Concrete calculator

How to estimate concrete in five steps

1. Pick the shape. Choose a slab, a footing or a round post hole. A slab and a footing are both boxes, so they use length, width and thickness. A post hole is a cylinder, so it uses a diameter and a depth.
2. Type the size. Enter the dimensions in feet and inches, or switch to meters and centimeters. If you have several identical pours, such as six post holes, type that number and the calculator multiplies for you.
3. Choose the bag size and the waste. Pick the 40, 50, 60 or 80 pound bag from the label on the bag you plan to buy. The waste starts at 10 percent, and you can change it.
4. Read the result. You get the volume in cubic feet, the amount to order with waste in cubic feet and cubic yards, and the number of bags, rounded up. If a pour is thinner than 2 inches, a note tells you that bagged mix is meant for 2 inches or thicker.
5. Add your own prices if you like. Type the price per bag or the price per cubic yard of ready-mix and a low to high range appears. Compare up to a few bids per cubic yard, then print a bilingual sheet to bring to your supplier or contractor.

What this calculator does, and what it does not

This page turns the size of a pour into a quantity: how much concrete it takes, how many bags that is, and how many cubic yards to ask for if you order ready-mix. It handles a flat slab, a footing trench and round post holes, in feet and inches or in meters.

It is an estimate, not a quote and not a plan. It does not know what you are building, what the ground is like or what your local rules say. It gives no advice on how thick a pour should be, how strong the mix needs to be, whether it needs reinforcement, how large a footing is or whether a permit is required. Those are decided by your building department and your contractor.

How the volume is worked out

For a slab or a footing, the calculator multiplies length by width by thickness, with everything in feet. A thickness typed in inches is divided by 12 first, so 4 inches becomes one third of a foot. The answer is in cubic feet.

For a round post hole, it takes the area of a circle, which is pi times the radius squared, and multiplies by the depth. Because the hole is usually measured in inches, the result is divided by 1,728, the number of cubic inches in a cubic foot.

Ready-mix is sold by the cubic yard, and one cubic yard is 27 cubic feet. So the calculator divides cubic feet by 27 to show the same amount in cubic yards.

A worked example: a 10 by 10 foot slab

Say a patio slab is 10 feet by 10 feet and 4 inches thick. Four inches is 0.3333 of a foot, so the volume is 10 times 10 times 0.3333, which is about 33.3333 cubic feet.

Add 10 percent for waste and the order is about 36.6667 cubic feet. Divide by 27 and that is about 1.3580 cubic yards. With 80 pound bags, each yielding 0.60 cubic feet, 36.6667 divided by 0.60 is 61.1, and the calculator rounds up to 62 bags, because nobody can buy part of a bag.

Try the same slab with 60 pound bags and the count goes up, because each bag fills less. That is why the bag size you pick has to match the bag you will carry out of the store.

What a yield per bag means

The yield is how much wet concrete one bag makes once you add water and mix. The calculator uses about 0.30 cubic feet for a 40 pound bag, 0.375 for a 50 pound bag, 0.45 for a 60 pound bag and 0.60 for an 80 pound bag.

Those are typical figures. Different mixes can differ a little, so check your bag label for the yield printed on it, and ask your supplier if the label is not clear. The bag size changes how many bags you carry and lift, not how much concrete the job takes.

Why waste is added

Almost every pour takes a little more than the drawing says. Ground is never perfectly flat, forms bow slightly, some mix sticks to the mixer or wheelbarrow, and a bit spills. Running short in the middle of a pour is a bigger problem than having a bag or two left over.

The calculator starts at 10 percent, a common rule of thumb, and you can type a lower or higher number. A tidy, small pour in a well-built form may need less. A rough excavation, a long pour or a tight schedule may call for more. Ask your supplier what they suggest for your job.

Bagged mix or ready-mix by the cubic yard

Bags suit small jobs: post holes, a small pad, a step or a repair. You mix as you go, you can stop and start, and you buy only what the job takes. As the amount grows, the bag count grows fast. The 62 bags in the example above are a lot to haul, mix and pour before the surface starts to set.

For larger pours, many people ask a supplier for ready-mix by the cubic yard, delivered in a truck. Suppliers have their own minimum loads, delivery charges and rules for short loads, so ask your supplier. The calculator shows the order in cubic yards to give you a number to start that conversation with.

How to read a contractor's quote

Compare quotes for the same work. A low number can leave out things another quote includes. The bid comparison on this page shows each bid per cubic yard so a small job and a bigger job can be compared on the same footing. Use the questions below as a starting point.

• How many cubic yards does the quote cover, and how was that worked out?
• Does it include digging, forms, base material and removal of the old surface?
• Is delivery included, and is there a charge for a short load or for waiting time?
• Who finishes the surface, and what finish is included?
• Who arranges the permit, if one is needed?
• What is in writing about the work, and for how long?

What an estimate cannot know

A calculator works only from the numbers you type. It cannot see soft ground, a sloping site, tree roots, buried pipes or an old slab that needs to come out. It does not know the weather, the temperature, the mix design or how the surface will be used.

Before you dig, call 811. It is the free number for having buried utility lines marked, and it is a pointer only; check with them about how and when. For thickness, strength, reinforcement, footing size and permits, ask your building department and your contractor. This page does not decide any of them.

Metric units

Switch to metric and type meters and centimeters. The volume comes out in cubic meters, and the calculator also shows cubic feet, cubic yards and bags so you can compare with a supplier who sells in either system.

The two conversions to remember: one cubic yard is 27 cubic feet, and one cubic meter is about 35.3 cubic feet. A pour of 4 by 3 meters and 0.10 meters thick is 1.2 cubic meters, and with 10 percent waste it is 1.32 cubic meters, about 46.6 cubic feet or 1.73 cubic yards.

Your answers and your privacy

The calculator runs in your browser. It has no place to type a phone number, an email or an address, nothing you enter is sent to a server, and nothing is saved when you close the page. Prices you type stay on your screen and are marked as typed by you. Print the sheet if you want to keep the numbers.

Frequently asked questions