How Our Calculators Work

This page sets out the formulas behind every HousyTools calculator in plain words, the default values each one starts with and where those defaults come from. All calculations run in your browser, and every tool also shows its own step by step working under the result.

General Principles

• Defaults are starting points. Each tool opens with realistic example values so you see a result straight away. Replace them with the figures on your tin, pack, bill or data sheet.
• Rounding up for things you buy. Tins, boxes, packs, rolls and bags are always rounded up, because you cannot buy part of a box.
• Wastage is shown, not hidden. Where a tool adds an allowance for offcuts or spillage, the percentage appears in the breakdown.

Paint and Decorating Calculators

Used by the tools in paint and decorating.

Paint quantity

• Wall area = 2 x (room length + room width) x wall height.
• Doors and windows are taken off: number of doors x area of one door, plus number of windows x area of one window. Defaults are 1.6 m² per door (a standard UK internal door is about 0.76 m by 1.98 m) and 1.5 m² per window.
• The ceiling (length x width) is added only if you tick it.
• Paint in litres = area x number of coats / coverage in m² per litre. The default coverage is 12 m² per litre, a typical figure printed on interior emulsion tins. Always use the figure on your own tin.
• Tins are suggested as the mix of 5 litre, 2.5 litre and 1 litre tins that covers the need with the least paint left over.

Example: a 4 m by 3.5 m room with 2.4 m walls, one door and one window has 36 m² of wall, less 3.1 m², so 32.9 m². Two coats at 12 m² per litre need 32.9 x 2 / 12 = 5.48 litres, so one 5 litre tin and one 1 litre tin.

The ceiling paint tool works the same way and adds one extra coat for a mist coat on new plaster. It counts the mist coat as a full coat to be safe.

Masonry, fence and deck coatings

• Masonry paint: area = wall length x height, less doors and windows. The smooth surface coverage from your tin (default 10 m² per litre) is multiplied by a surface factor: 1 for smooth render, 0.75 for brick or fine texture, 0.5 for pebbledash or rough cast.
• Fence paint: area = number of panels x panel width x panel height x sides painted. Defaults are 1.83 m by 1.83 m panels (6 ft by 6 ft) and 5 m² per litre, because rough sawn timber absorbs more.
• Deck stain: area = length x width plus any steps or rails, with a default of 8 m² per litre.

Wallpaper

• Length to cover = room perimeter, less any width of doors and windows you choose to skip.
• Each drop = wall height + 10 cm for trimming + one pattern repeat.
• Drops per roll = roll length / drop length, rounded down. Drops needed = length to cover / roll width, rounded up.
• Rolls = drops needed / drops per roll, rounded up. We suggest one spare roll from the same batch.
• The default roll is the standard European size of 53 cm by 10.05 m.

Paint cost and painting time

• Paint cost: litres are worked out as above, tins = litres / tin size rounded up, then cost = tins x price per tin + primer + sundries.
• Painting time = prep time + (area x coats / painting speed) + (coats minus 1) x waiting time between coats. The default speed is 10 m² an hour, a steady DIY pace with a roller.

Flooring and Tiling Calculators

Used by the tools in flooring and tiling.

Tiles

• Area to tile = length x width, less any area left untiled.
• Each tile covers (tile width + grout gap) x (tile height + grout gap), so the joint is included.
• Tiles = area / area of one tile with joint x (1 + wastage), rounded up, then divided by tiles per box and rounded up again.
• Wastage depends on the layout: 10% for a straight lay, 12% for brick bond, 15% for diagonal and 20% for herringbone.

Example: 3 m by 2.5 m (7.5 m²) of 600 by 300 mm tiles with a 3 mm gap. One tile with its joint covers 0.603 x 0.303 = 0.1827 m². 7.5 / 0.1827 x 1.10 = 45.2, so 46 tiles, which is 8 boxes of 6.

