Concrete Calculator

Slabs, footings and L-shaped patios all reduce to rectangles

The shape dropdown offers two options rather than three, and that is deliberate. A slab and a footing are the same calculation — length × width × depth — because a footing is really just a narrow, deep trench of the same bagged mix. No manufacturer publishes a separate “footing formula”; Quikrete and Sakrete both list footings as a standard use of the very product a patio slab gets poured from. The only thing that changes between a 10 ft × 10 ft patio and a long, narrow footing is which of the three numbers is the large one.

Thickness is entered in inches while length and width are in feet, because that is how the figures appear on a drawing and on a tape measure — a patio is 4 in thick, not 0.333 ft. The engine divides thickness by 12 before multiplying, so typing 4 gives you four inches of slab. Typing a depth in feet into a field that expects inches is the costliest input mistake available on this page, which is why the unit sits on the field itself rather than in a footnote.

A cubic yard is 27 cubic feet, because a yard is three feet on every side, and that single conversion is what turns a raw volume into an order quantity — ready-mix is quoted and delivered by the cubic yard, never by the cubic foot. Anything that is not a plain rectangle or circle should be cut into pieces and added, never averaged: an L-shaped patio is two rectangles, so run the tool once for each leg, make sure the two legs do not both claim the corner, and add the cubic yards. A slab with a thickened edge is a rectangle plus a perimeter footing. A curved bed is closer to part of a circle than to any rectangle you could average it into. Averaging the dimensions of an irregular pour is how people end up a third of a yard short with a truck already on site and the forms already wet.

Doubling a post diameter quadruples the concrete

Circular mode asks for a diameter because that is the number you can actually measure across a form tube or a post hole, but the geometry runs on the radius: π × r² × depth, where r is half of whatever you typed. The squaring is the part that catches people out. A 1 ft diameter post has a cross-section of π × 0.5² = 0.79 ft². A 2 ft diameter post is π × 1² = 3.14 ft². The hole looks twice as wide and swallows four times the mix.

The same effect bites at much smaller steps. Moving from a 12 in form tube to a 16 in one is not a 33% increase: the radius goes from 0.5 ft to 0.667 ft, so the area goes from 0.79 ft² to 1.40 ft² — the ratio is (16 ÷ 12)², or about 1.78, so roughly 78% more concrete per post at the same depth. Deck footings and fence posts are almost always specified by diameter, so it is worth running the tool once for each candidate size before buying a bundle of tubes, particularly when a dozen holes share the same depth and the error multiplies twelve times over.

Depth is in inches in this mode too, which trips people more often on columns than on slabs, because a post hole is usually talked about in feet — a 3 ft footing is 36 in. And the figure the tool returns is the volume of the whole cylinder. A timber or steel post set into that hole displaces part of it, and the calculator does not model the post, a belled base, or a gravel bed under the pour. If you are buying bags to an exact count for posts rather than ordering a truck, work the displaced volume out yourself and take it off before you round up.

Ten percent waste is the top of a published range, not a guess

The waste field defaults to 10%, and that is not a comfortable round number somebody invented. NRMCA’s CIP 31, “Ordering Ready Mixed Concrete”, recommends ordering 4–10% more than the plan-dimension estimate, and 10% is the top of that published range. The margin covers three things a tape measure cannot see: spillage while the concrete is being placed, a subgrade that is not quite level, and forms that get overfilled at the edges. A screeded, well-compacted, properly formed slab may only need the bottom of the range. A hand-dug footing over uneven ground earns the top of it.

Where the margin is applied matters as much as how big it is. The calculator adds waste to the raw cubic-foot volume first, and only then divides by 27 for cubic yards and by the bag yields for bag counts — so the truck order and all three bag counts you see are already waste-adjusted. Adding a second margin on top of the displayed figure double-counts the same contingency, and on a large pour that is a whole extra yard of concrete bought for nothing.

The field is capped at 30%, well beyond any published guidance, and exists only so you can model a genuinely rough pour or a site where spillage is guaranteed. Setting it to 0 gives you the bare plan-dimension volume, which is useful for checking a supplier’s quote against your own drawing but is not a quantity anybody should order from. The asymmetry is the thing to remember: surplus concrete is a nuisance and a disposal problem, but running out mid-pour leaves a cold joint through the slab and usually costs a short-load fee on the second delivery as well.

