How We Calculate

The rules every tool follows

• Published methods first. Where engineers, installers or manufacturers have a standard way to work something out, the tool uses it, simplified to questions a homeowner can answer.
• Cautious defaults you can change. When a figure is uncertain (paint coverage, waste, heat loss) the default leans slightly towards buying or sizing a little more, and you can replace it with your own figure.
• Dated prices. Default energy and fuel prices come from official or widely used sources and show when they were checked. Enter your own price for the most accurate result.
• The working is shown. Tools show the steps between your inputs and the answer, and each tool page explains the method in plain English.
• Calculated in your browser. Nothing you enter is sent anywhere. The same inputs always give the same result.
• Built with AI assistance, checked by a person. The tools are built and maintained with the help of AI (Claude, by Anthropic). The methods and figures are checked against the sources on this page, and each tool is tested before it goes live.
• Estimates. Results are a guide for planning and comparing, not a quote, a survey or professional advice.

Energy prices

• UK electricity and gas: the Ofgem price cap for 1 October to 31 December 2026 (Direct Debit averages: 26.32p per kWh electricity, 7.97p per kWh gas).
• Ireland: SEAI Domestic Fuel Cost Comparison (July 2026); heating oil from Irish price-comparison sites (October 2026).
• Other EU countries: Eurostat household electricity and gas prices, all taxes included (second half of 2025).
• Petrol, diesel and public chargers: UK Fuel Finder and Zapmap Price Index; EU Weekly Oil Bulletin (all October 2026, or August 2026 for Zapmap).

Standing charges are left out of running-cost comparisons, because you pay them whichever appliance or heater you use.

Energy and bills

Electricity Cost Calculator

Cost = power (kW) × hours of use × electricity price. Each appliance starts with a typical wattage and hours of use; replace them with the figure on its label or from a plug-in energy meter for a closer answer.

Replace or Keep

Compares the yearly running cost of your current appliance with a more efficient one, and works out the payback: purchase price ÷ yearly saving.

Heating Cost Comparison

Heat the room needs (floor area × heat per m² for its insulation level) ÷ the efficiency of each heating type (boiler efficiency or heat pump COP) × fuel price, per hour, per month and per winter, using the prices above.

Radiator & BTU Calculator

A simplified room heat loss in the way installers do it (BS EN 12831, as used in the CIBSE Domestic Heating Design Guide and the MCS heat load calculator): U-values for walls, windows, roof and floor from RdSAP 10; room temperatures from CIBSE; outside design temperatures from CIBSE Guide A (UK) and ASHRAE 2021 (Ireland and Europe). Radiator sizes are converted to the ΔT50 rating radiators are sold by (EN 442), so they also work for lower heat pump temperatures.

Heat Loss Calculator

The same U-values and design temperatures as the radiator calculator, applied to a whole home whose wall, window and roof areas are estimated from its type and floor area. Heat pump running costs use typical seasonal efficiencies (SCOP) for the flow temperature. Results include the Heat Loss Indicator that SEAI’s heat pump grant uses.

U-Value Calculator

BS EN ISO 6946: each layer’s resistance is its thickness ÷ its thermal conductivity (λ); the U-value is 1 ÷ the total, including the standard inside and outside surface resistances and BR 443 values for lofts and air gaps. Results are compared with the limits in England’s building regulations (Approved Document L 2021).

EV Charging Cost Calculator

Energy used = distance × the car’s real-world consumption (kWh per 100 km, or miles per kWh), plus charging losses, split between home and public charging at their prices. Compared with a petrol or diesel car at current pump prices.

Damp and air

Dehumidifier Picker

Moisture to remove each day from the room size, ceiling height and how damp it is, plus moisture from the people in the home and laundry dried indoors. The suggested size allows for the gap between the 30 °C / 80% test rating dehumidifiers are sold by and real, cooler rooms.

Dew Point & Mould Calculator

Dew point from room temperature and humidity (Magnus formula). Surface temperature measured, or estimated with the temperature factor method in BS EN ISO 13788. Mould risk when the humidity at the surface reaches 80%, the ISO 13788 criterion.