Grout and adhesive

• Grout in kg per m² = (tile length + tile width) / (tile length x tile width) x joint width x joint depth x grout density, with sizes in millimetres and density in kg per dm³. The default density is 1.6, typical for cement grout. A 10% allowance is added.
• Tile adhesive = coverage rate x area x 1.05 (5% allowance). Typical rates by notched trowel are 1.5 kg per m² for 3 mm, 2.5 for 6 mm, 4 for 10 mm and 5 for 12 mm. Back buttering large tiles adds 30%. The coverage printed on your bag always takes priority.

Laminate, vinyl and carpet

• Laminate packs = room area x (1 + wastage) / coverage per pack, rounded up. Wastage choices are 5%, 10% (default) and 15%. Underlay rolls = room area / roll coverage, rounded up.
• Sheet vinyl and carpet come in fixed roll widths (2, 3 or 4 m for vinyl, 4 or 5 m for carpet). The tool tries the roll both ways along the room. For each direction, strips = (width across + 2 x trim) / roll width, rounded up, and length = strips x (run + 2 x trim). It picks the direction that needs the least material. The default trim is 5 cm per edge.
• Carpet gripper rod = room perimeter less the total door width.

Skirting and levelling compound

• Skirting run = room perimeter less door openings. Boards = run x (1 + allowance) / board length, rounded up. The default allowance for mitres and offcuts is 10%.
• Self levelling compound in kg = area x average depth in mm x consumption per mm x (1 + allowance). The default consumption is 1.6 kg per m² per mm and the allowance is 5%.

Energy and Bills Calculators

Used by the tools in energy and bills.

Electricity running costs and bills

• Energy in kWh = power in watts / 1000 x hours of use.
• Cost = kWh x unit rate. A year is days used per week x 52.14 weeks.
• Bill from meter readings = (latest reading minus previous reading) x unit rate + days x daily standing charge, plus VAT only if your rates exclude it.
• Electric shower energy = shower rating in kW x minutes / 60.

Example: a 2,000 W heater used 2 hours a day uses 4 kWh. At 26.32p per kWh that is about £1.05 a day, or £384.25 a year if used every day.

Gas bills

Gas meters measure volume, but suppliers bill in kWh. We use the conversion set out in GOV.UK guidance on gas meter readings:

• kWh = cubic metres used x 1.02264 (volume correction) x calorific value / 3.6.
• For older imperial meters, which count hundreds of cubic feet, units are first multiplied by 2.83 to give cubic metres.
• The default calorific value is 39.5 MJ per m³. Your bill shows the exact figure, which is usually between 38 and 41.

Example: 100 m³ x 1.02264 x 39.5 / 3.6 = 1,122.1 kWh. At 7.97p per kWh plus 30 days of a 29.68p standing charge, the bill is £98.33.

Tariff comparisons, solar, heat pumps and EVs

• Economy 7: single rate cost = yearly kWh x single rate + 365 x standing charge. Economy 7 cost = day units x day rate + night units x night rate + 365 x standing charge. The tool also finds the night share at which both cost the same. Day and night rates are example values; use your supplier quote.
• Solar panels: system size in kWp = panels x panel watts / 1000. Yearly generation = kWp x yield per kWp (default 900 kWh, check PVGIS for your address). Benefit = units used at home x import rate + units exported x export rate. Simple payback = installed cost / yearly benefit, ignoring panel ageing and price changes.
• Heat pump running cost: electricity used = yearly heat demand / SCOP (default 3). It is compared with a gas boiler: heat demand / boiler efficiency (default 85%) x gas rate + 365 x gas standing charge.
• EV charging: energy into the battery = battery size x (charge to minus charge from). Energy from the grid = that figure / (1 minus charging losses), with a default loss of 10%. Yearly cost uses distance x consumption per 100 km.

Where energy rates come from

UK figures are the Ofgem energy price cap averages for a direct debit customer. Your own tariff may differ by region, payment method and supplier, so every tool has a “My own tariff” option. We update these presets whenever Ofgem announces a new cap period and recheck them at least every 3 months.

