What this site is
Loft Insulation Calc is a free calculator for homeowners who want to know how much insulation they need, what to buy and roughly what it will cost, before they talk to a merchant, an installer or building control.
You tell it what you're insulating, how big it is and what's there now. The UK section works out the depth needed to reach the U-values in the Building Regulations, how many rolls or boards to buy and a cost range, and it shows every step of the maths.
How it's built
- Free, with no sign-up. There are no accounts and nothing to unlock.
- Runs in your browser. The calculation happens on your device. Your inputs are kept only in the page address, so you can bookmark a result or share it.
- No cookies. We don't use cookies or cross-site tracking. Our privacy notice has the details.
- Sourced. UK targets come from Approved Document L, the U-value method from BS EN ISO 6946 and BRE's BR 443, product data from manufacturers' data sheets, installed costs from the Energy Saving Trust and prices from UK retailers. Every page lists its sources, and the methodology explains each formula.
Where it works
The site has separate sections for four countries, each with its own rules, products and units:
- United Kingdom: U-values from Approved Document L (England), in millimetres and pounds
- United States: R-values by climate zone, found from a ZIP code
- Germany: U-values for the GEG and federal funding rules
- France: thermal resistance targets for renovation grant schemes
The UK targets are England's. Wales and Scotland set their own, so check your nation's guidance.
Independence and how the site is funded
Loft Insulation Calc isn't owned by, or paid by, any manufacturer, retailer or installer. Brand names appear only because their published data sheets are the sources for our numbers.
The site is free to use. In future it may carry advertising or affiliate links to help cover its costs. If it does, they won't change the calculations, and we'll say so clearly. Our affiliate disclosure explains how that would work.
Contact
Found an error, or have a suggestion? Email info@loftinsulationcalc.com. Corrections to data or formulas are especially welcome.
Our terms of use explain what the estimates are and aren't.