Concrete and mortar bag calculator

Every concrete calculator on the internet answers a question it has quietly invented: how many bags of cement, sand and stone in some ratio nobody named. Mix ratios are not universal — they vary by country, by standard, by what the concrete has to hold up — so a page that hands you one without saying where it came from is guessing on your behalf. This one does not guess. It works in the only figure the industry publishes plainly: the yield printed on a bag. QUIKRETE states that an 80 lb bag of its Concrete Mix "yields approximately 0.60 ft3 (17 L)", and gives the same figure for four other bag sizes. Multiply out your slab, divide by that, round up. For mortar the same sheet counts in bricks rather than volume: one 80 lb bag "will lay up to 37 standard bricks or 13 standard ... blocks". Everything on this page is one of those two sums.

How it is calculated

bags = length × width × thickness ÷ yield per bag · mortar bags = units ÷ units per bag

A slab or a footing is a box, so its volume is the three dimensions multiplied together — in metres for cubic metres, in feet for cubic feet, and 1 cubic foot is 0.02831685 cubic metres. Divide that by what one bag makes and round up, because you cannot buy four fifths of a bag and because the mixer keeps some. The five bag yields offered here are QUIKRETE's own, one per bag size; if you are buying a different product, its data sheet will print the same figure and you can type it in instead. Mortar does not work by volume at all in the published figures: the sheet counts the units one bag lays, so the sum is a count divided by a count.

Source: QUIKRETE Concrete Mix product data sheet, product no. 1101 — "YIELD: A 40 lb (18.1 kg) bag yields approximately 0.30 ft3 (8.5 L); A 50 lb (22.6 kg) bag yields approximately 0.375 ft3 (10.6 L); A 60 lb (27.2 kg) bag yields approximately 0.45 ft3 (12.7 L); An 80 lb (36.2 kg) bag yields approximately 0.60 ft3 (17 L); A 90 lb (40.8 kg) bag yields approximately 0.675 ft3 (19.1 L)"

Questions people ask

Why does this not ask for a mix ratio?

Because there is no single right one to offer, and inventing one would be worse than leaving it out. Ratios such as 1:2:4 or 1:1.5:3 describe cement to sand to aggregate by volume, and which one you want depends on the strength required, the standard in force and the aggregate you have. A bagged mix has already answered all of that in the factory: the sheet quotes 2,500 psi at 7 days and 4,000 psi at 28 days for QUIKRETE Concrete Mix, tested to ASTM C39. If you are batching from raw materials you need a mix design, not a calculator, and the ratio should come from whoever specified the job.

How thick should a slab be?

Thicker than you think, and the product sheet sets a floor: QUIKRETE Concrete Mix "is designed for pouring concrete 2 in (50 mm) thick or more". Below that it is not a slab, it is a screed, and it wants a different product. A four inch slab — 100 mm — is the usual figure for a shed base or a patio, and it doubles the concrete of a two inch one, which is where most underestimates come from. Work the thickness out first and the plan area second.

How much water goes in?

Less than feels right while you are mixing. The sheet gives it per bag: "Final water content should be approximately 6 pt to 9 pt (2.8 L to 4.3 L) of water per 80 lb (36.2 kg) bag of concrete", aiming at a 2 to 3 inch slump. Extra water makes the mix easier to place and weaker once it sets, and it is the single easiest way to turn a 4,000 psi bag into something considerably less. The water is not counted in the yield figure on this page because the yield is already the finished, mixed volume.

Should I add anything for waste?

Yes, and the tool deliberately does not do it for you, because the right allowance depends on the job. A footing dug in soft ground swallows more than the trench you measured; a slab in tight formwork barely wastes any. What matters more is buying the whole job at once: a pour finished the next day has a cold joint through it, which is a plane of weakness you cannot see and cannot fix. Round up, add a bag, and return the unopened one.

Does the mortar figure allow for my joint width?

Only at the thickness the sheet assumes. QUIKRETE describes the bed as "approximately ½ in to ¾ in (13 mm to 19 mm) thick", and the 37 bricks or 13 blocks per bag are for laying at about that. A wider joint, a deeper bed, or filling block cores will get through bags faster, and the sheet does not publish a figure for those, so this page does not pretend to. Note also that "standard brick" is doing quiet work there: a British brick and an American modular brick are not the same size.

Can I use it for a different brand of bag?

Yes — pick "Something else" and type the yield off the bag in litres. Every manufacturer prints one, and they differ, because the bags differ in weight and in aggregate. What you should not do is take the QUIKRETE figure and apply it to a bag of something else with a different weight on it; the whole point of this page is that the yield is a property of the specific product, not a general fact about concrete. The mortar figures are 80 lb only, which is why that side of the tool does not offer bag sizes at all.

What about grout?

Deliberately absent. Tile grout quantity depends on tile size, joint width, joint depth and the density of the particular grout, and the coefficient that ties those together is different for every manufacturer. The figures circulating for it are copied between calculator sites without a data sheet behind any of them, and this repository does not ship a number it cannot point at. If a manufacturer publishes a coverage table in a form worth transcribing, grout gets its own page.

Sources

The documents this page reads its numbers out of, linked so you can check them yourself.

1. QUIKRETE Concrete Mix product data sheet, product no. 1101 — "YIELD: A 40 lb (18.1 kg) bag yields approximately 0.30 ft3 (8.5 L); A 50 lb (22.6 kg) bag yields approximately 0.375 ft3 (10.6 L); A 60 lb (27.2 kg) bag yields approximately 0.45 ft3 (12.7 L); An 80 lb (36.2 kg) bag yields approximately 0.60 ft3 (17 L); A 90 lb (40.8 kg) bag yields approximately 0.675 ft3 (19.1 L)"
2. NIST SP 811 (2008), Appendix B.8 — cubic foot (ft3) to cubic meter (m3) = 2.831 685 E−02
3. QUIKRETE Mortar Mix product data sheet, product no. 1102 — "YIELD: Each 80 lb (36.2 kg) bag of QUIKRETE Mortar Mix will lay up to 37 standard bricks or 13 standard (8 in by 8 in by 16 in [200 mm by 200 mm by 405 mm]) blocks"

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