How Much Concrete Do I Need? A Simple Guide for Any Project
The one unit that matters: cubic yards
Concrete is sold and mixed by volume, and in the United States that volume is the cubic yard. One cubic yard is a cube three feet on every side, which works out to 27 cubic feet. That number, 27, is the hinge of every concrete calculation, because you measure your project in feet and inches but you order in yards, so you are always dividing cubic feet by 27 to convert.
The reason this trips people up is thickness. A slab is wide and long in feet but only a few inches thick, so you have to convert that thickness to feet before you multiply. Four inches is one-third of a foot, or 0.333; six inches is half a foot, or 0.5. Get the thickness conversion right and the rest is easy multiplication. The concrete slab calculator does this conversion automatically, but knowing the mechanic lets you double-check any quote a supplier gives you.
The formula for slabs, footings, and posts
For a rectangular slab the formula is length in feet times width in feet times thickness in feet, all divided by 27, which gives cubic yards. Take a 10-by-10 patio at 4 inches: 10 times 10 times 0.333 is 33.3 cubic feet, divided by 27 is about 1.23 cubic yards. That is the raw volume before any waste allowance.
Footings and post holes use the same idea with a different shape. A continuous footing is just a long, narrow slab, so use length times width times depth divided by 27. A cylindrical post hole uses the volume of a cylinder: 3.1416 times the radius in feet squared, times the depth in feet, divided by 27. For example, a 12-inch-diameter hole (radius 0.5 feet) dug 3 feet deep is 3.1416 times 0.25 times 3, about 2.36 cubic feet, or roughly 0.087 cubic yards per hole. Multiply by the number of holes for a fence or deck. Because post holes and footings share materials and setup with slabs, planning them together avoids a wasted partial order, and the concrete slab calculator lets you total several pours before you decide between bags and a truck.
A quick reference table
Most projects fall into a handful of common sizes, so it helps to have a table you can glance at to sanity-check any calculation. The figures below show the raw concrete volume for common slabs at two standard thicknesses, followed by an approximate 80-pound bag count for the smaller jobs. These are pre-waste numbers; add 5 to 10 percent before you order, as the next section explains.
| Slab size | 4" thick (cu yd) | 6" thick (cu yd) | 80-lb bags (4") |
|---|---|---|---|
| 4 x 4 ft | 0.20 | 0.30 | ~30 |
| 8 x 8 ft | 0.79 | 1.19 | ~118 |
| 10 x 10 ft | 1.23 | 1.85 | ~185 |
| 12 x 12 ft | 1.78 | 2.67 | ~267 |
| 20 x 20 ft | 4.94 | 7.41 | truck |
The bag column makes the crossover obvious. An 80-pound bag yields only about 0.6 cubic feet, so even a modest 10-by-10 slab would take roughly 185 bags, which is why anything past about one cubic yard is almost always cheaper and easier delivered ready-mixed by the truck. Bags make sense for post holes, small pads, and repairs; the truck makes sense for real slabs.
The waste factor pros always add
Here is the mistake that catches first-timers: ordering the exact calculated volume. In the real world you never place exactly your calculated number. The ground is never perfectly level, so a slab specified at 4 inches is 4 and a half in the low spots. Some concrete sticks to the forms, the wheelbarrow, and the chute. A little is lost to spillage. Order the precise figure and you will come up short, and running out mid-pour is far worse than having a little extra.
The industry answer is a waste factor. For most slabs, add 10 percent to the calculated volume; for a very flat, well-prepared subgrade you can use 5 percent, and for rough or sloping ground use 10 to 15 percent. On our 10-by-10 patio, 1.23 cubic yards becomes about 1.35 with a 10 percent allowance, which you would round up to the nearest quarter yard the supplier sells. Rounding up is always the right call, because a few extra dollars of concrete is trivial next to a second delivery charge or a weak cold joint where a paused pour met fresh concrete. The concrete slab calculator lets you apply this waste factor automatically so your order matches what actually goes in the forms.
Bags, trucks, and what else to order
Once you know your waste-adjusted volume, deciding how to buy it is straightforward. Under about one cubic yard, bagged concrete mixed on site is practical: a 60-pound bag yields roughly 0.45 cubic feet and an 80-pound bag about 0.6, so divide your cubic feet by that yield and round up. Above a cubic yard, call a ready-mix supplier, who will deliver by the yard and often set a small-load minimum and fee, which is another reason to combine nearby pours into one delivery.
Concrete is rarely the only material a pour needs. Most slabs sit on a compacted base of gravel, typically 4 to 6 inches, both for drainage and to prevent cracking, and that gravel is ordered by volume just like the concrete. You may also need sand for leveling or a border of topsoil and mulch once the slab is finished and you are restoring the surrounding yard. It is worth sizing all of that in one pass so nothing is forgotten on delivery day; the gravel, mulch and topsoil calculator handles those base and finishing materials the same way, in cubic yards, so your whole material list is ready before the truck arrives.
Frequently asked questions
How do I calculate concrete for a slab?
Multiply length in feet by width in feet by thickness in feet, then divide by 27 to get cubic yards. Convert inches to feet first: 4 inches is 0.333, and 6 inches is 0.5. A 10-by-10 slab at 4 inches is 10 times 10 times 0.333 divided by 27, about 1.23 cubic yards before you add a waste allowance.
Why is concrete measured in cubic yards?
Concrete is mixed and sold by volume, and the cubic yard is the standard U.S. unit for ready-mix delivery. One cubic yard equals 27 cubic feet, a cube three feet on each side. Because you measure a project in feet and inches but order in yards, every calculation ends with dividing cubic feet by 27 to convert to the unit suppliers use.
How much extra concrete should I order?
Add a waste factor of about 10 percent for most slabs, 5 percent for very flat, well-prepared ground, and 10 to 15 percent for rough or sloping subgrade. Uneven ground, material stuck to forms and tools, and spillage all consume more than the exact calculation. Then round up to the nearest quarter yard, because running short mid-pour is far costlier than a little surplus.
How many bags of concrete are in a cubic yard?
It takes roughly 45 bags of 80-pound mix or about 60 bags of 60-pound mix to make one cubic yard, since an 80-pound bag yields about 0.6 cubic feet and there are 27 cubic feet in a yard. That is why bags suit small jobs like post holes and repairs, while anything over about a cubic yard is cheaper delivered by truck.
Do I need gravel under a concrete slab?
Usually yes. Most slabs sit on 4 to 6 inches of compacted gravel base for drainage and to reduce cracking from ground movement. That base is ordered by volume in cubic yards, just like the concrete, so size it in the same planning pass. The gravel, mulch and topsoil calculator handles the base and any finishing materials the same way.
When should I use a ready-mix truck instead of bags?
Switch to a ready-mix truck once your waste-adjusted volume passes roughly one cubic yard. Below that, mixing bags on site is practical and avoids delivery minimums. Above it, hand-mixing dozens or hundreds of bags becomes impractical and more expensive than delivery, and a single continuous truck pour also avoids the cold joints that come from mixing batch after batch.
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