Concrete bag yields by size

Pre-blended concrete mix is sold by weight, but a project is measured by volume. The bridge between the two is the bag yield: the volume of placed concrete the manufacturer says one bag produces once it is mixed. The concrete bag calculator, and the footing, column, and post-hole calculators that report bag counts, all run on the same small table of yields; the slab calculator prices ready-mix by the cubic yard, and its cubic-feet figure can be carried into the bag calculator. This guide sets out where those figures come from and how any volume becomes a count of whole bags. Every figure below is computed at build time by the site's engine from its sourced constants.

What a bag yield is

A yield is a coverage figure of the same kind as the square feet a gallon of paint covers: it converts a purchasable unit into a quantity of finished work — here, one bag of dry mix into cubic feet of mixed, placed concrete. The manufacturer publishes it on the product data sheet and states it as approximate, since real yield varies from bag to bag and with mixing, which is one reason every tool applies a waste allowance before rounding to whole bags. The yield does not depend on what you are pouring: a cubic foot of slab, footing, column, or the ring around a fence post takes the same bag, so the tools compute the geometry first, in cubic feet, and only then divide by the yield. That is why the bag calculator accepts a bare volume — anything that produces cubic feet can feed it, including the arithmetic in the cubic feet and cubic yards guide done by hand.

The four imperial sizes

The imperial bag table comes from the QUIKRETE Concrete Mix (Product No. 1101) product data sheet, "Yield" section, and covers the 40, 50, 60, and 80 lb bags. Each yield is a manufacturer figure, stated as approximate, and the tools present it as one: the yields are not editable inputs, because a typed-over yield would no longer be the manufacturer's.

The right-hand column is the engine's count for exactly one cubic yard (27 cu ft, by definition) with the waste allowance at zero; every size divides it evenly, so the counts are the bare ratios and every leftover is zero. The litre column restates each yield through the exact conversion 1 cu ft = 0.028316846592 m³.

One property of this table does most of the work in practice: every size yields the same volume per pound of dry mix. Dividing each yield by its bag weight gives 0.0075 cu ft per pound for all four sizes — an arithmetic consequence of the published figures, not a separately published number — so doubling the bag weight exactly doubles the yield. The rate also fixes the dry mix a cubic yard consumes before waste: 27 ÷ 0.0075 = 3,600 lb, whichever size you carry it in.

The two metric sizes

Outside the US the same mix is bagged by mass. The tools offer the two metric sizes published for this line in Canada, 25 and 30 kg, with yields from Target Products Ltd., the licensed manufacturer of QUIKRETE in western Canada. A 20 kg concrete-mix bag is not offered in that line, so none is invented here.

The metric yields are not scaled from the pound figures. A 30 kg bag is 66.1 lb (1 lb = 0.45359237 kg exactly, by the international definition of the pound), which at the imperial rate would imply about 0.496 cu ft — but the manufacturer publishes 0.52 cu ft. Pre-blended mixes are batched and rated per bag by the manufacturer, and the rated metric figures differ slightly from a pound-proportional estimate. The engine therefore keeps two separate tables and never converts between them.

From a volume to a bag count

Whatever tool you start from, the count comes from the five steps documented on the methodology page, specialised to concrete:

1. Geometry. The shape becomes cubic feet: cubic yards × 27; cubic metres ÷ 0.028316846592, the exact value of 0.3048³; litres ÷ 1,000 first.
2. Waste. volume × (1 + waste% ÷ 100). The concrete default is 10% — a trade convention, adjustable in every tool.
3. Bags. ceil(adjusted volume ÷ yield), once per size — the only rounding in the chain.
4. Leftover. bags × yield − adjusted volume, per size, in cubic feet or litres.
5. Cost. Bags of the one size you say you will buy, times the price you type. The site never supplies a price.

Waste is applied to the volume before the ceiling, not to the finished count: padding 6 bags by a further percentage and rounding again would add the margin twice on a small job.

Worked examples

A step or small repair: 3 cu ft

3 cu ft at 10% waste is 3.3 cu ft to buy.

The 40 lb row is an exact fit: 3.3 ÷ 0.3 is 11 with nothing left over, and the engine's exact-fit tolerance keeps binary floating-point noise from buying a phantom bag. The 80 lb row is not half of it: 3.3 ÷ 0.6 = 5.5, which rounds up to 6 bags with 0.3 cu ft spare.

