Why isn't there a default coverage number I can just trust?

Batt insulation coverage per package varies a lot by R-value and thickness — an R-13 package covers a different area than an R-38 package of the same physical size. Always use your specific product's printed coverage.

Does higher R-value always mean better insulation?

Higher R-value means more resistance to heat flow, but the right R-value depends on your climate zone and where in the house you're insulating (attic, wall, floor) — check local building code recommendations.

Should I compress batts to fit a smaller space?

No — compressing batt insulation reduces its effective R-value. If a stud bay is shallower than the batt's thickness, use a batt rated for that depth instead.

What does the coverage printed on a package actually measure?

Read that before entering it. This page divides your gross length times width by the figure you give it, so if the label states the batts' own face area — the pieces laid edge to edge — then measuring over the framing counts part of the joist or stud faces as well. A batt for 16 in (406 mm) centres is made about 15 in (381 mm) wide to fit its bay snugly, so its face covers about fifteen-sixteenths of the measured area, and the remaining 6% or so then works as a hidden allowance on top of the waste field; where the label states the framed area the pack serves at its intended centres, there is no such margin.

How do I count a loft or attic laid in two layers?

Run the page once for each layer. The first layer fills the bays between the ceiling joists, sized to their depth and spacing; the second is laid across the joists at right angles, covering their tops as well, and is usually a different thickness with its own coverage per pack. Both runs use the same length and width, and adding the two counts is right where adding the two coverage figures would be wrong. In the UK both layers are loft roll or quilt; in North America the cross layer should be unfaced, because a second vapour retarder laid over the first traps moisture between them.

Is the R-value on a North American pack the same number as a UK one?

No. A North American R-value is in ft²·°F·h/Btu; UK and European labels state thermal resistance in m²·K/W, and Canadian labels print it as RSI beside the imperial figure. One metric unit is about 5.68 imperial ones, so an R-13 batt is roughly 2.3 m²·K/W and an R-38 roughly 6.7 m²·K/W. No R-value enters this count, but reading one system's number as the other's picks the wrong pack, and with it the wrong coverage figure. The R-Value to RSI Calculator does the conversion.

Why do some walls waste more batt than the percentage allows?

Because the loss follows the number of bays, not the area. North American wall batts are sold pre-cut to suit standard stud heights, so a wall of that height takes one piece per bay with little or no trimming, while a wall of any other height leaves an offcut in every bay or needs a short piece added to each. A long, low wall — a knee wall in a loft or attic room, often called a dwarf wall in the UK — has many bays for its area and loses more than a flat percentage predicts. Where roll is cut to length on site, the loss collects at the end of each roll instead; fixed-length slabs, including UK wall slabs, lose material bay by bay like pre-cut batts. Count the bays and the pieces per bay, then set the waste field from that.

Insulation & Efficiency

Insulation Batt Calculator

Estimate how many packages of batt insulation you need to cover an area, using your product's stated coverage.

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Who is doing the work?

The standard allowance most suppliers and estimating guides assume for ordinary work.

Area Length

The length of the wall, floor, or attic area to insulate.

Measure the space you're insulating — an attic floor, a wall run, or a ceiling.

Tools needed: Tape measure

Area Width

The width of the area to insulate.

Measure perpendicular to the length.

Tools needed: Tape measure

Package coverage

The square footage one package covers, printed on the product label.

This varies significantly by R-value, thickness, and brand — check your specific product's packaging rather than relying on a generic default.

Waste Factor (%)

Extra material for cuts around obstructions (pipes, wiring, joists).

10% is standard for a straightforward area. Attics with lots of framing members and obstructions may need more.

Estimated insulation batt needed

12 packages

High confidence

Area to insulate: 1,014 sq ft
Area with waste factor: 1,115.4 sq ft

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Batt/roll insulation coverage per package varies by R-value, thickness, and brand — always use the square footage printed on your specific product's packaging for an accurate count

Inputs used

Area Length: 39 ft
Area Width: 26 ft
Package coverage: 100 sq ft
Waste Factor (%): 10

Intermediate steps

Area to insulate: 1,014 sq ft
Area with waste factor: 1,115.4 sq ft

Final result12 packages

What this calculation does not cover

This counts area against the coverage printed on your package. It does not check that the batt width suits your framing spacing or that the batt thickness fits the cavity depth — a 16 in (406 mm) batt in bays framed at 24 in (610 mm) centres leaves gaps that no package count corrects.
No R-value enters the calculation. Nothing here tests the finished assembly against the R-value your climate zone and local code call for, and no allowance is made for heat flow through the framing members themselves, which conduct more than the cavity beside them.
The area is the plain rectangle you enter. Nothing is deducted for windows, doors or the attic hatch, and nothing is set aside for the areas that must stay clear — recessed lights, flues and chimneys, and the eave gap soffit ventilation needs. Those clearances come from the manufacturer's instructions and local code, not from this count.
The waste factor is a flat percentage of the whole area, not a cutting plan. Irregular bays, hips and valleys, and heavily obstructed framing produce short offcuts that waste far more than a percentage of the total suggests.
This is a purchase quantity, not an energy or compliance calculation. It says nothing about air sealing — batts do not stop air leakage — nor about which way a faced batt's vapour retarder must face, which depends on your climate.

Estimated cost — your price

Price per package ($)

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Part of bigger jobs

This trade is one line of several job takeoffs. Run the whole job and every other trade comes back with it, off the same measurements.

Or plan the space itself: measure it once, doors and windows included, and this figure comes back worked out on that space, with what goes with it.

