About, and where the constants come from

Who runs it

One person, no editorial calendar. There is no login, no quote form and nothing to buy.

The site is paid for by display advertising, which is why there are ad slots on the pages that answer a question — and why there are none inside or above an answer. If an ad ever sits between you and a number, that is a defect, and the address below is where to send it.

The five constants, and where each comes from

Every figure on the site comes out of these. Nothing else is used, and no page holds a number that was typed in by hand.

• 0.6 cubic feet from an 80 lb bag. The yield published on the QUIKRETE Concrete Mix data sheet No. 1101, at the water ratio printed on the bag. Mixing it wetter does not stretch it; it makes weaker concrete.
• 0.45 cubic feet from a 60 lb bag. The same source, and it is three quarters of the 80 lb figure, as the weights are.
• 140 lb per cubic foot. The unit weight on the same data sheet, measured to ASTM C138, which puts a cubic yard at 3,780 lb. Delivered ready-mix is commonly heavier than a bagged mix; rather than quote two uncited numbers this site uses the one cited number everywhere.
• 10 cubic yards per mixer-truck load. Stated on the pages as a typical figure, because drums vary and a plant will tell you what it is sending. It is worked from the ordered volume, allowance included, since that is what has to fit on the truck.
• 60 bags of 80 lb as the bags-or-ready-mix line. That is 36 cubic feet, or 1.33 cubic yards. Below it you mix in an afternoon and pay only for what you open; above it you are buying pallets, lifting two tons, and racing the set on the batch you already placed.

Bag counts round up

A store will not sell you nine tenths of a bag, so a calculated 222.2 bags is stated as 223 bags. This is the one place the site deliberately disagrees with a plain division, and it is applied on every page, in every table, in the same direction.

Cubic yards and cubic feet are not rounded up — they are stated to two and one decimal places. Plants sell by the quarter yard and do their own rounding, and this site does not do it for them.

Where the 10% waste allowance appears, and where it does not

Two rules, on purpose, because the two things are bought differently.

• Ready-mix carries it. The ordered figure is the volume plus ten percent. A truck will not come back for a quarter yard, so the allowance has to be inside the order, and the truck count is worked from that ordered figure rather than from the bare volume.
• Bag counts do not. The headline count is the volume, because the question people ask is how much concrete a slab holds, not what to put in the trolley. The plus figure sits next to it on every page that states bags, so you can buy to whichever number you want.

What the numbers exclude

Everything except the concrete itself. In particular:

• Reinforcement. Mesh, bar, chairs and ties are not in any figure here, and the site never tells you whether you need them. That is a code question and a loading question.
• The base. Gravel is a separate order in separate units, and counting it as concrete is the error the whole site is designed around.
• Forms, labour, pumping and finishing. The price band is concrete delivered: your quoted price per yard on the ordered volume, taken fifteen percent either side of the middle. Delivery and any short-load fee are the plant's and are not in it.
• Local code. Thickness, reinforcement, frost depth and setbacks are set where you live, not here. The calculator will happily compute a two-inch driveway.
• Tax, and your plant's minimums. Both are real and neither is knowable from a page.

When a number here is wrong

Tell me and I will fix it, and say on the page that it changed. contact@concreteslabcalc.com is where it goes. The constants above are the only things that can be wrong in more than one place at once, so they are the ones worth arguing about.

This is an estimating tool, not an engineering document. Nothing on it is a substitute for a structural design or an inspection.