger cars. Step up to 5 or 6 inches when the slab will hold heavier loads, such as a hot tub, an outdoor kitchen, or truck and RV traffic. Going from 4 inches to 5 inches can raise the slab's load-carrying capacity by nearly 50 percent. Check your local building codes, since some set their own minimum.

Should I use bags or order ready-mix concrete?

It comes down to volume. For small jobs like a short walkway, a small patio, or a few footings, bagged mix lets you buy the exact amount and mix as you go. For larger pours like a driveway, ordering ready-mix by the cubic yard is easier than mixing bag after bag, per the Concrete Network. A rough line is one cubic yard, which is about 45 of the 80 lb bags. Above that, delivery usually wins on both time and effort.