Concrete Calculator
How much concrete a slab, a footing, or a round column actually takes — in cubic yards for the truck, and in 40, 60 and 80 lb bags for the hardware store. Measure in feet and inches; the conversions happen here.
Slabs, patios, and driveways
A footing and a slab are the same box measured differently; a column is a cylinder.
The long side, measured inside the forms.
The short side, also inside the forms. Irregular shapes: split the job into rectangles and add the results.
Four inches is the usual patio and shed-pad slab; six for a driveway that will carry vehicles.
Identical slabs in one pour. They are counted before the bag arithmetic, so nothing rounds up twice.
Ten percent covers spillage and a subgrade that is never quite flat. Kept as its own line below so you can argue with it.
33.33 cu ft of slabs plus a 10% allowance of 3.33 cu ft comes to 36.67 cu ft — or 62 bags of 80 lb premix.
- Volume of the slabs
- 33.33 cu ft
- Waste allowance (10%)
- 3.33 cu ft
- Order this much
- 36.67 cu ft
- In cubic yards
- 1.36 cu yd
- Before waste
- 1.23 cu yd
- Ready-mix delivery
- Above the 1 cu yd minimum
| Bag size | Yield per bag | Bags needed |
|---|---|---|
| 40 lb | 0.30 cu ft | 123 |
| 60 lb | 0.45 cu ft | 82 |
| 80 lb | 0.60 cu ft | 62 |
- Bag counts are rounded up. Running short mid-pour puts a cold joint through the element, which is a structural defect; running long costs you the price of the surplus.
- Over a cubic yard, ready-mix is normally cheaper than bags and far less work. Confirm the supplier's minimum load when you book the truck.
- Yields of 0.30, 0.45 and 0.60 cu ft are the manufacturers’ nominal figures for 40, 60 and 80 lb bags. Real yield moves with how wet the mix is worked.
From feet and inches to yards and bags
Every shape here reduces to a volume in cubic feet, and every answer is that volume expressed in a unit somebody sells in. A slab and a footing are the same box — a footing is simply described by its depth rather than its thickness, because one is dug and the other is poured. A column is a cylinder.
Vbox= L × W × (T ÷ 12) • Vcolumn= π × (D ÷ 24)² × H • yd³ = ft³ ÷ 27- L, W
- Length and width in feet, measured inside the forms
- T
- Slab thickness or footing depth in inches, hence ÷ 12
- D, H
- Column diameter in inches and height in feet — ÷ 24 turns a diameter in inches into a radius in feet
- ÷ 27
- Cubic feet to cubic yards: three feet in each of three directions
The element count multiplies the volume before anything else happens. Twelve deck footings are one order, not twelve, and rounding each footing up to its own whole bag would buy several bags of nothing. Waste is applied next, as a percentage of that total, and is reported on its own line rather than folded in — an allowance you cannot see is an allowance you cannot argue with.
Bag counts are then the total volume divided by the yield printed on the bag, rounded up. Never to the nearest bag: 61.1 bags is 62 bags, because the shop does not sell the other ninth.
A 10 × 10 patio slab, four inches thick
Four inches is 4 ÷ 12 = 0.3333 feet, so the slab is 10 × 10 × 0.3333 = 33.33 cubic feet. A 10% allowance adds 3.33 cubic feet, giving 36.67 cubic feet to order. Divide by 27 and that is 1.36 cubic yards — comfortably over a typical one-yard minimum, so a ready-mix truck will take the job without a short-load fee.
Bagged, the same pour is 36.67 ÷ 0.60 = 61.1, so 62 bags of 80 lb premix. That is 4,960 pounds — roughly two and a half tons to buy, carry, open, and mix, one wheelbarrow at a time, before the first of it starts setting. The 60 lb equivalent is 82 bags and the 40 lb is 123. The truck is the right answer here, and it is not close.
Now the other side of the crossover. Twelve deck footings in 12 inch tube forms, 4 feet deep, come to π × 0.5² × 4 = 3.14 cubic feet each, or 37.70 cubic feet for all twelve. With 10% that is 41.47 cubic feet — 1.54 cubic yards, or 70 bags of 80 lb premix. Drop to four footings and the same sum gives 0.51 cubic yards: under the minimum, into short-load-fee territory, and a job that 24 bags and a rented mixer will handle for less.
Short is a defect. Long is an invoice.
These two errors are not the same size, and that is the single most useful thing to know before placing an order. Come up short mid-pour and the concrete already in the forms keeps stiffening while you wait. If the next batch lands after that first lift has lost its plasticity, the two do not knit: the boundary becomes a cold joint, a plane through the element that never develops full strength, cracks preferentially, and lets water in. On a structural footing that is a defect you cannot sand out. On a driveway it is the line the slab will eventually fail along.
Come up long and the surplus goes back on the truck, or into a spare form, and you have paid for concrete you did not use. That is a bad afternoon and a small number of dollars. Weigh the two honestly and the waste allowance stops looking like padding and starts looking like insurance priced in single-digit percent.
The same asymmetry is why every bag count on this page is rounded up rather than to the nearest whole bag. Two-thirds of a bag is not something anyone sells, and being one bag short at the far end of a pour is exactly the scenario above.
