UK Home Insulation Calculator

How this calculator works

You enter property type, wall type, current loft insulation, annual heating bill, heating fuel and the measures you want to price: loft, cavity wall, solid wall, floor, windows and draught proofing. Your bill is divided by an assumed price per kWh for your fuel to estimate how much energy you use. Each ticked measure then applies a fixed percentage saving and a fixed cost from a lookup table in the page, with the cost scaled by property type.

The percentage savings are added together and applied to your bill to give an annual saving. Payback is the total cost, less any ECO4 amount, divided by that annual saving. Carbon saved is the energy saved multiplied by a fuel emission factor. The percentages, costs and fuel prices are assumed values in the code with no cited source or date, so they are not quotes or live prices. Adding savings together also overstates the result when you combine measures, because each one leaves less heat loss for the next to cut.

The two ECO4 tick boxes, on benefits or low income, only tell the calculator to treat loft, cavity wall and solid wall costs as covered. They do not check eligibility. ECO4 is funded by energy suppliers under rules set by government and administered by Ofgem, and the scheme rules and a survey decide what a household can actually get. Floor and window work are never treated as covered.

Use the result to see which measures might matter for your home, then read the insulation grant guide or check your eligibility.

Worked example

Example inputs, chosen only to show the arithmetic: a semi-detached house with cavity walls, 100mm of loft insulation, gas heating and a £1,200 annual heating bill. You tick two measures, a loft top-up to 270mm and cavity wall insulation, and leave both ECO4 boxes unticked.

1. Energy use: £1,200 ÷ £0.073 per kWh of gas = 16,438 kWh a year.
2. Loft, 100mm to 270mm: 15% saving. Cost is £300 (the loft figure labelled diy in the code) × 1.0 for a semi-detached house = £300. Payback on its own: £300 ÷ (£1,200 × 0.15) = 1.7 years.
3. Cavity wall: 33% saving. Cost is £575 for a semi-detached house. Payback on its own: £575 ÷ (£1,200 × 0.33) = 1.5 years.
4. Totals: 15% + 33% = 48% saving, and £300 + £575 = £875 cost. Nothing is deducted because both ECO4 boxes are unticked.
5. Annual saving: £1,200 × 0.48 = £576. Payback: £875 ÷ £576 = 1.5 years.
6. Energy saved: 16,438 kWh × 0.48 = 7,890 kWh. Carbon saved: 7,890 kWh × 0.182 kg ÷ 1,000 = 1.44 tonnes, shown as 1.4 tonnes a year.

How this is calculated and sources

Only the gas carbon factor (0.182 kg CO₂e per kWh) is labelled in the calculator's code with a source, the UK Government GHG Conversion Factors 2026 (DESNZ). The gas price, the percentage savings, the installation costs and the property-type multiplier have no source or date in the code, so they are assumptions used by this calculator. Adding percentages together overstates the saving when measures are combined. The output is an estimate, not a quote, and it does not check ECO4 eligibility.