Every BuildCalc calculator turns your measurements into a number you can shop with: gallons, bags, sheets, rolls, bundles or cubic yards. This page explains where our formulas and product figures come from, how results are rounded, how each calculator is tested, and where an estimate stops being enough.
Last reviewed: September 2026.
The short version
- Published sources only. Coverage rates, product sizes and code limits come from manufacturers, trade associations, government agencies, model building codes and university extension services. Every guide lists its sources at the end.
- Math you can check. Each guide shows the formula with a worked example, and the calculator's default values reproduce that example exactly.
- Numbers you can buy. Anything sold by the piece, bag, sheet or roll is rounded up to a whole unit, and waste allowances are shown so you can change them.
- Tested before publishing. Automated tests check every calculator against its worked example and against bad input.
- No prices, no sponsors. We don't quote material prices, which change often and vary by region, and no company pays to be used in a calculator or guide.
Where our numbers come from
We prefer primary sources: the organization that writes the standard, publishes the data or makes the product. These are the main references behind our 25 calculators:
| Topic | Main references |
|---|---|
| Paint, drywall, baseboard and wallpaper | PaintCare, Sherwin-Williams, Behr and Benjamin Moore coverage data; USG (Sheetrock) product data; International Residential Code Table R702.3.5; York Wallcoverings roll sizes |
| Floors, tile and plywood | Tile Council of North America; Ceramic Tile Foundation; Custom Building Products mortar coverage; flooring installation instructions from Pergo and Mohawk; APA – The Engineered Wood Association |
| Concrete, block, brick and pavers | QUIKRETE and Sakrete product data sheets; Brick Industry Association Technical Notes; NCMA TEK 02-01B; Concrete Masonry & Hardscapes Association |
| Decks, fences and stairs | American Wood Council DCA 6; International Residential Code stair rules; Trex installation guides; Simpson Strong-Tie; Southern Pine Inspection Bureau lumber sizes |
| Yard and garden | University extension services, including Penn State, Illinois, Purdue, Minnesota, Iowa State, Maryland, UConn and Texas A&M; NIST Handbook 44 for the cord; U.S. EPA Burn Wise; Federal Highway Administration |
| Roofing, insulation and cooling | GAF and Owens Corning product literature; ENERGY STAR; U.S. Department of Energy; IECC climate zone map |
Where a figure varies by product, the guide says so and the calculator shows the assumption it uses. Many calculators let you type in the number from your own product label, such as coverage per gallon, coverage per box, roll or pallet, or bricks per bag of mortar mix.
From measurement to shopping list
Most calculators follow the same steps:
- Measure. Each guide explains what to measure and how, in feet and inches.
- Work out the geometry. Area, volume, slope or spacing, depending on the job.
- Subtract what you won't cover. Doors, windows and other openings, where they make a real difference.
- Apply the product figures. Coverage per gallon, bag yield, sheet size, bundles per square and so on, from the sources above.
- Add a waste allowance. Shown as a percentage you can change, based on the guidance in each guide's sources.
- Round up. To whole units you can actually buy.
Example: a 20 x 8 ft brick wall is 160 sq ft. At 6.75 modular bricks per sq ft (Brick Industry Association), that is 1,080 bricks, or 1,134 with 5% extra for breakage and cuts. At 37 bricks per 80 lb bag of mortar mix (QUIKRETE), 1,134 ÷ 37 = 30.6, so the calculator says 31 bags.
Rounding and units
- Results are in US units: feet and inches, square feet, cubic feet and cubic yards, gallons and tons. The square footage calculator also gives square yards and square meters.
- Anything you buy as a whole item is always rounded up. You can't buy 30.6 bags, and running short usually costs more than one extra bag.
- The steps before rounding are shown with decimals, so you can see exactly where the final number comes from.
- Because of rounding and waste, a result can look a little higher than the raw area math. That is intentional.
How we test every calculator
Before a calculator is published or changed:
- Its default values are set to the worked example in its guide, and an automated test confirms the calculator returns the same result the guide prints.
- More tests run it across many combinations of inputs, including every option in its drop-down lists, and compare the results with values worked out from the guide's formula.
- Invalid input, such as blank fields, zero, negative numbers or doors larger than the wall, gets a clear message instead of a misleading number.
- The page is checked on phone, tablet and desktop screen sizes.
- When one calculator changes, the tests for all the others run again, so an update can't quietly change another result.
Your measurements stay on your device
The calculators run entirely in your browser. The numbers you type are not sent to our server and are not stored. How ads and cookies work on the rest of the site is explained in our Privacy Policy.
What an estimate can't do
- It can't see your project. Rooms that aren't square, uneven ground, rough surfaces and tricky cuts all change real quantities.
- Products differ. Always compare our number with the label or data sheet of the exact product you are buying.
- It isn't a design. For anything structural or safety related, such as decks, stairs, retaining walls, roofs and footings, your local building code and a qualified professional have the final word. Our code references are to model codes, and your local version may differ.
Read the full limits in our Disclaimer.
Corrections and updates
If a result looks wrong or a source has changed, please tell us through our contact page, with the page and the numbers you entered if you can. We check every report against our sources and fix confirmed errors. When a fix changes a formula, we update the calculator, the guide and its worked example together. We also review guides when codes or manufacturer specifications change. You can learn more about the site on our About page.