Methodology

This page lists every formula and constant the calculators use. The values shown here are read from the same code that runs the calculators, so they always match.

General rules

• Units: US customary. Lengths in feet or inches, areas in square feet, volumes in cubic feet and cubic yards, paint in US gallons.
• Waste is added before rounding. Rounding happens once, at the end, and always up, so you are never told to buy less than you need.
• Product figures are typical, not universal. When a number depends on a product, the calculator lets you change it and the page tells you to check your label.
• Testing: each calculator is covered by automated tests with hand-worked examples. Tables on the site are generated from the tested code.

Concrete

• Slab: length (ft) × width (ft) × thickness (in) ÷ 12
• Round column: π × (diameter in ÷ 24)² × height (ft)
• Post hole: (π × (diameter ÷ 2)² − post width²) × (hole depth − gravel base) ÷ 1,728, all in inches
• Cubic yards: cubic feet ÷ 27

Tile

• Coverage per tile: (width + grout joint) × (length + grout joint) ÷ 144
• Tiles: area ÷ coverage per tile × (1 + waste), rounded up
• Boxes: tiles ÷ tiles per box, or area × (1 + waste) ÷ sq ft per box, rounded up
• Grout: joint length per sq ft = 144 × (tile width + tile length) ÷ (tile width × tile length), in inches; volume = joint length × joint width × tile thickness; one pound of dry sanded grout is taken to fill 12 cu in, the low end of typical values, so the estimate leans high.

Paint

• Walls: 2 × (length + width) × height − openings
• Door: 21 sq ft (a standard 3 × 7 ft interior door). Window: 15 sq ft (a 3 × 5 ft average).
• Gallons: area × coats ÷ coverage. Default coverage: 350 sq ft per gallon, the middle of the 250–400 range printed on most interior paint labels.
• Buying: whole gallons plus quarts; three or more quarts round up to another gallon.

Square footage

• Rectangle: length × width. Triangle: base × height ÷ 2. Circle: π × (diameter ÷ 2)². Trapezoid: (side 1 + side 2) ÷ 2 × height.
• 1 sq ft = 0.09290304 m² (exact). 1 sq yd = 9 sq ft. 1 acre = 43,560 sq ft.
• Board feet: thickness (in) × width (in) × length (ft) ÷ 12, using nominal sizes.

Flooring

• Boxes: area × (1 + waste) ÷ sq ft per box, rounded up. Cost is charged on whole boxes.
• Default waste: straight 10%, diagonal 15%, herringbone 20%; plus 5% for rooms with closets, many corners or several doorways.
• The cutting-waste tables come from a row-by-row model: joints staggered at least 8 in, offcuts of 8 in or more reused, last row ripped to width.

Drywall

• Sheets: (walls + ceiling − openings) × (1 + waste) ÷ sheet area, rounded up, walls and ceiling counted separately.
• Screws: sheets hung across the framing; 12 in spacing on ceilings, 16 in on walls framed 16 in on center, 12 in on walls framed 24 in on center.
• Joint compound: 9 gal per 1,000 sq ft of board (USG: about two 4.5-gallon pails). Paper tape: 370 ft per 1,000 sq ft.

Gravel, sand and soil

• Volume: length × width × depth (in) ÷ 12; cubic yards = cubic feet ÷ 27; tons = cubic yards × lb per cubic yard ÷ 2,000.
• Weights used: Crushed stone (#57, clean) 2,700 lb/yd³; Crusher run / road base 3,000 lb/yd³; Pea gravel 2,600 lb/yd³; River rock 2,600 lb/yd³; Sand (dry) 2,700 lb/yd³; Topsoil 2,200 lb/yd³. A compaction allowance (crusher run 15%, sand and topsoil 10%) is added for bases that will be compacted.

Decks

• Rows: deck width ÷ (board width + gap), rounded up. Each row is filled with whole boards; offcuts of 2 ft or more are reused.
• Joists: deck length ÷ spacing + 1. Screws: rows × joists × 2.
• Board spans from IRC 2021 Table R507.7: 5/4 wood (1 1/4 in) 16 in (12 in diagonal); 2x wood (2 in) 24 in (24 in diagonal).

