Free tool
Concrete calculator, cubic yards
Work out how much concrete a slab, footing or set of post holes needs, in cubic yards and bags, with a waste allowance built in. Feet and inches, free, no sign-up.
- Volume before waste
- 1.48 cu yd
- Same volume in cubic feet
- 44.0 cu ft
- 60 lb bags (approx)
- 98
- 80 lb bags (approx)
- 74
Includes a 10% waste allowance. A 60 lb bag yields about 0.45 cubic feet and an 80 lb bag about 0.6, so a cubic yard takes roughly 60 or 45 bags. Past about half a yard, ready-mix delivered by the truck is usually cheaper. Figures update as you type.
Concrete is ordered by the yard, and a yard is a lot of concrete. Come up short on a pour and the truck is gone before the slab is finished; order a yard too many and you are paying to have it dumped and broken out. This calculator works out the volume for a slab, a footing or a set of columns from feet and inches, in cubic yards and bags, with a waste allowance built in. Here is how it works.
How to work out concrete for a slab
Length times width times thickness gives you cubic feet. Divide by 27 to get cubic yards, because a cubic yard is 3 feet by 3 feet by 3 feet.
Cubic yards = length × width × thickness ÷ 27.
The catch is the thickness, which you measure in inches while everything else is in feet. Divide the inches by 12 first. A 12 by 10 foot pad at 4 inches thick is 12 × 10 × 0.333 = 40 cubic feet, which is 1.48 cubic yards. The calculator does the conversion and adds your waste on top.
How to work out concrete for a footing
A footing is a long trench, so the run is in feet and the width and depth are in inches. Same math, same divide by 27.
Cubic yards = run × (width ÷ 12) × (depth ÷ 12) ÷ 27.
A 40 foot run of 16 inch wide footing at 8 inches deep works out at 35.6 cubic feet, or about 1.32 cubic yards. Measure the run around the whole perimeter, corners included, and use the trench you actually dug rather than the one on the drawing.
How to work out concrete for columns and post holes
Round holes and tube forms use the area of a circle times the depth, then multiplied by the number of holes.
Cubic feet = π × radius² × depth × number of holes.
A 10 inch tube set 4 feet deep holds about 2.18 cubic feet, so four of them come to roughly 8.7 cubic feet, or 0.32 cubic yards before waste. Switch the calculator to "Column", put the diameter in inches and the depth in feet, and it handles the rest.
Bags or ready-mix?
Bagged concrete goes a shorter way than most people expect. A 60 lb bag yields about 0.45 cubic feet and an 80 lb bag about 0.6, so it takes roughly 60 of the smaller bags or 45 of the larger ones to make a single cubic yard.
- Bags suit small work: a few post holes, a pad for an AC unit, a repair.
- Ready-mix, delivered by the yard, is normally cheaper, faster and more consistent for a slab or anything over about half a yard.
The calculator gives you the volume in cubic yards, which is how you order from the plant, plus a bag count for both common sizes so you can weigh it up before you decide.
Ordering from the plant
Most suppliers take orders in quarter yard steps, set a minimum load and add a short load fee for anything well under a full truck. That pricing works in your favor when you round up: going from 1.6 yards to 1.75 costs very little, and running short costs you the pour. Have your forms, rebar and screed ready before the truck books in, because wait time is usually charged by the minute once the drum has been on site a while.
Always allow for waste
Never order the exact calculated volume. Subgrade is never perfectly flat, trenches get dug a little wide, forms bow, and some concrete is always lost to spillage and what sticks in the chute. A waste allowance of 5% to 10% covers it. The calculator adds 10% by default and you can change it. A little left over is a driveway patch; a little short is a cold joint through the middle of your slab.
Working in meters instead?
If you order concrete by the cubic meter rather than the yard, use the concrete calculator in cubic meters, which takes meters and millimetres and gives you 20kg and 25kg bag counts.
From materials to a priced job
Knowing the quantity is half the job; the other half is turning it into a price that pays. Once the volume is settled, drop the materials into a clean, itemized quote with the quote generator, and make sure the labor, the pump and the finishing are priced properly with how to price a job so you actually make a profit.
This tool is for general estimating and gives approximate quantities. Bag yields, mixes and ground conditions vary, so check the coverage printed on the bag and your own measurements, and order a sensible amount over rather than exactly to the calculation.
How do I work out how much concrete I need?
Multiply length by width by thickness, all in feet, to get cubic feet, then divide by 27 for cubic yards. A 12 by 10 foot slab at 4 inches thick is 12 × 10 × 0.33 = 40 cubic feet, or about 1.5 cubic yards. For round post holes it is pi times the radius squared, times the depth, times the number of holes. This calculator does both and adds the waste.
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb. That is most of a pallet either way, which is why anything much over half a yard is usually cheaper and a lot faster ready-mixed.
Should I use bags or ready-mix?
Bags suit small pours: a few posts, a shed pad, a step. From about half a yard up, ready-mix delivered by the truck is cheaper, quicker and gives a more consistent mix. Ask about the short-load fee before you order, because most yards charge one under their minimum load.
How thick should a concrete slab be?
Four inches is standard for a patio, walkway or shed base. Go to five or six inches for a driveway or anything a vehicle parks on, and thicken the edges. Check what your local code and your inspector expect before you pour, because that is the figure that actually counts.
How much waste should I allow?
Around 5% to 10% covers spillage, uneven subgrade and holes that got dug a bit wide. The calculator adds 10% by default and you can change it. Ready-mix is ordered in quarter-yard steps, so round up: running a wheelbarrow short part-way through a pour is a far bigger problem than a little left over.
How much concrete does a post hole take?
A 10 inch hole dug 3 feet deep holds about 1.6 cubic feet before you allow for the post itself, so figure roughly three 80 lb bags a hole. Set the hole diameter, the depth and the number of holes and the calculator totals a whole fence run for you.
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