Concrete Calculator — Cubic Yards, Bags & Cost

How is concrete volume calculated?

Concrete is estimated by volume, and every mode in this calculator reduces to the same idea: work out the space the concrete has to fill in consistent units, add a waste allowance, then divide by the yield of whatever you are buying. The only thing that changes between a slab and a set of stairs is the geometry.

The stair formula is the one worth pausing on. A flight of solid poured steps is not n identical blocks — the bottom step stands one riser tall, the second stands two risers tall because it sits on the first, and so on. Summing that arithmetic series gives the n(n + 1) ÷ 2 term, which is why doubling the number of steps roughly quadruples the concrete. Four steps take ten riser-units of material; eight steps take thirty-six.

Bag yields are the other half of the arithmetic. Every standard concrete mix yields 0.0075 cubic feet of mixed concrete per pound of dry mix, which is why the four bag sizes sold in the US work out as exact multiples of one another: 0.30 ft³ for a 40 lb bag, 0.45 for a 60, 0.60 for an 80. If you are looking at an unfamiliar bag, divide its stated yield by its weight — you should get 0.0075.

A worked example: a 10 × 12 ft patio slab

Say you are pouring a patio 12 ft long, 10 ft wide and 4 inches thick, with the standard 10% waste allowance. The figures below are generated by the same code that runs the calculator above, so they cannot drift apart from it.

Two things fall out of that. The first is that 74 bags is roughly 5,920 lb of dry mix to move, open and mix — a full day for two people with a mixer, and genuinely hard work without one. The second is that at 1.63 cubic yards this job sits below almost every ready-mix supplier's minimum load, so a truck would carry a short-load fee. This is exactly the awkward middle band where the right answer depends on how much you value your Saturday.

Who this calculator is for

It is built for the person standing in the aisle deciding how many bags to put on the cart, and for the contractor who needs a defensible number on a quote. Concrete is unusually unforgiving of a bad estimate in one direction and merely annoying in the other, so the tool is deliberately biased towards telling you when you are cutting it fine.

• DIY slabs and pads — patios, shed bases, AC condenser pads, hot tub pads. Use slab mode and check the thickness warning.
• Fence and deck contractors — post-hole mode subtracts the post, which over twenty or thirty holes is the difference between one pallet and two. Pair it with the fence calculator or the deck calculator, which both feed post counts into the same footing arithmetic.
• Anyone pouring steps — the series formula catches the mistake almost every quick estimate makes.
• Deciding bags versus a truck — the tool prices both side by side and tells you which side of the roughly 4 cubic yard crossover you are on.

What this calculator does not account for

These limits are real and they are the difference between a planning estimate and a takeoff. None of them are handled by the arithmetic above.

• Subgrade that is not flat. A slab formed on a surface that dips an inch over its length can consume 15–25% more than the nominal thickness suggests. The waste slider is doing this job, and on a rough subgrade 10% is not enough.
• Strength, mix design and admixtures. This calculates volume, not psi. Air entrainment for freeze-thaw regions, fibre mesh, accelerators and retarders are all mix decisions that a supplier or an engineer makes, not volume ones.
• Reinforcement. Rebar, mesh, chairs and dowels are not included. Size those with the rebar and mesh calculator.
• The base course. Most slabs sit on 4–6 inches of compacted gravel, which is often a larger material order than the concrete itself. Work it out with the gravel calculator, and remember that compactable material settles about 20%.
• Forms, labour, pump hire and disposal. None are quantified here. On an awkward-access pour, a concrete pump can cost more than the concrete.
• Local code. Footing depth below the frost line, slab reinforcement requirements and inspection points are set by your jurisdiction, not by a national figure. Frost depth alone ranges from zero in southern Florida to eight feet in northern Minnesota.

How many bags of concrete for common slab sizes?

This is the table to bookmark. Every row is computed by the same concrete yardage calculator above — length × width × depth, with a 10% waste allowance — so a 12×20 driveway slab and a 40×60 pole-barn floor use the identical formula, just at different scale. Click a column heading to sort.

All rows include 10% waste and use 80 lb-bag ready-mix truck loads at 10 yd³ per truck. Coverage rates and bag yields verified against Quikrete's published data sheet, 2026-08-04.

What about footings, columns, sonotubes and ready-mix?

A concrete footing calculator and a sonotube (round column) calculator are really the same rectangular- or cylindrical-volume arithmetic as a slab, just with different inputs — this tool covers both as modes above rather than as separate pages, so switching between a footing, a round sonotube column and a slab keeps your waste factor and bag pricing intact. The ready-mix concrete calculator comparison sits in the results panel automatically: enter your dimensions once and it prices bags and a delivered truck side by side.

Bag counts per cubic yard, for reference

Counts are rounded up to whole bags, which is the only way you can buy them. Yields are the manufacturers' published figures for mixed concrete at the water ratio printed on the bag — adding water beyond that ratio increases volume and reduces compressive strength.