Insulation Calculator
Enter the loft or wall area and the coverage printed on your roll, pack or board. This calculator adds your chosen cutting allowance, rounds up to whole packs and shows the planned insulation depth. Check the product width against the clear joist bays separately: spacing alone does not tell us how much material is lost in cuts.
Your measurements
Check your order
Your wall and ceiling order
- Measure each surface and opening; check thickness and product coverage.
- Confirm sheet or pack sizes, cuts and the required fixing or finishing system.
- List corners, joints and finishing materials separately from the main surface quantity.
Tools for this job
Measure each cavity or surface before choosing pack quantities. Account for obstructions and follow the product instructions for fit without unwanted compression.
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- Check room dimensions and cuts
Short tape measure
STANLEY 25-foot tape measure
For checking lengths, widths and short runs before ordering. This is a 25-foot tape; use matching units in the calculator and check the scale on the selected model.
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How much loft insulation do I need?
Measure the loft floor, length × width, from inside the eaves. Add your chosen cutting allowance, divide by the coverage printed on the roll and round up. The same area calculation is used for batts and boards. Between joists, check the actual clear bay width against the product width and consider whether offcuts can be reused. A centre spacing of 450 mm does not automatically mean a 25% loss, so the centre selector is a planning reminder and does not change the result.
Coverage per roll changes with thickness far more than people expect, because a roll is a fixed volume of compressed wool. A 100 mm roll often covers around 8 m², a 200 mm roll around 5 m². Always take the figure from the label of the product you are buying, and enter that.
The formula
Coverage by product
| Product | Typical size | Coverage | Where it goes |
|---|---|---|---|
| Loft roll, 100 mm | 1140 mm wide, combi-cut into 2 or 3 strips | about 8 to 9 m² per roll | Between the joists |
| Loft roll, 170 mm | 1140 mm wide | about 5 to 6 m² per roll | Across the joists as the top-up layer |
| Loft roll, 200 mm | 1140 mm wide | about 4.5 to 5.5 m² per roll | Across the joists, or between deep joists |
| Mineral wool slab or batt | 1200 × 600 mm, 4 to 10 per pack | 2.9 to 7.2 m² per pack | Between studs and rafters |
| PIR board | 1200 × 2400 mm | 2.88 m² per board | Floors, walls, warm roofs |
| PIR board, cut for joists | 1200 × 450 mm | 0.54 m² per board | Check the clear bay width and cutting plan |
Worked example
A loft 8 m × 5 m with joists at 400 mm centres, insulated between the joists with 100 mm rolls that cover 8 m² each, and a 10% allowance.
Area = 8 × 5 = 40 m². With 10% for cutting = 44 m². Rolls = 44 ÷ 8 = 5.5, rounded up to 6 rolls, with 48 m² of labelled coverage. The 8 m² beyond the measured area includes the cutting allowance; it is not a promise of usable leftover.
The top-up layer is a second calculation: the same 40 m² with 170 mm rolls covering 5 m² each takes 44 ÷ 5 = 8.8, rounded up to 9 rolls, and brings the total depth to 270 mm.
Between the joists, then across them
The usual approach is two layers. The first fills the joist bays to the top of the joists, normally 100 mm. The second runs at right angles across the joists so the timber itself is covered, because uninsulated joists are a strip of cold bridge every 400 mm. The widely quoted target for a UK loft is 270 mm in total, and the two-layer method gets there with a 100 mm and a 170 mm roll.
If you plan to board the loft for storage, the second layer collides with the boards: 270 mm of wool cannot sit under a board screwed to 100 mm joists without being crushed flat. A raised-leg loft boarding system lifts the boards clear of the insulation, so plan the legs before the top-up layer goes down.
What not to do
- Do not push insulation into the eaves. Leave the ventilation gap at the edge of the roof clear, or the timbers get damp.
- Do not cover recessed downlights unless they are rated for it, and keep a gap around flues and chimney breasts.
- Do not lay it over electric cables that carry a heavy load, such as a shower circuit: cables need to run above the insulation or be derated.
- Do not insulate under a cold water tank. It needs the heat from below to stop it freezing; insulate the tank itself instead.
- For a stud wall, measure the framed area as length × height and use batts or slabs cut to the stud centres, then board over it with the drywall calculator. For an odd-shaped loft, split it into rectangles with the area calculator.
Sources and assumptions
The estimate divides area with the chosen allowance by entered roll, batt or board coverage. Thickness records the intended layer; it does not turn the estimate into a thermal-performance calculation.
- Knauf Insulation: UK solutions guide, Loft Rolls 40 and 44: The product tables give different pack areas by thickness and identify combi-cut widths of 570 mm or 380 mm, illustrating why actual product dimensions matter.
Before using this estimate
- Enter the pack coverage and account for usable offcuts in the actual joist bays. Centre spacing alone cannot determine cutting waste for every roll or slab.
- The calculator does not assess U-values, condensation, ventilation, fire protection or electrical clearances. Follow the insulation product and building assembly specification.
Source notes reviewed . See how we check calculations and handle corrections.
Planning your order
Check the inputs before buying: measure the surfaces you need, then apply an allowance and round to whole packs. The guides include worked examples you can follow alongside this calculator.
Published by OrderEnough. This is a planning estimate: check the product specification and the assumptions above before ordering. Last updated . Read how we source our figures on the methodology page.