EstimatorKitFree concrete calculator
Concrete for any shape, in bags and cubic yards
Choose the shape you are pouring — slab, footing, column, post hole or steps — give it the dimensions off your tape, and this returns the volume as whole bags in all three retail sizes and as the cubic yards a plant would batch. An 80 lb bag makes 0.6 of a cubic foot and a cubic yard is 27 of them, so 45 bags is a yard, which is also roughly where carrying bags stops being the cheaper idea. Both numbers are shown, along with the weight you would be lifting either way.
- 100% free
- No signup
- 5 pour shapes
- 40, 60 and 80 lb bags
- Quarter-yard ready-mix steps
One rectangle at one thickness — a patio, a shed base, an AC pad, a walkway.
Every dimension takes a tape reading as written: 12' 6 3/8", 150 in and 3.8 m all land on the same number.
Order this much ready-mix
2 cu yd
- Bare volume
- 1.78 cu yd
- With 10% allowed
- 1.96 cu yd
- Cubic feet
- 52.8
- Weight in place
- 3.96 tons
| Bag | Bags | To lift | Water | Spare |
|---|---|---|---|---|
| 40 lb | 177 | 7,080 lb | 66.4 gal | 0.3 cu ft |
| 60 lb | 118 | 7,080 lb | 66.4 gal | 0.3 cu ft |
| 80 lb | 89 | 7,120 lb | 66.8 gal | 0.6 cu ft |
Past the point where bags pay
1.96 cu yd is 89 eighty-pound bags — 7,120 lb through the checkout, into the truck and out again, and 45 mixer batches that all have to go in before the first one stiffens. The crossover sits near 1 cu yd, and it is a judgement rather than a specification: a wheelbarrow crew and a supplier's minimum load both move it.
If it does come on a truck, budget 18 barrow loads for anything the chute cannot reach: what a 6 ft³ contractor barrow takes in concrete — half full, and already 450 lb.
Bag counts use the yields printed on the bags — 0.6 cu ft from an 80, 0.45 from a 60, 0.3 from a 40 — and 27 cu ft to the yard. Water is scaled from the mixing water printed on an 80 lb bag of pre-mixed concrete: 6 pints, about 2.8 liters; it moves with how damp the sand in the bag already is, and how stiff you want the mix. Weight assumes 150 lb/ft³, which varies with aggregate type and the steel in the section — lightweight mixes run near 110. Yardage is rounded up because ready-mix plants batch and bill in quarter-yard steps.
How to work out how much concrete you need
Pick the geometry first — that is where the error usually is, not in the arithmetic.
Pick the shape you are actually pouring
A slab is a rectangle at one thickness, a footing is a run of constant section, a column is a cylinder, a post hole is a cylinder with the post and its drainage stone taken out, and a flight of steps is a stack where each tread carries every tread above it. Choosing the wrong one is the biggest single error on this page: measuring the wall instead of the footing under it, or the finished stair instead of the wedge of material behind it.
Measure the section, and type it as the tape reads
Footings and curbs need the width and depth of the concrete itself, in inches, and the total run in feet — walk the perimeter and add every leg. Fractions are fine as written: 12' 6 3/8" parses, and so does 150 in or 3.8 m. The one dimension people guess at is thickness, and it is the one the whole answer scales with, so measure the form board rather than assuming the slab is four inches.
Take the bag count or the quarter yard, not both
The panel shows the same pour three ways because the store has whichever size it has that morning, and the bags do not divide evenly: 40, 60 and 80 lb bags yield 0.3, 0.45 and 0.6 cubic feet. Under about a cubic yard, buy bags. Over it, order the yardage rounded up to the next quarter yard, and read off the barrow loads you will need for anything the chute cannot reach.
Technical specifications
| Shapes | Slab, footing or curb, round column, post hole, poured steps — five geometries, no duplicates |
|---|---|
| Bag yields used | 0.3 cu ft from a 40 lb bag, 0.45 from a 60, 0.6 from an 80 — the yields printed on the bags |
| Bags to a cubic yard | 90 forty-pound, 60 sixty-pound or 45 eighty-pound, from 27 cu ft per yard |
| Ready-mix rounding | Up to the next 0.25 yd³ — the smallest increment a plant will put in the drum |
| Post hole deduction | Post section and the stone base under it are both subtracted — over a fifth of a typical hole |
| Stair volume | Triangular stack, n(n+1)/2 tread-and-riser blocks, not a wedge |
| Placed weight | 150 lb per cubic foot, so 4,050 lb per cubic yard and 3 cu ft per barrow load |
| Kept after you close the tab | Nothing — the dimensions are never sent anywhere to be calculated |
Frequently asked questions
How many bags of concrete for a 10 x 10 ft slab?
Fifty-six 80 lb bags at 4 inches thick, or sixty-two once a 10% allowance is on. The bare geometry is 33.3 cubic feet and an 80 lb bag makes 0.6 of one, so 56 bags is the arithmetic and 62 is the order. That is 4,960 lb of bagged material to move twice — the same pour as ready-mix is 1.5 cubic yards, which is why a hundred square feet is roughly where people stop bagging.
