Concrete Calculator
Pick the shape you are pouring, enter its dimensions, and get the volume in cubic yards and cubic feet along with the number of bags it takes.
How many yards of concrete do I need?
Work out the volume in cubic feet, then divide by 27, because there are 27 cubic feet in a cubic yard. A 20 ft x 10 ft slab at 4 in thick is 20 x 10 x 0.333 = 66.7 cu ft, which is 2.47 cubic yards before any waste allowance. Add 10 % for an uneven subgrade and order 2.72 cu yd.
Concrete you need
At this size, compare bags against ready-mix delivery: 2.72 cu yd is 123 bags of 80 lb to mix by hand.
- Volume
- 73.33 cu ft66.67 cu ft before waste
- Bags of 80 lb
- 1230.6 cu ft per bag
- Total weight
- 4.92 tons9,840 lb of mix
The numbers above are a worked example for a 20 ft x 10 ft slab poured 4 in thick. Allow this page to run scripts in your browser to use your own dimensions, or follow the formula below and do it by hand.
How the concrete calculator works, step by step
Every concrete estimate is a volume. Find the volume of the shape in cubic feet, add an allowance, then divide by 27 to get cubic yards, or by the yield of a bag to get bags. The only thing that changes between a patio and a stair is the geometry in the first step.
Thickness is the number people get wrong, because it is the one dimension given in inches while the other two are in feet. A 4 in slab is 0.333 ft thick, not 4.
Dividing the diameter by 24 does two things at once: it halves the diameter to get the radius, and it converts inches to feet. A 12 in tube has a radius of 0.5 ft, and 3.1416 x 0.25 x 4 ft of height is 3.14 cu ft.
Stairs look complicated and are not. Slice the staircase into rectangular blocks the size of one rise by one run: the top step sits on one block, the next on two, and so on down. Adding those up is the triangular number n x (n + 1) / 2. Four steps means 1 + 2 + 3 + 4 = 10 blocks.
Worked example: a 20 ft x 10 ft patio, 4 in thick
- Volume: 20 x 10 x (4 / 12) = 66.67 cu ft
- With a 10 % allowance: 66.67 x 1.10 = 73.33 cu ft
- Cubic yards: 73.33 / 27 = 2.72 cu yd
- In 80 lb bags: 73.33 / 0.6 = 122.2, rounded up to 123 bags
- That is 9,840 lb of mix, close to 5 tons to carry, open and mix by hand
The last line is the one that decides the job. Nobody mixes 123 bags on a Saturday. Above about one cubic yard the answer is a ready-mix truck, and the calculator gives you the yardage to order.
Bag yields, and how many you get per yard
Bagged concrete mix yields less volume than the bag weighs, because the mix compacts and takes water. These are the published yields for standard concrete mix.
| Bag | Yield | Bags per cubic yard | Weight per cubic yard |
|---|---|---|---|
| 80 lb | 0.6 cu ft | 45 | 3,600 lb |
| 60 lb | 0.45 cu ft | 60 | 3,600 lb |
| 50 lb | 0.375 cu ft | 72 | 3,600 lb |
| 40 lb | 0.3 cu ft | 90 | 3,600 lb |
Yields per QUIKRETE, Concrete Mix product line (bag yields) - product line page, cross-checked against the manufacturer's own calculator. High-strength, fast-setting and crack-resistant mixes do not all yield the same. Read the bag you buy.
Notice that the weight per cubic yard is identical whatever the bag size: you are buying the same 3,600 lb of material either way. Small bags cost more per yard and are only worth it when you physically cannot carry an 80 lb sack.
How thick, and how much allowance
Thickness
| Use | Typical thickness |
|---|---|
| Patio, walkway, shed floor | 4 in |
| Driveway, cars and light trucks | 5 to 6 in |
| Garage floor | 4 to 6 in |
| Footing under a wall | 8 in deep, twice the wall width |
Thickness is where the money is. Going from 4 in to 6 in on a 20 ft x 10 ft slab adds 33 cu ft, which is half a yard, which is another 56 bags. Decide it before you order, not while the truck is waiting.