Solar and off-grid

Solar Panel Calculator

Yearly output = system size (kWp) × the sunshine for your area from PVGIS 5.3 (the EU Joint Research Centre’s solar database, best angle, 14% system losses) × roof direction × shading. Savings split output between what you use (by daytime use and battery) and what you export, at your electricity and export prices; payback includes grants where they apply.

Plug-in Solar Payback

The same PVGIS sunshine figures for balcony and plug-in kits, limited to the 800 W inverter output allowed in the UK and EU, with the share used in the home by time of day.

Off-Grid Power Planner

Daily energy = each appliance’s watts × hours, plus inverter losses. Battery = daily energy × days without sun ÷ the usable share of the battery (about 85% for lithium LiFePO4, 50% for lead-acid). Solar panels = daily energy ÷ (peak sun hours × system efficiency, 75% by default for wiring, controller and temperature losses).

Cable Size Calculator

For 12, 24 and 48 V DC circuits only (mains wiring is a job for an electrician). Voltage drop = 2 × length × current × the resistance of copper ÷ cable size. The fuse is the next standard size at least 1.25 × the current, and the cable is the smallest whose current rating (BS 7671 tables) covers the fuse and keeps the drop within your limit.

DIY and materials

The material calculators share one approach: measure the areas (several shapes, minus doors, windows and other gaps), add a waste allowance for cuts and breakages, then round up to the pack sizes you can buy. Waste allowances and coverage figures come from manufacturers’ and retailers’ guides, and are shown so you can change them.

Paint Calculator

Litres = area × coats ÷ coverage per litre, plus 10% for rollers and touch-ups. Coverage is set below makers’ “up to” figures, because real walls are rarely smooth and sealed.

Tile Calculator

Tiles = area × (1 + waste) ÷ one tile’s area: 10% straight or brick bond, 15% diagonal, 20% herringbone. Grout uses the formula published by Mapei, Weber and Sika; adhesive uses the bag’s coverage.

Wallpaper Calculator

Counts drops, not square metres, as decorators and wallpaper makers do: each drop is the wall height plus trimming, rounded up to whole pattern repeats.

Flooring Calculator

Area plus waste (10% straight, 15% diagonal or herringbone), in whole packs, with underlay, damp-proof membrane, beading and door bars.

Decking Calculator

Rows of boards across the deck plus 10% for cuts, at the board length that means buying the fewest metres; joists at your chosen spacing; two screws or one hidden clip per board per joist.

Concrete Calculator

Volume plus 10%. Dry ingredients = wet volume × 1.54 (they shrink when mixed), split by the mix ratio. Postcrete bags per post from Blue Circle’s table.

Gravel Calculator

Weight = area × depth × density (about 1.6–1.7 tonnes per m³, from suppliers’ coverage tables), in bulk bags or tonnes.

Square Metre Calculator

Standard geometry for rectangles, L-shapes, circles and triangles, with cut-outs taken off, shown in m² and ft².

Home and living

Laundry Symbols

The care symbols set out in ISO 3758 and used by GINETEX across Europe, with the matching machine settings.

Curtain Size Calculator

Pole length = window width plus an overlap each side. Curtain width = pole × the fullness for the heading (for example 2 for pencil pleat, 1.5 for eyelet), matched to standard ready-made sizes, with fabric for made-to-measure.

TV Size Calculator

Screen width and height from the diagonal (16:9). The best size fills a chosen share of your view: about 40° (THX’s cinema guidance), 30° (SMPTE) or 20° for a relaxed setup, and the tool checks you sit far enough back not to see the pixels at HD, 4K or 8K.

Keeping it up to date

Prices are checked and updated when the official figures change (for example each quarter for the Ofgem price cap). When a method or a default figure changes, the tool page and this page are updated, with the date at the top.

If you think a result is wrong, or know of a better source, email doyoursums@homesums.com.