Free tool · BuildMeA Projects
Insulation Calculator
Buy too few rolls and you're back at the merchant mid-job; buy too many and they're gathering dust in the garage. Enter the area you're insulating, pick your product, and get an honest number of rolls, boards or packs — with wastage already built in.
Two layers: the base area has been doubled for the 270mm top-up, so the number above already covers both the between-joist and cross-joist layers.
Every figure here is an estimate. Coverage varies by product, thickness and how the pack is cut on site — always round up and keep a spare roll or board in reserve.
How much loft insulation do I need?
Start with the floor area of the loft — length × width in metres — then divide by the coverage printed on the pack. A typical loft roll covers around 8 m², a rigid PIR board (2400×1200) covers 2.88 m², and a Rockwool-style slab pack covers about 5.5 m². Add roughly 10% for offcuts and awkward joist runs, then round up. This calculator does that maths for you and always rounds up so you're not left short a roll on the day.
What depth of loft insulation is recommended?
Current UK guidance recommends around 270mm of mineral-wool loft insulation — up from the 100mm many older homes were built with. Reaching 270mm usually means two layers: the first laid between the ceiling joists and the second laid across them at right angles, which also helps cut cold bridging through the timber. Tick the "top up to 270mm" box in the tool and it doubles the area to account for both layers.
How many insulation boards do I need?
For rigid PIR or PUR boards, work out the area you're covering, then divide by the coverage of a single board — a standard 2400×1200 board is 2.88 m². Add wastage for cuts around rafters, pipes and edges (10% is a fair starting point) and round up to whole boards. Boards are less forgiving than rolls because every cut leaves an offcut, so it's worth carrying a spare board rather than trimming it too fine.
Does this tool store my details?
No. There's no sign-up and nothing to submit — every calculation runs entirely in your browser as you type. Nothing you enter is uploaded, saved or sent anywhere, so you can use it as many times as you like without leaving a trace.
Please note: this tool gives a guide or estimate only. It may not be 100% accurate and is not professional advice — always check the figures before you rely on them. Nothing you enter is uploaded — the calculation runs entirely in your browser.
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