Insulation Calculator

Enter the area, the stud or joist spacing and the product coverage, and get the number of batts, rolls or bags you need.

Handles both batt and blown-in products, with the cavity count worked out from your framing spacing.

How to use the insulation calculator

Enter the wall, floor or attic area.

Set framing spacing and pack coverage.

Read batts, rolls or bags needed.

Why use the Build Tally insulation calculator

Cavity counting

Works from your real framing spacing.

Batts or blown-in

Both product types supported.

Your product's coverage

Uses the figure on the pack you are buying.

Attic and wall modes

Different defaults for each application.

Batts, rolls and blown-in

Batts are pre-cut to fit between studs and joists at 16 or 24 inch centres, so what matters is the number of cavities rather than the raw area — measure your actual framing spacing rather than assuming. Rolls are cut to length on site and suit long uninterrupted runs like attic floors. Blown-in loose fill is sold by the bag with a coverage table on the packaging that varies by target depth, which is why the calculator asks for the coverage of the product you are buying.

R-value and where it comes from

R-value measures resistance to heat flow, and the level you should aim for depends on your climate zone and which part of the building you are insulating — attics are typically specified much higher than walls. Those figures come from your local energy code, not from a general rule of thumb, and they change. Look up your zone's requirement before buying, because under-specifying is a mistake you only discover through a heating bill.

Fit matters as much as the number on the pack

Compressed or gappy insulation performs well below its rated value. Batts should fill the cavity without being squeezed, be split around wiring and pipes rather than stuffed behind them, and be cut slightly oversize so there are no gaps at the edges. In attics, keep insulation clear of recessed light fittings unless they are rated for contact, do not block soffit vents, and be aware that air sealing before insulating often does more for comfort than adding depth.

Read the guide: How Much Paint Do I Need? Gallons Per Room, Worked Out

Frequently asked questions

How much insulation do I need?

Count the cavities — area divided by framing spacing — or the area for blown-in, then divide by the coverage on the pack. Enter your figures above.

How many batts in a bag?

It varies by product and R-value. Use the coverage figure printed on the packaging, which is what the calculator asks for.

What R-value do I need?

It depends on your climate zone and the part of the building. Check your local energy code — attics are specified considerably higher than walls.

Is 16 or 24 inch spacing standard?

Sixteen inch on centre is most common in residential wall framing. Twenty-four inch appears in some walls and many roof and floor systems — measure to confirm.

Can I put new insulation over old?

In an attic, often yes, provided the existing material is dry and not compacted. Unfaced batts are used for the second layer so no vapour barrier is trapped between them.

Does compressed insulation still work?

Not as well. Compressing a batt into a smaller cavity reduces its effective R-value, so cut it to fit rather than squeezing it in.