I checked every figure in my own calculators. Three were wrong.
Every material calculator on the internet gives you a number. Almost none of them tell you where the number came from.
That is not a small thing. A bag yield, a coverage rate, a waste allowance — these are the entire calculation. The arithmetic around them is trivial; a middle schooler can multiply area by depth. What you are actually trusting when you use one of these tools is a handful of constants that somebody typed in once, and that everyone else has been copying ever since.
So after building ten of them, I went back and tried to find a primary source for every figure I had used. Not another calculator. Not a blog post. The manufacturer’s data sheet, or a university extension service, or a published industry standard.
Three of my numbers were wrong.
What held up
Most of it was fine, which is the boring and reassuring part.
Concrete bag yields. QUIKRETE publishes a data sheet for their standard Concrete Mix (no. 1101) that states the yield for every bag size: 0.30 cubic feet for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb, 0.60 for 80 lb. All four matched what I had. The sheet also lists a 90 lb bag at 0.675 cu ft, which I had not included and have since added.
Mulch coverage. Two cubic feet is the shelf standard, a cubic yard is 27 cubic feet, so 13.5 bags to the yard. A 2 cu ft bag covers 8 square feet at three inches deep. Confirmed against retail listings.
Mulch depth. Penn State Extension and the University of Maryland Extension both put landscape mulch at two to three inches, four at the absolute maximum, and both are emphatic about not piling it against trunks. That matched.
Paint coverage. Behr publishes 250 to 400 square feet per gallon per coat; Glidden says up to 400 on a primed, smooth, non-porous surface. My four surface categories all sat inside that band.
Roofing. A square is 100 square feet, and most shingle lines are packaged three bundles to the square. Straightforward.
What was wrong
The door was 21 square feet, and it should have been 20
When you paint a room you deduct the doors and windows from the wall area. The standard figures used by the manufacturers’ own calculators are 20 square feet per door and 15 per window.
I had written 21 for the door, with a note explaining that a standard US interior door is 3 ft by 6 ft 8 in — which is exactly 20 square feet — “rounded up slightly for the casing.”
That reasoning is backwards. Deducting more area means calculating less paint. If I was going to be imprecise, the error should have gone the other way. The extra square foot was a small thing on one door and a real thing in a hallway with six of them, and it was pointing in the direction that leaves you short.
Fixed: 20 square feet, which is both the industry figure and the actual geometry.
Primer does not cover as far as paint
My FAQ said that primer covers roughly the same area per gallon as finish paint, so you could use the same wall area and count it as one extra coat.
Manufacturer guidance puts primer at 200 to 300 square feet per gallon — meaningfully less than the 350 or so you get from a finish coat. That makes sense once you say it out loud: primer’s job is to soak into and seal a porous surface. Going into the surface is the entire point.
Anyone following my original advice would have bought about a third too little primer.
A four-inch driveway is fine for cars
My slab thickness table said patios and walkways at four inches, and driveways at five or six.
The actual industry position is that residential driveways run four to six inches depending on what parks on them. Four is sufficient for passenger cars. You go up to five or six when trucks, an RV or a trailer will be on it, or where the subgrade is poor.
This one is different in kind from the other two. Nobody’s project fails because they poured a driveway thicker than it needed to be — it fails their budget. A 600 square foot driveway at six inches instead of four is about four extra cubic yards of concrete, which is several hundred dollars spent on nothing. Being conservative is not free, and presenting a conservative figure as the standard is its own kind of wrong.
The one I refused to copy
Roof calculators need a slope factor: the number you multiply a building’s footprint by to get the actual sloped roof area.
The geometry is not in dispute. A pitch of rise inches per 12 inches of run makes a right triangle with legs of 12 and the rise, so the factor is the hypotenuse over the run:
slope factor = √(rise² + 12²) ÷ 12
For a 12:12 roof — 45 degrees — that gives exactly the square root of two, about 1.414.
At least one manufacturer’s guide gives 1.7 for a 12:12 roof. That is not a rounding difference; it is 20 percent more material. The most likely explanation is that some published contractor tables fold a waste allowance into the slope factor, so the “factor” is really doing two jobs at once. That is a defensible thing for a roofer’s cheat sheet to do, and an indefensible thing to copy into a calculator that also has a separate waste field, because then you are applying waste twice.
So the site derives the factor from the pitch and applies waste separately, and says so on the page.
What this means if you use these tools
You will not check every number in every calculator you use, and you should not have to. But two habits are cheap:
Sanity-check one figure against the product. Before you buy 70 bags of anything, read the yield or coverage off the bag and see whether it matches what the calculator assumed. A 0.05 cubic foot difference in bag yield sounds like nothing and is half a yard of concrete across a large pour.
Prefer a calculator that shows its arithmetic. Not because you will redo it, but because a tool that shows the formula is one where an error is visible. A tool that shows only an answer is a tool where a wrong constant can sit undetected for years, being copied by the next person who builds one.
How often does this need redoing
Some of these figures are permanent. There will always be 27 cubic feet in a cubic yard, a roofing square will always be 100 square feet, and the slope factor is geometry.
The rest can move. Manufacturers reformulate products and revise coverage rates. Bag sizes on the shelf change — garden soil moving from 2 cubic feet to 1.5 is exactly the kind of quiet shift that makes a bag count wrong. Extension services revise their recommendations, though on a timescale of decades rather than seasons.
The plan here is an annual review against the same primary sources, plus updating whatever anyone reports in between. Each calculator page carries the date its figures were last checked, so you can see how stale the answer you are looking at might be.
If you find one that does not match the product in front of you, that is the most useful message this site can receive. The label wins.