How much concrete does a fence post hole take?
About 1.5 cubic feet for a 12 inch hole 30 inches deep — three 80 lb bags or four 60 lb ones. The reason it is not the full 1.96 cubic feet the hole would hold is that two things are already in it: the post itself, which is 3.5 inches square if it is a nominal 4x4, and the four inches of drainage stone the post should be standing on so its end is not sitting in water. Together they take just over a fifth of the hole, and a calculator that ignores both sends you home with an extra bag per post.
Why do poured steps take so much more concrete than they look?
Because the material behind the steps is a staircase, not a ramp. Each step sits on top of everything below it, so the cross-section is the triangular number of the step count rather than a plain triangle — four steps at 7 by 11 inches on a 4 ft wide flight is 21.4 cubic feet, where treating the same flight as a wedge from the top nosing to the bottom gives 17.1. That is a fifth short, and it gets worse as the flight gets shorter: with three steps the plain triangle misses by a quarter.
What does a cubic yard of concrete weigh?
About 4,050 lb, or a little over two short tons, at the 150 lb per cubic foot that normal-weight concrete is designed around. Two numbers follow from it. A cubic yard in a half-ton pickup is illegal by a factor of four, and a contractor wheelbarrow that nominally holds six cubic feet carries three of concrete before it stops being pushable — 450 lb in the barrow and all of it in front of the wheel.
How much concrete does a ready-mix truck hold?
A standard drum is rated for about 10 cubic yards, though axle weight limits keep many trucks to 8 or 9 on the road. Orders are taken in quarter-yard increments and invoiced the same way, which is why this page rounds up to the next quarter rather than to a whole yard: on a 3.6 yard pour that is a saving of a third of a yard, and on a 1.1 yard pour it is the difference between paying for one yard and paying for two.
How much extra concrete should I order?
Five to ten percent, and the reason is the ground rather than spillage. Forms bow outward under the weight of a pour, subgrade that was raked flat is never flat, and every dip is volume: half an inch of average over-depth under a 12 by 12 slab is 6 cubic feet, which is an eighth of the whole pour. The allowance on this page is a field rather than a hidden multiplier, so you can see the bare figure and the padded one side by side and argue with either.
Is there a limit to how much I can mix by hand?
The limit is the clock, not your back. Batches have to go in against wet concrete, and a pour that stops long enough for the previous batch to stiffen has a cold joint through it — a plane the two halves never bond across. A 3.5 cubic foot drum mixer takes two 80 lb bags at a time, so ten bags is five batches and a manageable hour, while forty bags is twenty batches that one person cannot keep ahead of on a warm day.
About bags, yards and the crossover between them
Concrete is bought in two units that do not meet in the middle. The bag says cubic feet because that is what a bag makes; the plant quotes cubic yards because that is what a drum holds; and 27 cubic feet to the yard is the only bridge between them. Pre-mixed bags yield less than their weight suggests — 80 lb of dry mix becomes 0.6 of a cubic foot once water is in it, so a yard is 45 bags, 3,600 lb of dry mix and 23 batches through a portable mixer. That arithmetic is the whole reason the crossover exists, and it is why the answer to “how much concrete” is a different kind of number above and below it: under a yard you are buying a stack of bags, over it you are buying a delivery slot.
Where estimates actually go wrong is a step earlier, in what got measured. A footing is not the wall on top of it, so the section to enter is the concrete’s own width and depth. A post hole is not a cylinder, because the post occupies part of it and should be standing on stone rather than in the bottom of the mud. A flight of steps is not a wedge: each tread rests on the ones below, so the volume climbs as the triangular number of the step count and a flight always takes more than it appears to. Those three shapes account for most of the short orders we can see a reason for, which is why they are separate entries here rather than one length-times-width-times-depth box. Once the pour is priced, the concrete cost calculator takes the volume from this page and adds the short-load fee, the pump and the finishing labor that turn a yardage into an invoice.
Two neighboring materials get mistaken for this one. Bricks and blocks are bedded in mortar, which is a different mix at a fraction of the volume, and estimating a wall by its cubic content instead of its unit count is how orders come back tenfold wrong — the brick calculator counts the units and the joints instead. Pavers are dry-laid on bedding sand with no concrete anywhere in the section, priced by the paver calculator and its bedding sand. And if what you are pouring is a slab rather than a shape, the concrete slab calculator does the rest of the order this page leaves out: the vapor barrier, the mesh, the form lumber and the control joints that decide where it cracks.
What happens to the dimensions you type
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — measurements, quantities, the prices your supplier quoted you — is uploaded, logged or kept, which is also why the calculators carry on working on a site with no signal.
There is no quote button on this page and no supplier waiting at the other end of one. Most of the calculators ranking for this search belong to somebody who sells concrete, which is a perfectly good reason to publish one and also a reason their defaults tend to round upward.