The three bag counts are alternatives, not a shopping list

The 40, 60 and 80 lb figures are three answers to one question, so buy one of them and ignore the other two. They differ only in how the same volume is packaged: a 40 lb bag yields about 0.30 ft³ of mixed concrete, a 60 lb bag about 0.45 ft³, and an 80 lb bag about 0.60 ft³. Those yields come from Quikrete’s Concrete Mix (No. 1101) technical data sheet and Sakrete’s High-Strength Concrete Mix data sheet, and the two state identical figures — which is why this page treats them as a genuine cross-brand standard rather than one manufacturer’s rounding.

Because the yields scale cleanly, so do the counts: two 40 lb bags come to 0.60 ft³, exactly one 80 lb bag, and three 40 lb bags match two 60 lb bags. Choosing between them is a handling decision, not a quantity one. An 80 lb bag roughly halves the number of trips from the pallet and the number of bags you have to cut open, which matters when the count runs into three figures. A 40 lb bag is what you want if you are carrying mix down steps, working on your own, lifting above shoulder height, or turning it over in a wheelbarrow rather than a powered mixer.

Every count is rounded up to a whole bag, because a part bag still has to be bought and mixed, and the rounding happens after the waste factor rather than before it. On a small pour that round-up is a visible share of the total, so the 40 lb column can look disproportionately padded next to the 80 lb one. One thing the bag figures deliberately do not include is mixing water, which is added on site and forms no part of the yield the manufacturers publish.

Where a 10 × 10 patio stops being a bag job

Take the tool’s own default: a 10 ft × 10 ft patio slab, 4 inches thick, at the default 10% waste. Raw volume is 10 × 10 × (4 ÷ 12) = 33.33 ft³. With waste that becomes 36.67 ft³, or 1.36 cubic yards once divided by 27. In bags that is 62 at 80 lb, 82 at 60 lb, or 123 at 40 lb. Sixty-two 80 lb bags is 4,960 lb of dry material, and every pound of it moves twice — pallet to mixer, mixer to forms — fast enough that the first batch has not set before the last one lands.

That is why the result panel starts warning as the figure climbs. Between roughly 0.75 and 1.5 cubic yards it flags that you are close to a typical ready-mix minimum; past about 1.5 cubic yards it says plainly that a delivered truck is usually cheaper and far less work than hand-mixing. Both are advisory, because the minimum itself is not a universal rule. Some plants set around a one cubic yard minimum with a short-load surcharge below it, some suppliers advertise no minimum at all on volumetric mixer trucks that batch on site, and others will not roll a truck for less than three or four yards.

So the crossover is a phone call, not a constant. Under about a cubic yard, bags nearly always win. Over two, the truck nearly always does. In the band between, the answer turns on your own local plant’s minimum, its short-load fee, and whether you have the hands and the mixer to place a hundred-plus bags without stopping. The one option to rule out is splitting the difference: a slab poured in two sessions hours apart carries a cold joint straight through it, and no delivery saving repays that.

A volume figure cannot tell you how thick to pour

This calculator answers “how much” after you have decided “how thick”, and no amount of arithmetic bridges the two. Common residential practice is 4 in for patios and walkways and 6 in for driveways, rising to 8–12 in where vehicles or heavy equipment will stand; the helper text under the thickness field carries the first two. Local building codes set their own minimums, though, and a code minimum beats a rule of thumb every single time — check before pouring anything load-bearing or permitted.

Everything downstream of thickness sits outside this tool as well. It does not size a footing for a given load, specify a compressive-strength (psi) mix, place reinforcement, position control joints, or apply a frost-depth requirement — and frost depth in particular is a local figure that can double how deep a footing has to go in a cold climate. Reinforcement is not deducted from the order either: rebar and wire mesh displace a negligible amount of concrete and sit inside the pour rather than reducing how much of it you need, so order the full calculated quantity and let the steel take its place in the middle of it.

Those limitations are not hedging. A volume estimate correct to the cubic foot is still the wrong quantity if the slab is too thin for what will sit on it, the mix is the wrong strength, or the footing stops above the frost line. For anything load-bearing, permitted, or tied into a structure — foundations, structural footings, retaining walls — confirm the dimensions, the mix design and the reinforcement with a structural engineer, your ready-mix supplier, or the local building department before you order. This page estimates material; it does not design the pour.

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Sources

• Concrete Mix (No. 1101) — Technical Data Sheet (per-bag yields: 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb, 0.60 ft³ for 80 lb) — Quikrete
• High-Strength Concrete Mix — Technical Data Sheet (independently confirms the same 40/60/80 lb yields) — Sakrete
• CIP 31 — Ordering Ready Mixed Concrete (recommends ordering 4–10% more than the plan-dimension estimate) — National Ready Mixed Concrete Association (NRMCA)
• Estimating Bagged Concrete Jobs — Sakrete