Insulation Calculator

How many insulation batts you need

Standard fiberglass batts give about R-2.9 to R-3.8 per inch of thickness, according to DOE figures reported by Buildipedia. A 3.5 inch batt is about R-11, and a 6 inch batt is about R-19.

An insulation calculator counts how many fiberglass or mineral wool batts it takes to fill a wall. Enter the wall length and height in feet. The tool divides the area by 9.7 square feet per batt, rounds up to a whole batt, and multiplies by your price per batt.

You cannot buy a third of a batt, so the count always rounds up. That is why 32.99 shows as 33.

The formula in plain words

1. Wall area = length × height.
2. Batts = area ÷ 9.7.
3. Round up to the next whole batt.
4. Cost = batts × price per batt.

The 9.7 comes from a batt about 15 in wide and 93 in long. Those inches are 1,395 square inches, which is 9.7 square feet. The width is 15 in so the batt squeezes snugly into a stud bay spaced at 16 in on center.

Worked example: a long garage wall

A garage wall is 40 ft long and 8 ft tall. Batts cost $12 each.

• Area: 40 × 8 = 320 sq ft
• Batts: 320 ÷ 9.7 = 32.99
• Rounded up: 33 batts
• Cost: 33 × $12 = $396

Worked example: a shorter basement wall

A basement wall is 30 ft long and 9 ft tall.

• Area: 30 × 9 = 270 sq ft
• Batts: 270 ÷ 9.7 = 27.84
• Rounded up: 28 batts
• Cost: 28 × $12 = $336

Rounding added just 0.01 of a batt on the garage wall and 0.16 on this one. That is small next to what you will trim off at the top plate and around outlets.

Batts needed for a 40 ft run by wall height

Wall height	Area	Batts
8 ft	320 sq ft	33
9 ft	360 sq ft	38
10 ft	400 sq ft	42

A taller ceiling jumps the count quickly because the extra height applies across the whole length.

For other wall sizes: a 60 ft by 8 ft wall needs 50 batts, and a 100 ft by 8 ft run needs 83 batts.

How to measure

Measure the wall from the floor to the underside of the ceiling or roof deck, and across its full length. Do each wall separately, then add the batts.

For a gable end, measure the rectangle first, then add the triangle on top: half the base times the height.

Open stud bays are what you are filling. If a wall has unusually wide or narrow bays, a standard batt will leave gaps or need trimming. Batts come in several widths and lengths, so check what the package says it covers.

Batts versus other insulation types

This page counts batts, which come in precut lengths. Blown-in and spray foam are sold and installed differently, so the 9.7 figure does not apply to them. If a contractor is quoting blown-in, ask for coverage per bag at the R-value you need. If the wall is an existing finished wall, getting insulation into it is a different job than filling open studs.

When the studs are exposed, batts are the least fuss for a do-it-yourselfer. Buy one pack more than the count if the price is low, since a damaged batt in the last bay is annoying.

Common mistakes

• Buying by wall area without checking the package. Some packs list coverage in square feet, and some list the number of batts.
• Leaving gaps. Insulation that is crushed, bunched up, or missing in corners loses much of its value. Cut batts to fit snugly without stuffing them.
• Not subtracting big openings. A wall with several windows needs fewer batts than a solid wall of the same size.
• Forgetting the rim joist and attic. This tool covers walls only.
• Skipping safety gear. Fiberglass irritates skin and lungs, so wear gloves, a mask, and eye protection.

Related calculators

After insulation comes the board. The drywall calculator counts sheets for the same wall, and the paint calculator estimates the finish coat.