Concrete Calculator
Use this free concrete calculator to work out how much concrete you need for a slab, footing or post, in cubic yards, cubic metres and bags. Type in your measurements, pick a shape and a waste allowance, and the volume and bag count update straight away.
Volume: cubic feet
Volume: cubic yards
Volume: cubic metres
Bags needed:
80 lb bags:
60 lb bags:
40 lb bags:
1 cubic yard = 27 cubic feet; 1 cubic metre is about 35.31 cubic feet. Nothing you type is saved or sent anywhere.
How to use this concrete calculator
Step 1: Pick the shape. A slab is a flat area such as a patio or a path. A footing is the strip of concrete under a wall, and a column is an upright post or pier. The maths is the same for each one, so you only need the length, the width and the depth.
Step 2: Type in your measurements and pick your units. Use feet and inches if you measured in feet, or metres and centimetres if you measured in metres. Add a waste allowance of 10 percent if the ground is uneven, the formwork is rough, or you want a safety margin.
Step 3: Read the result. The concrete calculator shows the volume in cubic feet, cubic yards and cubic metres, and how many 40 lb, 60 lb and 80 lb bags you need. The bag counts are rounded up, because you cannot buy part of a bag.
Worked example: a 10 ft by 10 ft slab, 4 inches deep
Take a patio slab that is 10 feet long and 10 feet wide, with a depth of 4 inches. First, turn the depth into feet by dividing by 12, which gives 0.33 feet. Then multiply length by width by depth: 10 times 10 times 0.33 gives 33.33 cubic feet of concrete.
To get cubic yards, divide by 27, because one cubic yard holds 27 cubic feet. That gives 1.23 cubic yards. To get cubic metres, multiply the cubic feet by 0.0283168, which gives 0.94 cubic metres. A single number can look small, but a 10 by 10 slab still needs more than fifty bags.
For bags, divide the volume by the volume of one bag. An 80 lb bag holds about 0.6 cubic feet, so 33.33 divided by 0.6 is 55.55, rounded up to 56 bags. A 60 lb bag holds 0.45 cubic feet, so you need 75 bags. A 40 lb bag holds 0.3 cubic feet, so you need 112 bags. Add 10 percent waste and the volume rises to 36.67 cubic feet, or 1.36 cubic yards, with 62 bags of 80 lb concrete.
How thick should a slab be
Four inches is the usual thickness for a patio, a walkway and other light foot traffic. It is enough for people, garden furniture and a barbecue, and it keeps the amount of concrete, and the cost, down. Most home patios are poured at this depth, on a compacted base of gravel or hardcore.
Five to six inches is the right choice for a driveway, a garage floor and anywhere a car, a van or heavy equipment will sit. The extra depth spreads the load over a wider area, so the slab is far less likely to crack under weight. A thicker slab also gives you more room for steel mesh or rebar, which adds strength.
Footings are usually deeper than a patio slab, because they carry the weight of a wall, while a fence post or a deck post may only need a narrow column of concrete around the post itself. Whatever the job, measure the depth in a few places before you order. If the ground is not level, the depth will vary, and the thinnest spot is the one that matters most for strength.
Bags vs ready-mix
Bagged concrete suits small jobs. If you need under about one cubic yard, mixing bags in a wheelbarrow or a small mixer is simple, and you only buy what you need. It is also handy when the site is hard to reach, because you can carry bags in one at a time. The work is slower, though, and mixing fifty or more bags by hand is hard on the back.
Ready-mix concrete suits larger jobs. Above about one cubic yard, a truck delivery is usually cheaper per cubic yard than bags, and far quicker. The concrete arrives mixed, so you can pour, level and finish it without stopping to mix each batch. That matters with a big slab, because concrete starts to set, and a slow pour can leave cold joints where one batch meets the next.
Ready-mix does need access for a truck, and most suppliers charge a delivery fee, sometimes with a minimum order. Some also charge extra for a small load, so ask for the full price, including delivery, before you decide. For a job of one cubic yard or less, bags are often the simpler and cheaper choice, even if the concrete itself costs a little more per bag.
Frequently asked questions
How much concrete do I need for a 10x10 slab?
A 10 foot by 10 foot slab that is 4 inches deep needs 33.33 cubic feet of concrete, which is 1.23 cubic yards or 0.94 cubic metres. That is 56 bags of 80 lb concrete, 75 bags of 60 lb concrete, or 112 bags of 40 lb concrete, before any waste allowance.
How many bags of concrete make a cubic yard?
One cubic yard is 27 cubic feet. An 80 lb bag holds about 0.6 cubic feet, so you need 45 bags for a cubic yard. A 60 lb bag holds 0.45 cubic feet, so you need 60 bags, and a 40 lb bag holds 0.3 cubic feet, so you need 90 bags.
What size bag should I buy?
Buy the largest bag you can handle comfortably. An 80 lb bag is the best value and needs the fewest bags, but it is heavy to lift and mix. A 60 lb bag is a good middle choice, and a 40 lb bag is easier to carry, but you will need more of them for the same slab.
Should I add extra for waste?
Yes, 10 percent covers spills, uneven ground and formwork gaps. The ground is rarely perfectly level, and formwork can bow out a little under the weight of wet concrete. It is better to have one bag left over than to run short halfway through a pour.
Is this exact?
No, it is an estimate. Uneven ground and formwork change the real amount, so order a little extra. Measure the length, width and depth in a few places, use the largest figures if you are unsure, and check the result against the amount your supplier suggests.