A walkway pad: 8 ft × 4 ft × 4 in

The slab engine gives 32 sq ft × (4 ÷ 12) ft = 10.67 cu ft, or 0.4 cu yd. At 10% waste the bag engine works from 11.73 cu ft.

One cubic yard at the default 10% allowance

The yield table gives the zero-waste counts for a cubic yard; with the default allowance, 27 cu ft becomes 29.7 cu ft. The allowance alone adds 5 bags at 80 lb and 9 at 40 lb — not quite the same extra concrete: 3 cu ft at 80 lb against 2.7 cu ft at 40 lb, because the 80 lb division does not land on a whole bag: it leaves 0.3 cu ft of rounding where the 40 lb row leaves 0.

Six post holes: 10 in across, 30 in of concrete

The post hole calculator works from hole dimensions you have already decided on and subtracts the post. Each 10 in × 30 in hole holds 1.36 cu ft; a dressed 4×4 (3.5 in per the PS 20 lumber standard) displaces 0.21 cu ft, leaving 1.15 cu ft per hole. All 6 holes need 6.91 cu ft; at 10% waste the bag engine works from 7.6 cu ft.

A metric volume: 0.35 m³

In metric mode the bag calculator converts first: 0.35 m³ ÷ 0.028316846592 = 12.36 cu ft, and 10% waste brings it to 13.6 cu ft (385 L). The 30 kg row divides by the published 0.52 cu ft, not the smaller pound-proportional estimate.

Why counts round up, and why doubling the weight does not halve the count

Stores sell whole bags, so each count is a ceiling: the smallest whole number of bags whose combined yield reaches the waste-adjusted volume. Rounding to the nearest bag would leave short any pour whose division rounds down, so purchasable quantities round up, never to the nearest. The leftover column measures that rounding — concrete bought beyond the waste-adjusted need — and is always less than one bag's yield. Because every size rounds independently, the tidy relationships in the yield table hold only before the ceiling: an 80 lb count is exactly half a 40 lb count before rounding, and half after it only when both divisions land on whole numbers. In the 3 cu ft example, 40 lb bags come to 11 and 80 lb bags to 6, not 5.5; on a full cubic yard the rounding is a smaller share of the whole, 99 against 50.

The misreadings that inflate a bag count

Thickness in feet instead of inches. The slab and footing tools take thickness in inches; the column tool takes its cross-section in inches and its height in feet. A 10 × 10 ft slab at 4 in is 33.33 cu ft on the slab calculator; carried into the bag calculator at 10% waste, that is 62 bags of 80 lb mix. Type 4 meaning feet and the slab engine faithfully computes 400 cu ft, 12 times the volume, which the bag calculator turns into 734 bags of 80 lb mix. If a bag count looks wildly high, check the thickness field first.

Cubic yards in the cubic-feet field. One cubic yard entered as 1 cu ft produces 2 bags of 80 lb mix instead of 50; switch the bag calculator's unit selector to cubic yards or multiply by 27 first.

What the yield does not include

A yield converts a bag into placed concrete, and the counts assume the shape you entered is the shape that gets built. As the methodology page lists, the tools do not model subgrade preparation, formwork, reinforcement layout, compaction, moisture or temperature effects, regional product variation, or delivery minimums, and the waste allowance is a single flat percentage. A gravel base under a post or slab is not concrete: enter only the depth the concrete will fill. And the boundary every page on this site keeps: a bag count is a purchasing quantity and nothing more. Whether the mix, the thickness, the hole, or the reinforcement suits the job is a structural and code question for a licensed engineer or your building department — the tools and this guide make no structural, code-compliance, or engineering determination.

Frequently asked questions

Related: the concrete bag calculator recomputes every table above for your own volume; the cubic feet, cubic yards, and ordering concrete guide covers the volume arithmetic that feeds it; the post hole calculator and concrete slab calculator produce the cubic feet from dimensions. All guides are listed on the guides index.

Every bag count in this guide is computed at build time by the same engine that runs the calculators, and nothing you enter in the tools leaves your browser. The full estimation pipeline is documented on the methodology page.