To order the right amount, you do it in two steps: measure the space in the shape it actually is, then convert that measurement into the unit the material is sold in, and add a waste factor. Concrete sells by the cubic yard, drywall by the sheet, paint by the gallon, gravel by the ton. Each has its own coverage rate and its own typical waste. Get the conversion right and you make one trip and one order. Get it wrong and you either pay for material you did not need or, worse on a pour, come up short with a truck already leaving. Here is how to convert a measurement into an order for the materials you use most.
The two-step every order comes down to
Measure first, then convert. Measuring in feet and inches is the easy part. The mistakes happen in the convert step, because every material is sold in a different unit and most have a coverage rate that is a rule of thumb, not a law of physics. So the habit is: figure the area or volume of the real space, look up what one unit of the material covers or yields, divide, then add waste and round up. Never order straight off the raw measurement.
Concrete by the cubic yard
Concrete is sold by the cubic yard, and there are 27 cubic feet in a yard. The formula is length times width times thickness, all in feet, divided by 27:
Cubic yards = (length ft x width ft x thickness ft) / 27
The thickness is where people get it wrong. A 4-inch slab is 0.33 feet (4 divided by 12), not 4. So a 20 ft by 24 ft slab at 4 inches is 20 x 24 x 0.33 / 27 = about 5.9 cubic yards. Add about 10% for spillage, uneven subgrade, and the truck's leftover, and round up, because a short pour is a cold joint and a ruined day.
For small work by the bag, an 80-pound bag yields about 0.6 cubic foot and a 60-pound bag about 0.45 cubic foot, so it takes roughly 45 80-pound bags or 60 60-pound bags to make a single yard. Bags win for post holes and patches. Past about half a yard, ready-mix is cheaper and far less work. Confirm the yield printed on the bag, because it changes by product.
Rebar and wire mesh for that slab
Reinforcement is a grid, so you count it as linear feet. For a bar grid, figure the number of bars running each direction (slab dimension divided by your spacing, plus one), multiply each count by the length it runs, and add the two totals. Then add for overlaps, because bars are spliced with a lap where they meet, and that lap eats length. Welded wire mesh is sold by the sheet or roll, and you add for the overlap between sheets too, so order more coverage than the flat slab area.
One hard line: the size and spacing of reinforcement is a structural call, set by the plans, the code, or an engineer. This guide tells you how to count the steel once someone has told you the grid. It does not tell you what grid to use. Do not eyeball rebar in a structural slab or footing. Get the spec from the drawings or an engineer, then count it here.
Gravel base and aggregate by the ton
Base rock, road base, and sand are usually sold by the ton, but you measure the space by volume, so you need to convert cubic yards to tons. Most compacted aggregate runs roughly 1.3 to 1.5 tons per cubic yard, but it depends on the material and its moisture, so this is the one to confirm with your supplier before you order. Figure your volume the same way as concrete (length x width x depth in feet, divided by 27 for cubic yards), then multiply by the supplier's tons-per-yard figure. Order a little heavy, because you compact base and compaction eats volume.
Drywall, mud, tape, and screws
Drywall is sold by the sheet, and a standard 4 ft by 8 ft sheet covers 32 square feet. Add up all the wall and ceiling area you are covering, divide by 32, and add 10 to 15% for cuts and waste. Bigger 4x12 sheets cover 48 square feet and cut down on seams if you can handle them.
For the finish, joint compound and tape are rules of thumb: a common figure is roughly a gallon of ready-mix mud per 100 to 160 square feet of board over a full three-coat job, and about a same-length roll of tape per sheet or two, but coverage swings with how many coats and how heavy a hand you have, so confirm on the bucket. Drywall screws run on the order of a pound per several sheets depending on framing spacing. When in doubt on the finish materials, buy round and keep the receipt.
Thinset, mortar, and grout
Bag coverage for thinset, mortar, and grout is not a fixed number, because it depends on the trowel notch, tile size, and joint width. A deeper notch and a bigger tile burn material faster. That is why the coverage is printed right on the bag for each trowel size, and that printed figure is the one to use. Measure your area, read the coverage for your trowel notch off the bag, divide, and add waste for the tile you break and the buttering you do. Do not carry a single mental number for "a bag of thinset," because it is wrong as often as it is right.
