Concrete Volume & Bag Calculator
Cubic yards and bag counts for slabs, footings, piers and steps — and whether the pour can be finished before the concrete starts setting.
What a cubic yard actually costs you in bags
The yield per pound is the same for every bag size, so this table is really about how many times you bend down.
The number that decides the job isn't the bag count
Concrete starts setting about an hour after the water goes in. The limit on a bagged pour is not how many bags you can lift — it is whether you can mix, place, screed and float the whole thing before the first batch stiffens. Miss that and you get a cold joint: a plane where fresh concrete met concrete that had already begun to set. It never bonds properly, and it is where the slab eventually cracks. Heat, wind and dry air all shorten the window further.
Which rules out more jobs than people expect. A 10 × 10 ft patio at 4″ sounds like a weekend job. It is 1.23 cubic yards — 56 eighty-pound bags, 4,480 lb of material, about 28 mixer batches and over two hours of mixing before any placing. One person cannot pour it monolithically, and effort doesn't fix it, because the constraint is chemistry rather than stamina. Either order ready-mix or split the work at a designed construction joint with a keyway and dowels.
And the arithmetic trap: thickness is in inches, everything else is in feet. Four inches is a third of a foot. Multiplying by 4 instead of 0.333 overstates the order twelvefold — which is how people end up with a truck they can't use and a short-load fee attached to it.
- Fast-setting mix makes this worse, not better. It goes off in twenty to forty minutes, which is exactly right for setting a post and walking away, and exactly wrong for anything you have to screed and float.
- A cubic yard is 45 eighty-pound bags — 3,600 lb. The yield per pound is constant across sizes, so 60 lb bags are 60 of them and 40 lb bags are 90. Bag size changes the lifting, not the total.
- Pier volume goes with the square of the diameter. A 12″ sonotube takes 44% more concrete than a 10″ one at the same depth, and a 24″ takes four times a 12″.
- Steps stack as a triangular number. Three steps is six units of tread × rise × width, not three, because each step sits on the ones below it. Four steps is ten.
- The waste allowance isn't padding. Subgrade is never as flat as the drawing, forms bow outward under the pressure of wet concrete, and some ends up on the ground. Running out mid-pour is far worse than a leftover wheelbarrow, because the shortfall arrives while the clock is running.
- Concrete weighs about 150 lb per cubic foot. That 10 × 10 patio is two and a half tons — which matters as much for what you're pouring onto as for what you're carrying.
How to use
- Pick a shape and enter its dimensions — thickness in inches, everything else in feet.
- Add more pieces if the job has footings or piers as well as a slab.
- Check the working-time verdict before the bag count, since that is what decides whether bags are viable at all.
- Order the volume shown with waste included, not the bare calculated figure.
Frequently asked questions
How many bags of concrete are in a cubic yard?
Forty-five eighty-pound bags. A cubic yard is 27 cubic feet and an 80 lb bag of concrete mix yields 0.60 cubic feet, so a single yard is 3,600 lb of material. The yield per pound is the same across bag sizes, so it is 60 bags at 60 lb and 90 bags at 40 lb — the smaller bag changes how many times you bend down, not the total weight you move.
How many bags do I need for a 10x10 slab?
At four inches thick, 56 eighty-pound bags — about 4,480 lb, before any waste allowance. The volume is 33.3 cubic feet, or 1.23 cubic yards. That figure surprises most people, because a ten foot square patio does not feel like a ton-and-a-quarter job, and it is the point at which bagging stops being sensible.
Can I mix that many bags by hand?
Not as a single monolithic pour, and the reason is chemistry rather than stamina. Concrete stays workable for roughly an hour after the water goes in. Fifty-six bags is about 28 mixer batches and over two hours of mixing before any placing, screeding or floating — so the first batch has stiffened long before the last is down. Adding people helps less than you would hope, because one mixer is still one mixer.
What is a cold joint and why does it matter?
It is the plane you create when fresh concrete is placed against concrete that has already begun to set. The two never bond properly, so the joint stays a weakness through the full depth of the slab and is where cracking eventually shows up. It is the specific failure that a pour running past its working time produces, and it is invisible until it is not.
How thick is four inches in feet?
A third of a foot — 0.333. This sounds too obvious to mention until you notice how many concrete orders go wrong on it. Slab thickness is quoted in inches while length and width are in feet, so multiplying by 4 rather than 0.333 overstates the volume twelvefold. That is how people end up with a truck they cannot use and a short-load fee to go with it.
When should I switch from bags to ready-mix?
Somewhere around a cubic yard, and the working-time check will tell you more precisely than a rule of thumb. Below that, bags are usually easier than arranging a delivery. Between about one and three yards is genuinely awkward: most suppliers charge a short-load fee in that range, but bagging it means dozens of bags and over a ton of lifting. Get the short-load fee quoted rather than assuming it is prohibitive — it often is not.
Should I use fast-setting concrete to save time?
Only for posts. Fast-setting mix goes off in twenty to forty minutes, which is exactly what you want when you are setting a fence post and walking away — and exactly wrong for anything that has to be screeded and floated, because the window closes before you have finished placing. It fails jobs that standard mix handles comfortably.
How much extra concrete should I order?
Between five and fifteen per cent, depending on how rough the ground is. Ten per cent is the usual allowance for a slab on grade. This is not padding: subgrade is never as flat as the drawing, forms bow outward under the pressure of wet concrete, and some of it ends up on the ground. Running out mid-pour is far worse than a leftover wheelbarrow, because the shortfall arrives exactly when the clock is running.
How much concrete does a sonotube pier take?
A twelve inch tube four feet deep takes 3.14 cubic feet, or about six eighty-pound bags. The thing to watch is that volume goes with the square of the diameter, not with the diameter — so a twelve inch tube takes 44 per cent more than a ten inch one at the same depth, and a twenty-four inch tube takes four times as much as a twelve. Nine piers of a modest size add up faster than the drawing suggests.
How do I calculate concrete for steps?
Steps stack as a triangular number, which catches people out. Each step sits on the ones below it, so three steps is six units of tread times rise times width rather than three, and four steps is ten. Working out one step and multiplying by the number of steps understates a four-step flight by more than half.
How much does concrete weigh?
About 150 lb per cubic foot, or 4,050 lb per cubic yard — over two tons. A 10 by 10 foot patio at four inches is roughly two and a half tons of finished concrete. That matters as much for what you are pouring onto as for what you are carrying, particularly on a deck, a balcony or made-up ground.
Does this tell me how thick the slab should be?
No — these are quantities, not a structural design. Thickness, reinforcement, subbase and control joint spacing depend on what the slab carries and on the ground beneath it, and a driveway is a different problem from a garden path. Footing depth is governed by the frost line where you are, which is a local code question rather than a calculation.
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