The one-cubic-yard crossover
Ready-mix suppliers set a minimum load, commonly one cubic yard, and bill a short-load fee on anything below it. The economics are obvious once stated: the truck, the driver, the fuel, and the wash-out are the same whether the drum carries one yard or nine. On a small pour that fee can rival the cost of the concrete, which is why the result panel flags any order that lands under a yard.
Below the crossover, bagged premix is usually cheaper and always more flexible — you buy what you need, when you need it, and an extra bag costs an extra bag. Above it, ready-mix wins on price per yard and wins overwhelmingly on labour: a cubic yard is forty-five 80 lb bags, and premixed concrete gives you tens of minutes of working time, not hours. Somewhere around a yard, the wheelbarrow stops being a plan.
What this calculator assumes
- Dimensions are measured inside the forms, and the subgrade is flat. Real excavations are deeper in places than the tape suggests, which is what the waste allowance is for.
- Bag counts are always rounded up to a whole bag, never to the nearest one. Two-thirds of a bag of premix is not a thing you can buy, and running short mid-pour is far more expensive than running long.
- Bag yields of 0.30, 0.45 and 0.60 cubic feet for 40, 60 and 80 lb bags are the manufacturers’ stated nominal values, not a standard. Actual yield moves a few percent with how wet the mix is worked and how well it is consolidated.
- The one cubic yard ready-mix minimum is a common industry figure, not a rule. Minimums and short-load fees vary by supplier and region — the flag is a prompt to phone and ask, not a quote.
- No deduction is made for rebar, mesh, conduit, or embedded items. They displace a fraction of a percent of the volume, comfortably inside the waste allowance.
- Nothing here addresses mix design, strength class, air entrainment, reinforcement, or footing depth. Strength changes what a yard costs, not how many yards there are; footing depth is set by frost line and soil bearing under local code.
Concrete calculator FAQ
How many 80 lb bags of concrete are in a cubic yard?
Forty-five. A cubic yard is 27 cubic feet and an 80 lb bag yields a nominal 0.60 cubic feet, so 27 ÷ 0.60 = 45 bags. The same arithmetic gives 60 bags at 60 lb (0.45 cu ft each) and 90 bags at 40 lb (0.30 cu ft each). Forty-five 80 lb bags is 3,600 pounds of material to lift twice — once into the car, once into the mixer — which is the practical reason a cubic yard is usually the point where people call a truck instead.
Why is concrete sold by the cubic yard when I measured in feet?
Because that is the unit ready-mix trucks are metered and billed in throughout the United States. The geometry of your slab naturally produces cubic feet, so the conversion has to happen somewhere: 27 cubic feet to the cubic yard, since a yard is three feet in each of three directions. This calculator shows both, so you can check the arithmetic in the unit you measured and place the order in the unit the supplier speaks.
What actually happens if I run out of concrete halfway through a pour?
The concrete already placed begins to stiffen. If the next batch arrives after that first lift has lost its plasticity, the two do not knit together and the boundary between them becomes a cold joint — a plane of weakness running through the finished element that will not develop full strength and tends to crack and admit water along its line. It is a structural defect, not a cosmetic one, and it cannot be fixed after the fact. Ordering more than you need costs you the surplus. That asymmetry is the whole argument for rounding up.
Is it cheaper to use bags or to order a ready-mix truck?
Roughly speaking, bags win below about a cubic yard and ready-mix wins above it. Suppliers set a minimum load — one cubic yard is a common figure — and charge a short-load fee on anything under it, because the truck, the driver, and the drive are the same cost whether it carries one yard or nine. That fee can approach the price of the material itself on a small job. Above a yard or so, mixing by hand also stops being realistic: a cubic yard is about forty-five 80 lb bags, and premixed concrete has a working time measured in tens of minutes.
How thick should a concrete slab be?
Four inches is the usual thickness for a patio, a walkway, or a shed pad. Six inches is common where vehicles will park or drive. Footings are a different question entirely — their depth is set by the frost line and the bearing capacity of your soil, both of which are governed by local building code and neither of which any calculator can tell you. Check the code before you dig, not after you pour.
How much waste should I allow?
Five to ten percent covers the ordinary losses: spillage, material stuck in the wheelbarrow, and a subgrade that is a little deeper in places than the tape said. Ten percent is the default here. Go higher if you are pouring against uneven ground or excavating by hand, where over-dig is real and often larger than people expect; go lower if you are pouring into tight forms over a screeded base. The allowance is shown as its own line so you can change it and see exactly what it is costing you.
How do I calculate concrete for an L-shape or an irregular slab?
Split it into rectangles, run each one through the calculator, and add the cubic feet before you convert to yards or bags. Do not add up bag counts from separate runs — each one rounds up to a whole bag, so three sections would order three partial bags you do not need. For a curved or genuinely irregular edge, take the area by whatever method you trust and multiply it by the thickness in feet; the tool's slab mode is doing nothing more than that.
Sources and review notes
- American Concrete Institute — the body behind the codes, specifications, and terminology US concrete construction runs on, including what a cold joint is and why continuous placement matters
- NIST Special Publication 811 — the definitions behind the two conversions on this page: 12 inches to the foot, 27 cubic feet to the cubic yard
The geometry here is exact and has no data behind it to go stale. The two figures that can move are the bag yields, which are manufacturer-stated nominal values printed on the bag rather than a published standard, and the one cubic yard delivery minimum, which is a widely used industry convention that varies by supplier. Check both locally before you order.