Buy for the surfaces—not the room name.

Manufacturers print 350 to 400 square feet per gallon, and that number assumes one coat on a smooth, primed, non-porous surface with a roller. Real conditions cut into it: bare drywall drinks the first coat, textured or knockdown walls add surface area you cannot see in the tape measure, and spraying loses material to overspray. If the wall has never been painted, plan on the low end or prime first. The calculator uses 350 as a conservative default; if your can says otherwise, put its number in.

Subtract them if you are not painting them. A standard interior door is roughly 21 square feet of opening and a double-hung window about 12, so a bedroom with one door and two windows loses around 45 square feet — enough to change the can count on a small room. What you should not subtract is trim, outlet plates or a fireplace surround: those are small, and the offcuts and touch-ups absorb them.

Because that is how tile is sold. A box covers a fixed area — commonly around 15.5 square feet — and the store will not break one open for you. The rounding is not waste, it is your spare stock. Keep the surplus sealed and labeled: tile patterns get discontinued within a couple of years, and a single cracked tile with no match means recutting a whole field.

Ten percent is the baseline for a straight grid layout in a rectangular room. Go to 15% if the room has jogs, a diagonal layout, or tile larger than 15 inches. Go to 20% for herringbone or any pattern where every perimeter piece is a compound cut. Rooms with lots of small obstructions — a bathroom with a toilet flange, a vanity and a curbed shower — sit at the high end even with a straight layout.

For standard-size tile with a 1/4 by 3/8 inch notched trowel, budget about 1 pound of thin-set per square foot, which puts a 50 lb bag at roughly 50 square feet. Large-format tile needs a medium-bed mortar and a bigger trowel, which pushes consumption up by about half. Grout is much lighter: roughly 0.08 pounds per square foot at a typical joint width, so a 10 lb bag covers a small bathroom floor. Wider joints and thicker tile both increase grout consumption.

Volume first, then weight. Wall area in square feet times thickness in inches, divided by 12, gives cubic feet. A typical render mix carries about 18.7 pounds of Portland cement per cubic foot of mortar, so a 94 lb sack covers roughly 5 cubic feet of mortar, and sand runs about 0.9 cubic feet per cubic foot of finished mortar. If you are using a pre-blended stucco or plaster in bags, skip the mix math entirely and use the coverage table on the bag — it is per bag at a stated thickness.

No, and this is the most common error in a roofing estimate. A pitched roof is longer than the footprint under it. A 4:12 pitch adds about 5% to the area, 6:12 adds about 12%, and 9:12 adds about 25%. If you measure the plan and buy for the plan on a steep roof, you will be short by several rolls. Measure the actual slope length, or multiply the footprint by the pitch factor before you use this calculator.

One hundred square feet of finished roof surface. It is the unit contractors quote in, and roll products are sized around it: a standard roll of roll roofing covers about one square net after the overlap. Two thousand square feet of roof is 20 squares. Note that the net coverage is what matters — a roll may contain more material than it covers, because the overlap between courses is consumed twice.

Follow the manufacturer, but the common figures are 3 to 4 inches on the side laps and 6 inches on the end laps, which works out to roughly the 10% allowance this calculator uses by default. Overlaps are not the place to economize: nearly every low-slope roof failure starts at a seam. Add extra material for turn-ups at parapets, curbs and penetrations, which are not in the flat area at all.

Take the room perimeter, subtract about 3 feet for each door opening, add 8% for miters and coped corners, and divide by the stick length. The catch is that the arithmetic ignores where the joints land. A 14 ft wall needs one 16 ft stick, not two 8 ft ones — a scarf joint in the middle of a long wall is visible forever, no matter how well you cut it. Lay the walls out on paper before you commit to a stick length.

For anything with a color, a lot number or a discontinuation risk — tile, custom-tinted paint, patterned trim — yes. The marginal cost of one extra box is small; the cost of matching it three years later is a whole room. For commodity items with no shade variation, such as Portland cement or sand, buy what you need and pick up more if you run short, since bagged cement goes off in storage once it takes on humidity.

Yes, the underlying math is unit-agnostic — area divided by coverage. The conversions worth remembering: 1 US gallon is 3.785 liters, 1 square meter is 10.764 square feet, and a coverage of 10 m² per liter is the same as 407 square feet per gallon. If you are reading a European or Latin American datasheet that quotes m² per liter, multiply by 40.7 to get square feet per US gallon.