Sources and constants
Where our numbers come from
A calculator is only trustworthy if you can see what is inside it. This is every figure the site uses, split into the ones that cannot change and the ones you should check against your own materials.
Units, and where they come from
Everything here is US customary. Volumes come back in cubic feet and cubic yards, areas in square feet, and weights in tons — short tons of 2,000 pounds, not metric tonnes, which are about ten percent heavier. Where the calculators print a tonnage they give the metric equivalent alongside it.
Bag sizes are US retail standards, set out below. If you are shopping somewhere that sells bark by the 70-liter bag and aggregate by the 25-kilo sack, the volumes and the areas still hold, but the bag counts will not: work from the cubic meters instead, which is one cubic yard to 0.765.
You can enter measurements in meters and centimeters as well as feet and inches — the unit selectors next to each field. Prices are whatever currency you type; the calculators only ever compare two of your own numbers, so they never need to know which one it is.
Fixed: geometry, which cannot go out of date
These are derived, not sourced, and they are exact.
- Roof slope factor — √(rise² + 144) ÷ 12 for a rise over a 12 inch run. A 6-in-12 pitch gives 1.118033989.
- Framing fraction — one member width per spacing. A 1.5 inch stud at 16 inches on center is 9.375 percent of the wall.
- Board coverage — board width plus gap, converted to feet.
- Posts per run — sections plus one, less shared posts where runs meet.
- Post hole volume — π r² × depth, less the square section of the post over the same depth.
- A roofing square — 100 square feet, by definition. Siding is sold by the same unit.
- Siding courses — wall height divided by the exposure; lineal feet is wall area divided by the exposure.
- Drywall sheets — area divided by sheet area. Screws are the framing members a sheet crosses multiplied by the rows of screws down each of them. Joint tape is sheet perimeter halved, because interior joints are shared between two sheets.
- Studs and joists — the run divided by the spacing, rounded up, plus one, because there is a member at each end whatever the length: a 15 ft wall at 16 inches on center is 11.25 spaces and 13 studs. Then two more per corner, and four per opening for the king and jack studs.
- Deck fasteners — two at every point a board row crosses a joist.
- Stair risers — total rise divided by the maximum riser, rounded up; treads are one fewer than risers; the stringer is the hypotenuse of the rise and the run. The comfort check is the carpenter's rule of thumb that twice the riser plus the tread lands between 24 and 25 inches.
- Paint openings — 21 square feet a door and 15 a window, which are standard averages rather than your doors.
- Drip edge — eaves plus rakes, in 10 ft sticks.
Densities are loose and delivered-damp
Material weight is quoted per cubic foot in the condition the material is actually in when it reaches you: loose, not compacted, and holding some moisture. That is the honest state for estimating a delivery, because that is what the yard weighs on the truck.
It also means our figures sit slightly above textbook dry densities. Crushed stone and sand are taken at 100 lb per cubic foot, screened topsoil at 76, compost at 45, and wood mulch at 22. Set concrete is taken at 150.
Densities used
| Material | lb per cu ft | lb per cu yd |
|---|---|---|
| Wood mulch | 22 | 594 |
| Compost | 45 | 1,215 |
| Screened topsoil | 76 | 2,052 |
| Sand | 100 | 2,700 |
| Crushed stone and gravel | 100 | 2,700 |
| River rock | 100 | 2,700 |
| Decomposed granite | 100 | 2,700 |
| Concrete, set | 150 | 4,050 |
Bag sizes are US retail standards
Bag volumes come from the sizes carried by US big-box stores and landscape yards: 2 and 3 cubic foot mulch bags, 0.5 cubic foot (50 lb) stone and sand bags, 0.75 and 1.5 cubic foot soil bags, and 1 and 1.5 cubic foot compost bags.
Concrete bag yields follow the manufacturers' stated figures: 0.60 cubic feet for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb.
These do vary between brands and regions. The coverage figure printed on the bag in front of you always beats ours.
Defaults you can change
These are fields on the calculators, pre-filled with the common value. They vary by manufacturer and product, so the figure on your wrapper beats the figure we chose. That is the entire reason they are inputs.
- Bundles per square — 3 for most shingles, 4 for heavy laminated.
- Underlayment roll coverage — varies widely between synthetic and felt.
