How we calculate
Every result on this site comes from simple geometry and published reference data. Here's exactly what we use, so you can check it yourself.
Volume first, then everything else
Our material calculators start the same way: work out the area or volume of the space you're covering or filling, in square or cubic feet. Everything else — cubic yards, cubic meters, tons and bags — is a conversion of that number.
Calculators that count pieces — pavers, tiles, blocks, deck boards, pickets, drywall sheets, wallpaper strips — divide the area or length by the size each piece covers, including joints or gaps, and round up. Roof calculations multiply the footprint by a pitch factor, √(1 + (rise ÷ 12)²), to get the true sloped area.
You can type each measurement in feet, inches, yards, meters or centimeters. We convert everything to feet internally using exact factors (1 in = 1/12 ft, 1 yd = 3 ft, 1 m = 3.2808399 ft).
Conversions we use
| From | To | Factor |
|---|---|---|
| Cubic feet | Cubic yards | ÷ 27 |
| Cubic feet | Cubic meters | × 0.0283168 |
| Square meters | Square feet | × 10.7639 |
| US (short) tons | Metric tonnes | × 0.907185 |
Bags
Bag counts divide the volume (including the extra allowance) by the volume one bag produces, and always round up — you can't buy part of a bag. For premixed concrete we use the yields published in the QUIKRETE Concrete Mix data sheet: 0.30 ft³ for a 40 lb bag, 0.375 ft³ for 50 lb, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb. Mulch, soil and stone bags are sold by volume (for example 2 cu ft), so we use the volume printed on the bag.
Weight and density
Gravel, sand and soil are often sold by the ton, so we convert cubic yards to tons with a density — the weight of one cubic yard. Density varies with the type of stone, the size of the pieces and above all moisture, so we show a typical value and let you change it. When you order more than a few tons, ask your supplier which conversion they use and enter it.
The extra allowance
Real projects never match the drawing exactly: bases are uneven, forms bow, loose materials settle, and pieces are lost to cuts and breakage. Each calculator adds a default allowance that you can change — typically 5 to 10%, and more for complex layouts such as diagonal flooring or hip roofs — and most also show the quantity without it.
Where our reference data comes from
- Manufacturer data sheets for bag yields, mortar coverage and product limits (for example, QUIKRETE's concrete and mortar mix data sheets).
- Building codes — the International Residential Code — for limits such as stair dimensions and deck joist spacing.
- Government programs such as ENERGY STAR and the U.S. Department of Energy for air conditioner sizing and insulation R-values.
- University extension services for horticultural guidance such as mulch depth.
- Industry associations for construction practice such as paver base and slab thickness.
Where a value varies from product to product — the density of a particular gravel, the coverage of a paint or the number of shingle bundles per square — we use a typical figure, say so, and let you enter the one on your product.
Pages that rely on specific reference data list those sources at the bottom; pages based purely on geometry show the formula and a worked example instead. We review the reference data when manufacturers or codes change and note the date of the last update on each page.
What our calculators don't do
They estimate quantities; they don't design structures. Footing sizes, slab reinforcement, frost depth and drainage depend on your soil, climate and local building code. For structural work, follow your plans or ask a qualified professional.
Found an error or have a better source? Email hello@sizemyproject.com — we correct mistakes quickly and note what changed.