Heating and Insulation Calculators

Used by the tools in heating and insulation.

Radiator size (room heat loss)

The radiator size calculator uses a simplified room heat loss method. Heat loss in watts is the sum of fabric loss and ventilation loss:

• Fabric loss for each surface = area x U value x temperature difference between inside and outside.
• Ventilation loss = 0.33 x air changes per hour x room volume x temperature difference.
• Watts are converted to BTU per hour by multiplying by 3.412.

The default outside design temperature is minus 3°C, in line with the design conditions used by the MCS heat load calculator for much of the UK. Room temperatures and air change rates are:

Typical U values used, in W per m² per K:

Surfaces next to another heated room are counted as zero loss. External walls are taken in the order length, width, length, width.

Radiator output at lower flow temperatures (Delta T)

• Mean water temperature = (flow temperature + return temperature) / 2.
• Delta T = mean water temperature minus room temperature.
• Actual output = rated output at Delta T 50 x (Delta T / 50)ⁿ, where n is the radiator exponent from the data sheet. The default n is 1.3, typical for steel panel radiators.

Example: a radiator rated at 1,500 W, running at 55°C flow and 45°C return in a 21°C room, has a mean water temperature of 50°C and a Delta T of 29. The factor is (29 / 50)^1.3 = 0.493, so it gives about 739 W, roughly half its rated output.

Boiler and heat pump size

• Boiler size uses your known heat loss if you have one, or 1.5 kW per radiator if not. A combi boiler is sized by hot water demand: 24 to 27 kW for one bathroom and one shower, 28 to 34 kW for up to two, and 35 to 42 kW above that. System and regular boilers add about 3 kW to reheat a cylinder, with an upper figure 20% above the lower one.
• Heat pump size by floor area = area x watts per m² for the insulation level (35, 50, 70 or 100 W per m²). By gas use = yearly gas kWh x boiler efficiency / equivalent full load hours (default 2,200 hours).

Underfloor heating and loft insulation

• Heated area = floor area x share free of fixed units (default 80%).
• Electric mats: watts = heated area x mat rating (100, 150 or 200 W per m²). Current = watts / 230 V. Daily energy = kW x heating hours x thermostat on time.
• Wet systems: pipe = heated area x (1000 / pipe spacing in mm) + 2 x distance to the manifold. Loops are counted at about 100 m each.
• Loft insulation: rolls for the layer between joists are needed when existing insulation is shallower than the joists, and a cross layer when the target depth is above the joists. Each layer = loft area x (1 + allowance) / roll coverage, rounded up. The default target is 270 mm, the depth recommended by the Energy Saving Trust.

Units and Currency

• Every tool works in metric by default. Tools where people still measure in feet and inches have an imperial switch that converts metres to feet (x 3.28084), square metres to square feet (x 10.7639), millimetres and centimetres to inches, and kilometres to miles (x 0.621371). EV efficiency switches from kWh per 100 km to miles per kWh.
• Values are stored in metric behind the scenes, so switching units does not change the result, only how it is displayed.
• Money is shown in pounds (£) or euros (€). Unit rates and standing charges are entered in pence or cents, for example 26.32p per kWh or 28.96c per kWh, and results are shown to two decimal places.
• Some tools, such as the radiator output and boiler size calculators, use metric and watts only, because that is how the products are rated.

Limitations

• Results are estimates for planning. Real coverage depends on surface, product, application method and skill.
• The heat loss method is simplified. It does not replace a room by room survey, such as an MCS MIS 3005 heat loss calculation for heat pumps, or a sizing check by a Gas Safe registered engineer for boilers.
• Energy costs use the rates you enter. They do not include discounts, debt repayments, exit fees or regional differences unless you add them.
• Solar payback ignores panel degradation, inverter replacement, maintenance and future price changes.

For the rules behind our sources and updates, see the editorial policy. If a result looks wrong, please report an error.