- Nails per square — 4 per shingle standard, 6 in high-wind zones.
- Insulation coverage per bag — no default at all, because none would be right. Read the wrapper.
- Board and picket widths — actual dimensions of common lumber: a 2x6 is 5.5 in, a 2x4 is 1.5 by 3.5 in, a 2x8 is 7.25 in.
- Drywall sheet sizes — 4×8, 4×10 and 4×12 ft, which are standard. Joint compound coverage has no default at all, because none would be right.
- Siding exposure — set by the product and how it is lapped, so it is always yours to enter.
- Maximum riser and minimum tread — the presets are common values, not permissions. See below.
What the extra allowance is for
Every calculator adds a percentage on top of the exact volume, defaulting to 10 percent. It is covering three separate things: material lost in handling, settlement after watering and compaction, and the fact that ground is never as flat as the measurement assumes.
You can turn it off. We would not, except on a job where running over is worse than running short.
What goes under a paver patio
The paver calculator quotes two things you do not see once the job is finished. The base is measured over the paver area plus a 6-inch collar on every side, because the perimeter needs something to sit on, and it is quoted loose rather than in place: crushed stone loses about a fifth of its volume under a plate compactor, so four inches finished takes five inches delivered. On a wide patio the collar is a few percent; on a narrow walkway it is closer to forty, which is why it is calculated from your dimensions rather than added as a flat percentage.
The bedding sand gets neither allowance. It is screeded to one inch and never compacted before the pavers go down, so the figure is the plain volume.
What we deliberately do not publish
Required R-values by climate zone. They are set by whichever energy code your jurisdiction has adopted, which is not always the current one, and they change on a code cycle. A table here would go stale quietly and someone would insulate to a withdrawn number. Ask your building department.
Frost depths, span tables, footing sizes and load ratings. All local, all structural, all consequential. We count materials; we do not size them. That covers header sizing over an opening and how tall a stud wall may go at a given spacing.
Stair code limits. Maximum riser height, minimum tread depth, headroom, handrail height and guard spacing differ by jurisdiction and between residential and commercial work. The stair calculator takes the riser maximum and the tread depth as inputs for exactly this reason: the presets are common values, and they are not a substitute for the figure that applies where you are building.
Prices. Every price field on this site is one you type in. Nothing is fetched, nothing is stored, and no figure we published would survive contact with your local yard.
The sod calculator gives you square feet, pieces and pallets, but no weight. A pallet's weight depends on how thick the grower cuts the soil and how wet it is on the day, and the figures in circulation disagree by more than a factor of two — 1,500 to 3,000 pounds is the range growers themselves quote. A number that low by a factor of two is worse than no number, because it is the figure someone uses to decide whether to hitch a trailer. Ask your grower, and plan on the heavy end if you cannot.
Where these numbers will be wrong
Moisture is the biggest variable. Soaked mulch can weigh half again what dry mulch does, and wet sand is heavier still. Weight figures should be treated as approximate; volume figures are far more reliable.
Compaction is the second. Decomposed granite and crushed stone lose roughly a fifth of their loose volume when properly compacted, so on those jobs the ten percent default is not enough. Raise the allowance, or enter the loose depth you will spread rather than the compacted depth you want to end up with.
And nothing here is structural advice. Slab thickness, footing dimensions and reinforcement are governed by local building code, and a calculator cannot know your frost line or your soil.
How the arithmetic is checked
The calculator engine has a test suite that works every engine against hand-calculated examples: a 20 by 10 ft bed of mulch at 3 inches, a 10 by 10 slab at 4 inches, a 12 inch post hole, a 12 by 14 room in two coats, 200 sq ft of tile at 12 by 24, a paver base with its collar, a 40 by 20 gable at 6-in-12, a 16 by 12 deck in 5.5 inch boards, 800 sq ft of drywall, a 100 ft fence at 8 ft centers, and a stair flight checked to confirm the riser count rounds up rather than down. It also works the geometry collectors themselves — the shape formulas, the unit conversion, and the rule that a half-filled measurement card contributes nothing at all. A hundred and eighty-two checks, run on every change to the engine, against the same configuration the pages ship with — so a density or bag size edited without updating this page fails the build's tests.
Rounding is always up, and always at the end. Partial bags, bundles, boards and sheets do not exist at a trade counter.