Loft Insulation Quantity Calculator

Who this is for

Work out how many rolls the loft needs. Measure each area, take off what you will not cover, then set the new layers against your own target depth.

Homeowners topping up or laying loft insulation themselves, and anyone checking an installer's quantity or a grant scheme's quote against their own measurements.

What this helps you track

• The gross and net area of each part of the loft
• The hatch, chimney, tanks and boarded areas taken off
• Rolls or packs for each layer from the coverage on the packet
• The depth reached against the target you have set
• Storage boarding and hatch insulation costed with the rest

How to use it

1. Write the property, the depth of insulation already there as you measured it, and the target depth you are working to.
2. Split the loft into rectangles, measure each between the eaves in metres, and enter what you are leaving uncovered as deductions.
3. Copy the net area into each layer, with the layer's thickness and the coverage per roll or pack from the packet.
4. Rolls, cost and the summary are worked out in the browser and the workbook; on paper, divide the area by the coverage and round up.
5. Follow the maker's instructions when laying it, and keep the packet labels with the sheet.

Formats

• Printable PDF (A4 and US Letter)
• Fillable PDF with form fields
• Fill in online, saved in your browser
• Excel / Google Sheets workbook

The printable is laid out inside a box that fits both A4 and US Letter, so the two files are identical and neither one is scaled. The fillable PDF has real form fields. The workbook uses plain formulas only — SUM, IF, COUNTIF and the like — so it opens in Excel, LibreOffice and Google Sheets without anything breaking, and every calculated cell is grey so you can see which figures it worked out.

How to print

Download the A4 or US Letter file to match the paper in your printer, open it in a PDF reader and print at 100% — not "fit to page", which shrinks the writing space. Both files leave a 14 mm margin, which clears the unprintable edge on home printers. To print the online version with your entries in it, use the Print this sheet button, which prints only the sheet and none of the page around it. More on printing.

Questions

How deep should loft insulation be?

The sheet does not say. The right depth depends on the material, where you live and any rules or grant conditions that apply, so enter your own target from official guidance, your installer or the scheme you are using.

How do I measure the existing depth?

Push a ruler down through the insulation to the ceiling below in a few places and use a typical reading. Write the lowest reading in the notes if the depth varies a lot across the loft.

Why copy the net area into each layer?

Each layer covers the same floor, so each needs its own rolls. Laying one layer between the joists and another across them means two rows with the same area but different thicknesses and coverage.

What goes in the thickness column?

The thickness of each insulation layer, from the packet. Leave it blank for boarding, hatch kits and draught strip, so the depth reached adds up insulation only.