Concrete calculator
Concrete calculator: how much concrete do you need?
Multiply length × width × thickness in feet for cubic feet, then divide by 27 for cubic yards: a 10 by 10 foot slab 4 inches thick is 33.33 cubic feet, or 1.23 cubic yards. Choose slab, round column, post holes or steps below for cubic yards, cubic feet and bags at the yield printed on your bag, then add your prices to compare ready-mix with bags.
What are you pouring?
Includes 10% extra.
Ready-mix or bags?
How to work out concrete for any pour
- Measure in feet, or note the units. Thickness and post holes are usually in inches: divide by 12.
- Get the volume. Rectangles: length × width × thickness. Round: 3.1416 × radius × radius × height. Post holes: the hole minus the post. Steps: width × tread × riser × (1 + 2 + … + steps).
- Convert. Cubic feet ÷ 27 = cubic yards. Bags = cubic feet ÷ the yield on the bag.
- Add extra so you don’t run short mid-pour.
- Compare ready-mix and bags at your local prices, including delivery and small-load fees.
Base gravel under the slab? The gravel calculator. Building a block wall on the footing? The concrete block calculator. Converting units or working in meters? The cubic yard calculator.
Frequently asked questions
- How do I calculate how much concrete I need?
- Multiply length × width × thickness, all in feet, for cubic feet, then divide by 27 for cubic yards. A 10 by 10 foot slab 4 inches thick is 10 × 10 × (4 ÷ 12) = 33.33 cubic feet, or 1.23 cubic yards. Add extra for spillage and an uneven base.
- How many bags of concrete make a cubic yard?
- Divide 27 by the yield printed on the bag, in cubic feet. If a bag says it yields 0.60 cubic feet, a cubic yard takes 27 ÷ 0.60 = 45 bags; at 0.45 cubic feet it takes 60 bags. Bag yields differ by product, so use the number on your bag.
- How many bags of concrete do I need for a fence post?
- Work out the hole's volume (3.1416 × radius × radius × depth), take away the part of the post that sits in the hole, and divide by the bag's yield. A 10 inch hole 30 inches deep around a 3.5 inch square post holds about 1.15 cubic feet of concrete. The post hole mode does this for any number of posts.
- How do I calculate concrete for a sonotube or round column?
- 3.1416 × radius × radius × height, in feet, gives cubic feet. A 12 inch tube 4 feet tall holds 3.1416 × 0.5 × 0.5 × 4 = 3.14 cubic feet. Multiply by the number of tubes.
- How do I calculate concrete for steps?
- Each step sits on top of the ones below it, so the total is width × tread depth × riser height × (1 + 2 + … + number of steps). Three steps 4 feet wide with 11 inch treads and 7 inch risers come to 4 × (11 ÷ 12) × (7 ÷ 12) × 6 = 12.83 cubic feet. Add any landing as a slab.
- How much does a yard of concrete cost?
- It depends on your area, the mix and the size of the order; small loads often carry extra fees. Ask local ready-mix suppliers for their price per yard and any delivery or small-load charges, enter them, and the calculator prices your job and compares it with bags.
- Should I order extra concrete?
- Running short in the middle of a pour is a real problem, and forms and bases are rarely perfect, so most people order some extra. The calculator adds whatever percentage you choose.
- How thick should a concrete slab be?
- That depends on what it carries, the base under it and local building codes, so check with your local building department or a concrete contractor, then enter the thickness they give.
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