Tell me the shape and dimensions of your pour and I’ll work out the volume in cubic yards and how many bags of concrete that is, with a 10% waste allowance built in.
Concrete calculator
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Estimate for planning, with a 10% waste allowance. Not an engineering spec. For structural footings or columns, follow your permit/engineer’s mix and dimensions.
How I calculate this
Everything starts as a volume in cubic feet, then converts to bags or cubic yards.
Volume is length x width x thickness, converted to cubic feet then bags.
For a slab, that’s length times width times thickness, with thickness converted from inches to feet (divide by 12). For a wall footing, it’s length times width times depth, with the width and depth converted from inches. For a round column, it’s the area of the circle (pi times radius squared) times the height, times how many columns you’re pouring.
Then I add 10% for waste, spillage, and uneven sub-grade, because you do not want to be one bag short with a half-poured slab going off in the sun. To turn that into bags I divide by what each bag yields: an 80 lb bag makes about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. Cubic yards is just cubic feet divided by 27. I round bags up, always.
A worked example
Say you’re pouring a 10 by 10 foot slab, 4 inches thick. Volume is 10 x 10 x (4/12) = 33.3 cubic feet. Add 10% and you’re at 36.7 cubic feet, which is 1.36 cubic yards. Divide by 0.60 and you need 62 eighty-pound bags. The calculator shows that, plus what it would be in 60 and 40 lb bags.
A few things I’d tell a friend
Here’s the honest one: 62 bags is a brutal amount to mix by hand. Once your pour crosses about one cubic yard, bagged concrete stops making sense, both for your back and your wallet. Call a ready-mix supplier and have a truck deliver it. The calculator pops up a note when you cross that line, and I wrote a separate piece on concrete bags vs ready-mix to help you decide. Under a yard, bags are fine. Also: buy a couple extra bags beyond the estimate. Returning two dry, unopened bags is easy; stopping a pour to drive to the store is not.
FAQ
How many 80 lb bags make a cubic yard? About 45, since a cubic yard is 27 cubic feet and each 80 lb bag yields roughly 0.60. A 60 lb bag yields ~0.45 (about 60 per yard) and a 40 lb bag ~0.30 (about 90 per yard).
How thick should a slab be? Four inches is standard for patios, walkways, and shed floors. Driveways and anything bearing vehicle weight usually want 5 to 6 inches. Check local code for anything structural.
When should I order ready-mix instead of bags? Roughly once you pass one cubic yard. Below that, bags are practical; above it, a truck is cheaper and far less labor.
Does this include rebar or gravel base? No. This calculates the concrete only. Plan your gravel sub-base separately with the gravel calculator.
Built and maintained by Sam Walsh. Last updated June 2026. This is a planning estimate, not an engineering specification.
Tell me your room’s dimensions and I’ll tell you how many gallons of paint to buy, including a waste allowance so you don’t run short two-thirds of the way up a wall.
Paint calculator
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Estimate for planning. Coverage varies by paint, surface, and color change. Buy a little extra rather than risk a second trip.
How I calculate this
The math is simple, but the assumptions are where most calculators quietly differ, so here are mine in the open.
First I find your wall area: perimeter times height, or 2 x (length + width) x ceiling height. Then I subtract the openings you don’t paint. I use 21 square feet for a standard door (about 3 by 7 feet) and 15 square feet for a standard window (about 3 by 5 feet). If you tick “also painting the ceiling,” I add length x width on top.
I multiply the paintable area by your number of coats, then divide by paint coverage. I use 350 square feet per gallon per coat. Cans often claim 350 to 400, but coverage drops on porous, textured, or previously unpainted surfaces, and on a big color change, so I use the low end on purpose. Then I add 10 percent for waste, roller loss, and touch-ups, and round up to whole gallons because you can’t buy 3.1 gallons.
A worked example
Say your room is 14 by 16 feet with 9-foot ceilings, one door and two windows, two coats, ceiling not included. Walls come to 2 x (14 + 16) x 9 = 540 square feet. Subtract one door and two windows (21 + 30 = 51) and you’re left with 489 square feet of actual wall. Two coats makes 978 square feet. Divide by 350 and you get 2.79 gallons; add 10 percent and it’s 3.07. Round up and you need 4 gallons. That’s what the calculator shows, and you can check every step against your own numbers.
A few things I’d tell a friend
If this is your first time painting a room, I think the standard 10 percent waste is a little tight. Bump your own estimate to 15 percent if you’re new to a roller, because the learning tax is real. Buy your gallons in one batch with the same mix code so the color matches, and keep the lid label in case you need more. If you’re going from a dark wall to a light one, plan on the extra coat rather than hoping two will cover it; it usually won’t.
FAQ
How much does a gallon of paint cover? Around 350 to 400 square feet per coat on smooth, previously painted walls. I use 350 in the calculator because real surfaces rarely hit the top number.
Do I count doors and windows? No, subtract them, which the calculator does for you using standard door and window sizes. If yours are unusually large, knock the count down to match the area you’re actually leaving unpainted.
One coat or two? Two, almost always, unless you’re repainting the same color over a clean, sound surface. Color changes and bare drywall need two.
Does this include primer? No. Treat primer as its own coat with similar coverage. If you’re priming bare drywall, run the calculator again with coats set to 1 for the primer.
Built and maintained by Sam Walsh. Last updated June 2026. This is a planning estimate, not a guarantee of exact coverage.
