How we calculate

All maths runs in metric internally and is converted for display, so imperial and metric results always agree. Quantities are rounded up to whole bags, tins or boxes. Every formula is covered by automated tests with hand-checked worked examples.

Concrete

Volume = length × width × thickness × number of pours, plus your extra allowance. See Concrete Bag Yields Compared for published yields across four countries. US bag yields come from the manufacturer's data sheet (Quikrete Concrete Mix No. 1101): 40 lb ≈ 0.30 cu ft, 50 lb ≈ 0.375, 60 lb ≈ 0.45, 80 lb ≈ 0.60. Metric bags use the same mix density, about 0.468 litres of concrete per kg of dry mix (so 20 kg ≈ 9.4 L). Ready-mix is rounded up to the next 0.25 cu yd or 0.2 m³. Any bag can be replaced with your own yield.

Post holes

Concrete per hole = (π × radius² − post cross-section) × (hole depth − gravel base). A square post's cross-section is width²; a round post's is π × radius². The calculator rejects a post too large for the hole. Hole-size warnings follow Quikrete's post-setting guidance: a hole about 3 times the post width and about 6 in of gravel in the bottom.

Mulch, gravel and topsoil

Volume = the sum of each area × depth, plus a settling allowance (10% for mulch, topsoil and compost; 5% for gravel and sand). Weight uses editable densities in tonnes per m³: gravel and sand 1.6, topsoil 1.3, compost 0.6, bark mulch 0.4. One US ton per cubic yard equals about 1.187 t/m³. For bags sold by weight, volume per bag = bag weight ÷ density. US weights are in short tons (2,000 lb).

Paint

Wall area = 2 × (length + width) × height, less 21 sq ft (1.9 m²) per door and 15 sq ft (1.4 m²) per window. Paint = area × coats ÷ coverage. Default coverage is 350 sq ft per US gallon or 10 m² per litre. Tin sizes are chosen to minimise cost using a neutral price curve in which larger tins cost less per litre; no retailer prices are stored.

Flooring

Area is the sum of each rectangle. Waste: 10% straight, 15% diagonal, 20% herringbone, or your own figure. Boxes = area with waste ÷ coverage per box, rounded up.

Deck boards

Rows = (deck width + gap) ÷ (board width + gap), rounded up. Boards per row = deck length ÷ board length, rounded up; when the deck is shorter than a board, each board is cut into as many full-length pieces as fit. Spare boards are a percentage you set (5% by default). Joists = deck length ÷ joist spacing, rounded up, plus one. Screws = rows × joists × 2. Default joist spacing is 16 in (US, Canada), 400 mm (UK) and 450 mm (Australia), all editable.

Pavers

Pavers = area ÷ ((paver length + joint) × (paver width + joint)), plus a cutting allowance by pattern: 5% for stack, running bond and basketweave, 10% for herringbone and random layouts. Base and bedding sand = area × depth × (1 + compaction allowance, 20% by default). Weight uses 1.6 t/m³. Base and sand depths start at typical patio values (4 in / 100 mm base, 1 in / 30 mm sand) and should be checked against the paver maker's guide.

Fences

Sections per run = (fence length − gate widths) ÷ number of runs ÷ panel width, rounded up. Posts = sections + one per run + one per gate. Rails = sections × rails per section. Pickets = fenced length ÷ (picket width + gap), rounded up. Concrete uses the post hole formula above for every post. Default panel widths: 8 ft (US, Canada), 1.83 m (UK), 2.4 m (Australia).

Drywall and plasterboard

Area = 2 × (length + width) × height, less 21 sq ft (1.9 m²) per door and 15 sq ft (1.4 m²) per window, plus the ceiling (length × width) if included. Sheets = area × (1 + waste) ÷ sheet area, rounded up. Screws are a rough guide at one per square foot of board (10.8 per m²), rounded up to the next 10.

Tile and grout

Tiles = area ÷ ((tile length + joint) × (tile width + joint)), plus waste. Grout volume per m² = (A + B) ÷ (A × B) × joint width × tile thickness, with A and B the tile sides, all in mm, giving litres. Weight = volume × 1.8 kg/L (editable) × 1.1 for what's lost in application. Bags are rounded up.

Sod and turf

Pieces = total area × (1 + cutting allowance) ÷ the area of one piece, rounded up. Pallets = the same area ÷ what one pallet covers. Default pieces: 16 × 24 in (US), 2 × 5 ft rolls (Canada), 1 m² rolls (UK, Australia). Pallet sizes vary by grower and are only used if you enter one (450 sq ft is suggested for the US).

Pack sizes

Each country's bag, tin and box sizes are common retail sizes, not a complete list. If yours differs, choose Other size and enter what it makes.

Limits

These are planning estimates, not engineering advice. Structural work, footings, drainage and anything covered by building regulations or codes should be checked locally.