Loft planner

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Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1

Batt/roll insulation coverage per package varies by R-value, thickness, and brand — always use the square footage printed on your specific product's packaging for an accurate count

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Protection this work requires

Stop and check first. Textured ceiling coatings, board behind panels, floor tiles and bitumen adhesive in older buildings — built before asbestos was banned or phased out where you are — can contain asbestos, and drilling or sanding them releases fibres. Do not disturb it — have it sampled first. This is not a job for better PPE.

Tools and safety for this job

To fit batt insulation. Generic types, no brands, no prices.

Mineral and glass wool irritate skin, eyes and airways: cover your arms, wear a P2/N95 respirator while cutting, and rinse in cold water — hot water drives fibres in.
Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
Knives cause more site injuries than any power tool: cut away from your body, change blades often, and wear cut-resistant gloves to EN 388 level C for repeated cutting.

Show the 5 tools this job needs

Essential

Tape measure
Utility knife

Recommended

Staple gun
Step ladder

Optional

Dust extractor or vacuum with a fine filter

Also needed as materials: replacement blades, staples.

what each insulation tool is for, and the spec that decides which to buy where one does.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Choosing a Framed Exterior Wall Build-Upuses this calculator
Stud size and spacing, cavity insulation, vapour control, sheathing and cladding: what each choice changes in the order, and what it never does.
Converting an Integral Garage Into a Roomuses this calculator
Building up from a slab poured to drain: the floor stack, the infill under the old lintel, and holding the damp line unbroken across the threshold.
Insulating a Suspended Timber Ground Flooruses this calculator
The void under a period floor is a drying machine. Insulate between the joists without switching it off, and the floor gets warm instead of rotten.

Called something else where you work? R-value and U-value · Batt, quilt and insulation roll — the term in each market, how close the equivalence really is, and the standard that governs it.

Still deciding? Batt vs Blown-In Insulation · Mineral Wool vs Fiberglass Batts · Spray Foam vs Fiberglass Batts — the factors that actually differ, with no invented prices.

Already have some? How much attic will my packages of insulation batts cover?

Worked example: Loft insulation top up — step 6 of 7

The data behind it: Production rates by operation

How to calculate insulation batt in 5 steps

Area LengthThe length of the wall, floor, or attic area to insulate.
Area WidthThe width of the area to insulate.
Package coverageThe square footage one package covers, printed on the product label.
Waste Factor (%)Extra material for cuts around obstructions (pipes, wiring, joists).
Read the resultThe tool computes the result from those figures and shows the formula, its sources, and a confidence rating alongside it.

Estimated insulation batt needed by area length

Page defaults, not your figures above.

Area Length	packages
20 ft	6
30 ft	9
40 ft	12
50 ft	15
60 ft	18
70 ft	21

Frequently asked questions

Why isn't there a default coverage number I can just trust?: Batt insulation coverage per package varies a lot by R-value and thickness — an R-13 package covers a different area than an R-38 package of the same physical size. Always use your specific product's printed coverage.
Does higher R-value always mean better insulation?: Higher R-value means more resistance to heat flow, but the right R-value depends on your climate zone and where in the house you're insulating (attic, wall, floor) — check local building code recommendations.
Should I compress batts to fit a smaller space?: No — compressing batt insulation reduces its effective R-value. If a stud bay is shallower than the batt's thickness, use a batt rated for that depth instead.
What does the coverage printed on a package actually measure?: Read that before entering it. This page divides your gross length times width by the figure you give it, so if the label states the batts' own face area — the pieces laid edge to edge — then measuring over the framing counts part of the joist or stud faces as well. A batt for 16 in (406 mm) centres is made about 15 in (381 mm) wide to fit its bay snugly, so its face covers about fifteen-sixteenths of the measured area, and the remaining 6% or so then works as a hidden allowance on top of the waste field; where the label states the framed area the pack serves at its intended centres, there is no such margin.
How do I count a loft or attic laid in two layers?: Run the page once for each layer. The first layer fills the bays between the ceiling joists, sized to their depth and spacing; the second is laid across the joists at right angles, covering their tops as well, and is usually a different thickness with its own coverage per pack. Both runs use the same length and width, and adding the two counts is right where adding the two coverage figures would be wrong. In the UK both layers are loft roll or quilt; in North America the cross layer should be unfaced, because a second vapour retarder laid over the first traps moisture between them.
Is the R-value on a North American pack the same number as a UK one?: No. A North American R-value is in ft²·°F·h/Btu; UK and European labels state thermal resistance in m²·K/W, and Canadian labels print it as RSI beside the imperial figure. One metric unit is about 5.68 imperial ones, so an R-13 batt is roughly 2.3 m²·K/W and an R-38 roughly 6.7 m²·K/W. No R-value enters this count, but reading one system's number as the other's picks the wrong pack, and with it the wrong coverage figure. The R-Value to RSI Calculator does the conversion.
Why do some walls waste more batt than the percentage allows?: Because the loss follows the number of bays, not the area. North American wall batts are sold pre-cut to suit standard stud heights, so a wall of that height takes one piece per bay with little or no trimming, while a wall of any other height leaves an offcut in every bay or needs a short piece added to each. A long, low wall — a knee wall in a loft or attic room, often called a dwarf wall in the UK — has many bays for its area and loses more than a flat percentage predicts. Where roll is cut to length on site, the loss collects at the end of each roll instead; fixed-length slabs, including UK wall slabs, lose material bay by bay like pre-cut batts. Count the bays and the pieces per bay, then set the waste field from that.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.