Waste allowance
Ten percent is the working default and it is not padding. It covers the subgrade being lower than you think in one corner, the forms bowing slightly under the weight, spillage during the pour, and the material left in the mixer. On a dug footing where the trench walls are rough, 15 % is more honest. Running short mid-pour is the expensive failure: the joint between yesterday's concrete and today's is a cold joint, and it is a permanent weakness.
Cases the simple formula does not cover
Slabs that are not rectangles
Break the pour into rectangles, calculate each, and add the volumes. For a circular pad, area is 3.1416 x radius squared with the radius in feet, then multiply by the thickness in feet. For a triangle, base x height / 2.
Thickened edges and haunches
Many slabs are poured with a thickened perimeter, typically 8 to 12 in deep and 12 in wide. Calculate it as a continuous footing on the perimeter run, then add the slab at its normal thickness over the whole area, and subtract nothing: the small overlap at the edge is well inside the waste allowance.
Deducting what you are not pouring
Subtract large voids only: a sunken planter, an inspection pit, a column footprint over about 4 sq ft. Do not subtract the volume of rebar or of a pipe sleeve; that is noise against the allowance.
Post holes
A post hole is a round column minus the post, and it is easier to get right on the fence calculator, which does the subtraction and rounds up per hole rather than once for the whole job.
What this calculator does not size
Reinforcement, base gravel, vapour barrier, and the strength class of the mix are separate decisions, and above a certain size they belong to an engineer or your local building department rather than to a web page. A slab that will carry a structure, a retaining wall footing, and anything in a seismic or expansive-soil area are all cases where the quantity is the least interesting part of the problem.
Frequently asked questions
How many bags of concrete are in a cubic yard?
Forty-five 80 lb bags, or sixty 60 lb bags, or ninety 40 lb bags. A cubic yard is 27 cu ft and an 80 lb bag yields 0.6 cu ft, so 27 / 0.6 = 45. Whatever the bag size, a yard is about 3,600 lb of mix to carry.
How much concrete do I need for a 10x10 slab?
At 4 in thick, 10 x 10 x 0.333 = 33.3 cu ft, which is 1.23 cu yd before waste. With a 10 % allowance that is 37 cu ft, so 62 bags of 80 lb. At 6 in thick it goes to 50 cu ft, 1.85 cu yd and 92 bags. That is already truck territory.
How do I calculate concrete in cubic yards?
Work the volume out in cubic feet with every dimension in feet, then divide by 27, because a cubic yard is 3 ft x 3 ft x 3 ft. The usual mistake is leaving the thickness in inches: divide it by 12 first.
How much concrete does a 12 inch tube form take?
A 12 in diameter tube takes 3.1416 x (12 / 24)^2 = 0.785 cu ft per foot of depth. A 4 ft deep pier is 3.14 cu ft, which is six 80 lb bags. Six such piers come to 19 cu ft, or 0.7 cu yd.
At what point should I order ready-mix instead of bags?
Around one cubic yard. That is 45 bags of 80 lb, 3,600 lb to move and mix, and a pour that has to stay wet from the first bag to the last. Below a yard, bags win on cost and convenience. Above it, the truck wins on every count, and most suppliers have a short-load minimum plus a fee.
How much concrete do I need for stairs?
Multiply the width by the rise and the run of a single step, all in feet, then by the triangular number of the step count. Four steps at 7 in rise and 11 in run, 4 ft wide, gives 4 x 0.583 x 0.917 x 10 = 21.4 cu ft, which is 0.79 cu yd before waste, and 40 bags of 80 lb once the 10 % allowance is applied.
Does the calculator include waste?
Yes, and it shows both figures: the raw geometric volume and the volume with the allowance applied. The default is 10 %, which you can change. Bag counts always round up, because a part-used bag is not a unit you can buy.