Paint by the gallon
A gallon of paint covers roughly 350 to 400 square feet in one coat on a smooth, sealed surface. Most jobs are two coats, so budget close to double, and budget more on bare, porous, or rough surfaces, which drink the first coat. Figure your wall area (perimeter times height), subtract big openings if you want to be tight, and divide by the coverage for the number of coats. Primer covers on the same order. The exact spread rate is on the can and varies by product and sheen, so confirm it there.
The waste factor, material by material
Waste is not padding, it is arithmetic. Order to the calculated quantity and you will come up short every time, because real material gets cut, broken, and wasted. The rough factors:
- Simple, square layouts: add about 10%.
- Diagonal, patterned, or cut-heavy work (diagonal tile, herringbone, complex rooms): add 15% or more.
- Brick, block, and pavers: add for breakage and for the cull pile.
- Concrete: add about 10% for spillage and uneven subgrade, then round up, because you cannot buy a half-truck of it mid-pour.
Match the waste to the job. A plain rectangle is 10%. A cut-up bathroom in a diagonal pattern is not.
Round up, and know what you cannot buy more of mid-job
The last move is always the same: round up, and round up harder on anything you cannot easily get more of once you have started. Coming up a few sheets short of drywall is a quick trip to the yard. Coming up short on a concrete pour, or on a dye lot of tile that is now out of stock, is a real problem. So the rule scales with the pain of running out. The on-site Calculators do these conversions for you, but knowing the math means you can sanity-check the number before the truck rolls.
Common questions
How much concrete do I need for a slab?
Multiply length by width by thickness, all in feet, then divide by 27 to get cubic yards, and add about 10% for waste. The trap is the thickness: a 4-inch slab goes in as 0.33 feet, not 4. So a 10 ft by 12 ft slab at 4 inches is 10 x 12 x 0.33 / 27 = about 1.5 cubic yards before waste. Round up, because a short pour cannot be fixed once the truck leaves. Use the on-site Calculator to check yourself.
How many bags of concrete make a cubic yard?
About 45 80-pound bags, or roughly 60 60-pound bags, make one cubic yard. An 80-pound bag yields about 0.6 cubic foot and a 60-pound bag about 0.45 cubic foot, and a yard is 27 cubic feet, so you divide 27 by the bag yield. Bags are worth it for small patches and post holes. Once you pass about half a yard, ordering ready-mix is usually cheaper and far less work. Confirm the yield printed on the bag.
How do I convert cubic yards of gravel to tons?
Multiply the cubic yards by the material's tons-per-yard figure, which for most compacted aggregate is roughly 1.3 to 1.5 tons per cubic yard. Measure the volume first (length x width x depth in feet, divided by 27), then multiply by that factor. The exact density depends on the material and moisture, so get the tons-per-yard number from your supplier before ordering, and order a little heavy because compaction eats volume.
How much paint do I need per square foot?
Budget one gallon per 350 to 400 square feet per coat, and most jobs are two coats. Figure your wall area as perimeter times ceiling height, then divide by the coverage and multiply by the number of coats. Bare, porous, or rough surfaces drink the first coat, so add for them. The exact spread rate is on the can and varies by product and sheen, so confirm it there.
How many drywall sheets do I need?
Add up all the wall and ceiling area you are covering, divide by 32 square feet for standard 4-by-8 sheets, and add 10 to 15% for cuts and waste. Larger 4-by-12 sheets cover 48 square feet and reduce the number of seams if you can handle them. Then budget joint compound, tape, and screws off the sheet count, confirming the finish-material coverage on the bucket, since it swings with the number of coats.
The honest bit
- The geometry here is fixed, but the coverage rates are not. Bag yields, thinset and mortar spread, aggregate density, and paint coverage all vary by product, so the number printed on the bag or given by your supplier beats any rule of thumb in this guide. Confirm before you order.
- Rebar and mesh SIZE and SPACING are structural decisions set by the plans, the code, or an engineer. This guide only tells you how to count steel once the grid is specified. Do not size reinforcement from a rule of thumb.
- These are estimating rules of thumb, not engineering. Structural quantities, loads, and spans come from the code tables or an engineer.
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