Enter your room dimensions and tile size and I’ll tell you how many tiles to buy, including enough extra for cuts and breakage so you don’t come up short mid-job.
Tile calculator
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Estimate for planning. Always order from the same dye lot. For irregular rooms or many obstacles, add an extra 5% on top of the pattern waste.
How I calculate this
The formula is straightforward: divide your room’s square footage by the area of one tile, then add a waste allowance. The waste part is where I see people get burned, so I want to be clear about the numbers I use.
First, I find your room area in square feet (length x width). If you enter the dimensions separately, I multiply them; if you already know the square footage, I use it directly.
Next, I figure out the area of one tile in square feet. A 12×12-inch tile is exactly 1 sq ft. An 18×18-inch tile is 2.25 sq ft (18 x 18 = 324 square inches, divided by 144 = 2.25 sq ft). A 12×24-inch tile is 2 sq ft. A 24×24-inch tile is 4 sq ft. If you enter a custom size, I do the same conversion: multiply the two dimensions in inches and divide by 144.
I divide the room area by the tile area to get the raw tile count, then add waste:
Straight lay (tiles set in a standard grid): 10% waste. This is the industry standard for simple rectangular rooms with a straightforward grid pattern.
Diagonal or herringbone lay: 15% waste. Angled cuts at every wall produce a lot of unusable triangular pieces. For complex patterns (chevron, mosaic with lots of cuts), I’d push this to 20%, which you can do by selecting “Complex pattern” from the pattern dropdown.
After adding waste I round up to whole tiles, because you can’t buy a fraction of a tile. If you enter a box coverage (how many square feet your chosen tile’s box covers), I divide the total into boxes and round up there too.
Grout joint width affects coverage slightly, but for planning purposes a 1/8-inch joint on a 12×12 tile adds less than 2% to area, so I don’t inflate the formula for it. The waste factor already covers the real-world shortfall.
Say you’re tiling a kitchen floor that’s 10 feet by 12 feet, using 12×24-inch tiles in a straight lay.
Room area: 10 x 12 = 120 sq ft. One 12×24 tile: 12 x 24 = 288 square inches, divided by 144 = 2 sq ft per tile. Raw tile count: 120 / 2 = 60 tiles. Add 10% waste: 60 x 1.10 = 66 tiles. Round up: 66 tiles.
If your tile comes in boxes that cover 16 sq ft each, you need 120 sq ft (with waste, call it 132 sq ft) divided by 16 = 8.25 boxes. Round up to 9 boxes.
That 9th box is not overkill. I’ve done enough tile jobs to know that one box of the same dye lot sitting in the garage is worth every dollar, either for repairs down the road or because the last row didn’t go as cleanly as expected.
Recommended order quantities per 100 sq ft of room area, including 10% waste. These are tiles to buy, not raw tile counts.
Tips before you buy
The dye lot matters more than most people realize. Tiles from different production runs can vary slightly in shade, and you won’t notice until they’re side by side on the floor under bright light. Order everything you need from one lot. The lot number is printed on the box. If you have to order more later and the lot is gone, you may be stuck.
For diagonal lay, add 15% minimum and don’t argue with it. Diagonal cuts at both ends of every row eat material fast. On a small bathroom floor I once thought 10% would cover it at 45 degrees. It didn’t. I made two hardware store runs.
If the room has an L-shape, alcoves, or a lot of obstacles (toilet flanges, door casings, a freestanding tub), I’d add another 5% on top of whatever the pattern calls for. The calculator gives you a clean-room number; irregular rooms burn more.
Measure twice. Tile is not forgiving of math errors, and the return policy on cut tile is typically zero. If you’re unsure how to get an accurate room measurement, see how to measure a room before you fill in the calculator.
Tile size also affects perceived room size. Large-format tiles (24×24 or bigger) make a small room feel more open because fewer grout lines interrupt your eye. In a bathroom under 50 sq ft, a 12×12 can look busier than you expect. This doesn’t change the math, but it’s worth knowing before you commit.
How much extra tile should I buy? For a straight grid layout in a square or rectangular room, 10% overage is standard. For diagonal, herringbone, or any 45-degree pattern, use 15%. For irregular rooms or complex patterns like chevron, go to 20%. Always round up to full boxes, not individual tiles.
Does grout joint width change how many tiles I need? Slightly, but not enough to change your tile count in most cases. A 1/8-inch grout joint on 12×12 tiles adds roughly 1-2% to your effective coverage area, which the waste factor already absorbs. If you’re using wide grout joints (3/8 inch or more) with very small tiles, it’s worth noting, but for standard 12×12 and up the formula here is accurate for planning.
What are the most common tile sizes? The most popular floor tile sizes are 12×12, 12×24, 18×18, and 24×24 inches. Large-format tiles (24×24 and bigger) are increasingly common for open-plan floors. Smaller tiles (4×4, 6×6) show up on bathroom walls and backsplashes. The calculator handles any size you enter.
What if I don’t know the box coverage? Leave the box field blank and the calculator gives you the total tile count. Then check the box at the store: it will state the coverage in square feet per box. You can do the division in your head: total tiles divided by tiles per box, rounded up.
Can I use this for wall tile too? Yes. The math is identical: room area (wall height x width) divided by tile area, plus waste. For a shower surround with many obstacles (valve, niche, shampoo shelf), I’d use 20% rather than 10%, because the cuts are relentless.
Built and maintained by Sam Walsh. Last updated July 2026. Tile counts are planning estimates based on standard waste factors; actual usage depends on your room shape, layout, and installer technique.