Stairs

• Risers: total rise ÷ target riser, rounded up; riser height = rise ÷ risers; treads = risers − 1.
• Stringer length: risers × √(riser² + tread²).
• Checks against IRC R311.7: riser ≤ 7 3/4 in, tread ≥ 10 in, width ≥ 36 in. Comfort rule: 2 × riser + tread between 24 and 25.5 in.

Roofing

• Pitch multiplier: √(1 + (rise ÷ 12)²). Roof area = footprint × multiplier. Hip length per foot of run: √(2 + (rise ÷ 12)²).
• Bundles: squares × (1 + waste) × 3, rounded up. Waste: gable 10%, hip 15%.
• Underlayment: one roll per 1,000 sq ft (a typical synthetic roll). Drip edge: 10 ft pieces.

Mulch and soil

• Cubic feet = area × depth (in) ÷ 12; cubic yards = cubic feet ÷ 27; bags = cubic feet ÷ bag size (1.5, 2 or 3 cu ft), rounded up. A top-up counts only target depth minus existing depth.
• Depth guidance from Iowa State University Extension: 3 to 4 in of wood chips or bark on well-drained sites, 2 to 3 in on heavy soils, 6 in clear of trunks.

Fences

• Sections = (length − gate openings) ÷ post spacing, rounded up. Posts = sections + 1 + one per gate. Rails: 2 per section up to 4 ft tall, 3 above.
• Pickets = fence line ÷ (picket width + gap) + 5%. Concrete per post uses the post-hole formula above, with a 6 in gravel base and 10% extra.

Brick and block

• Units per sq ft and mortar per 1,000 units (3/8 in joints) from BIA Technical Note 10: Modular brick (7 5/8 × 2 1/4 in) 6.75 per sq ft, 8.1 cu ft mortar per 1,000; Queen brick 5.5 per sq ft, 12.2 cu ft mortar per 1,000; King brick 4.55 per sq ft, 14.2 cu ft mortar per 1,000; Utility brick 3 per sq ft, 12.3 cu ft mortar per 1,000. Concrete block: 1.125 per sq ft.
• Allowances: 5% extra units, 20% extra mortar (BIA suggests at least 5% and 15 to 25%).
• Bags: one 80 lb bag of QUIKRETE Mason Mix lays up to 37 standard bricks or 13 blocks (data sheet). Other brick sizes are converted through mortar volume.

Wallpaper

• Strip length = wall height + 4 in, rounded up to whole pattern repeats; half-drop adds half a repeat. Strips per roll = roll length ÷ strip length, rounded down.
• Strips = (wall width − 3 ft per door) ÷ roll width, rounded up. Rolls = strips ÷ strips per roll, rounded up. Windows are not subtracted.

Carpet

• Drops = room dimension across the roll ÷ roll width, rounded up; cut length = the other dimension + 3 in per end. Both directions are tried and the one using less carpet is shown.
• Pad = room area + 5%. Tack strip = perimeter − doorways.

Sources

• Concrete bag yields: labels and product data sheets of standard concrete mixes sold at US home centers.
• Paint coverage: spread rates on labels and technical data sheets of interior latex paints from major US brands.
• Slab, footing and stair guidance: the International Residential Code (IRC), as adopted by your state or city. Stair limits checked against a county summary of IRC R311.7.
• Deck spans: IRC 2021 Tables R507.6 and R507.7, as printed in Anne Arundel County's deck guide; AWC DCA 6.
• Drywall: USG joint compound estimate; screw spacing per ASTM C840 and GA-216 as summarized by Walls & Ceilings.
• Gravel weights: a quarry supplier's published figures and typical supplier ranges.
• Shingles: GAF on bundles per square.
• Brick and mortar: BIA Technical Note 10; QUIKRETE Mason Mix data sheet.
• Mulch depth: Iowa State University Extension.
• Wallpaper roll sizes: Selected Wallpapers and maker labels.
• Unit conversions: NIST SP 811, Appendix B.

Corrections

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