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Calculation methods and limits

These tools provide planning estimates. Results depend on your inputs and the exact product. Each calculator has its own guide, worked example and source links.

Last reviewed 14 October 2026

Shared calculation policy

Tools start from your measurements or entered usage rates, subtract excluded areas where supported, and add your chosen allowance. Purchase counts for boxes, bags, tins, rolls, cartridges, packs or panels round up to whole units. Some intermediate values are displayed with fewer decimals than the underlying calculation.

Prefilled values are editable starting examples, not verified specifications for the item you will buy. Replace coverage, pack quantity, dimensions and price with the product label or quotation. Results are preliminary planning estimates, not engineering designs, suitability guarantees or quotations.

Cooling & air quality

  • Aircon sizing

    Room area × a 700–800 BTU/h/m² baseline, adjusted for climate, sun, ceiling, occupants and kitchen, then rounded to the nearest 100 BTU/h. Roof, glazing and insulation prompt a site check rather than an invented multiplier.

  • Aircon running costs

    Watts × hours × time fraction ÷ 1,000 · Daily energy × days × chosen average rate

  • Air purifier sizing

    Room volume × selected air changes per hour (ACH) gives target CADR in m³/h, rounded up to the next 10.

  • Purifier filters and running costs

    Watts × hours/day × days/year ÷ 1,000 · Recurring cost = electricity + filters; total includes device price only when entered

  • Ventilation and extractor fan sizing

    Volume = length × width × height · Hourly airflow = volume × chosen ACH

Paint, walls & floors

  • Tiles

    Net tiled area × (1 + allowance), divided by tile or box coverage, then rounded up to whole boxes.

  • Paint quantity

    Net wall area plus any chosen ceiling, multiplied by coats, divided by coverage and increased by allowance; tins round up to whole units.

  • SPC flooring

    Net floor area plus cutting allowance divided by box coverage, rounded up; a selected underlay is calculated separately.

  • Vinyl flooring

    Net floor area plus cutting allowance divided by box coverage, rounded up; check the actual vinyl format and installation method.

  • Laminate flooring

    Net floor area plus cutting allowance divided by box coverage, rounded up; check underlay requirements.

  • Tile adhesive

    Net area × product usage in kg/m², or a rate converted from coverage per bag; add allowance and round up to whole bags.

  • Grout

    Use an entered kg/m² rate or a formula from tile size, joint width/depth and entered factor; add allowance and round up to whole bags.

  • Wallpaper rolls

    Count strips for each wall using roll width; account for pattern repeat and trimming in each drop, then divide by full drops per roll and round up.

  • Skirting and trim

    Perimeter 2 × (4 + 3) = 14.00 m; subtract 0 m. · Add 10% and divide by 2.4 m stock lengths; round up to 7.

  • Gypsum boards and finishing supplies

    Area 4 × 2.4 − 0 = 9.60 m². · Add 10% then divide by 2.88 m² per board and round up = 4. · Joint compound 9.60 ÷ 70 m²/bag rounded up = 1 bags. · Taped joint length is blank, so tape rolls remain unknown.

  • Silicone and sealant cartridges

    Joint length × rectangular or triangular cross section gives millilitres; add allowance and round up to whole cartridges.

  • Epoxy coating quantities

    20 m² × 2 coats × 0.3 kg/m² × (1 + 10%) = 13.20 kg. · Divide by 5 kg per kit and round up = 3 kits.

  • Wall panelling and cladding

    Wall width 4 ÷ effective width 0.2, rounded up = 20 runs. · Add 10% to 20 runs and round up = 22 panels.

Solar & electricity

Roofing & insulation

Water & bathrooms

  • Household water storage and backup days

    Daily use = people × litres/person + other use · Nominal tank capacity = daily use × reserve days ÷ usable fraction · Round tank count up

  • Tank volume

    Base = length × width · Volume = base area × water height; 1 m³ = 1,000 L

  • Shower water and heating costs

    Water per shower = L/min × minutes; monthly water = per-shower water × daily showers × days · Heating kWh = litres × temperature rise °C × 0.001163 ÷ efficiency · Cost = m³ × entered water rate plus kWh × entered electricity rate

  • Rainwater collection

    1 m² of roof × 1 mm of rain = 1 L · Potential capture = roof rainfall × capture fraction; stored volume is capped by empty tank space

  • Pond volume and liner size

    Water = length × width × depth × 1,000 · Each liner dimension = pond dimension + 2 × depth + 2 × anchor allowance

  • Pool volume and filling cost

    Average depth = (shallow + deep) ÷ 2 · Volume = length × width × average depth; 1 m³ = 1,000 L · Fill cost = m³ × entered price per m³ when known

Windows & furnishings

Garden & outdoor projects

  • Soil and compost quantities

    Rectangular or circular planter volume, or known litres, plus allowance; split any compost share and round up bags when buying bagged materials.

  • Gravel, sand and decorative stone quantities

    Multiply length × width for area · Multiply by finished depth in metres, then add allowance · Convert to litres and round bags up

  • Paving quantities

    Count slab positions on each axis with joints · Multiply rows by slabs per row · Add cutting allowance and round up full packs

  • Raised-bed materials

    Round courses up from depth and board face width · Pack long and short cuts into stock boards, subtracting the entered saw cut whenever an offcut remains · Multiply all bed volumes, add soil allowance, and round bags up

  • Lawn and turf quantities

    Subtract excluded area from the rectangle · Calculate actual area of one turf piece · Add cutting allowance and round pieces up

  • Fence materials

    Subtract gate opening and use the measured left run · Round bays up per run and count both end posts · Count palings from width and gap, then add allowance

  • Mulch and garden-bed top-up

    Subtract areas without mulch · Multiply area by depth in metres · Add allowance, divide by litres per bag, round up

  • Deck boards and compatible fixings

    Count rows using board width and gap · Count whole cuts per stock board after the entered saw-cut loss, then add allowance · Count joist lines from entered spacing and multiply fixings per crossing

  • Deck and fence stain or wood finish

    Multiply surface area by faces and coats · Divide by coverage stated on the product · Add allowance and round cans up

  • Garden drainage trench materials

    Multiply trench cross-section by length · Subtract the pipe cylinder using its outside diameter · Add aggregate allowance and calculate fabric under base and both sides

Building materials

  • Concrete quantities

    4 × 3 × 0.1 = 1.200 m³. · Convert to litres and add 10% = 1320.0 L; ÷ 17 rounded up = 78 bags.

  • Brick and block quantities

    5 × 2.4 − 0 = 12.00 m². · Module (0.6+0.01) × (0.2+0.01) = 0.1281 m²; with 10% allowance = 104 units.

  • Masonry mortar quantities

    Net area 12.00 m²; joints occupy 6.3% of the unit module. · Area × 0.1 m depth × joint fraction × 15% allowance = 87.3 L; 6 bags.

  • Plywood and sheet-material quantities

    For 0.6 × 1.2 m pieces, use only the entered orientation; with 0.003 m kerf, the grid yields 1 pieces/sheet. · 10 pieces plus 10% = 11; ÷ 1 rounded up = 11 sheets.

Measurements & conversions

Guides and sources

Open a calculator above for its full method, worked example, checks and source links. When your product or conditions differ, use the product label, manufacturer instructions and an appropriate professional site check.