Pick a calculator
IRC R305.1 / R304: enter the floor areas you measured (a 5-ft pole and a 7-ft pole tell you where each contour falls).
IRC R310: measure the net clear opening with the window fully open.
How attic sizing works
A gable attic is a triangle in cross-section: full height only at the ridge, zero at the eaves. Building codes draw two contour lines across that triangle. At 5 feet of ceiling height, floor area starts counting toward habitable space (IRC R304.3); below 5 ft it counts as zero. At 7 feet, the space qualifies as standing room, and at least half of the required floor area must reach it (IRC R305.1 exception 1).
The setback from each eave to a height contour is simple: setback = height ÷ (pitch/12). On an 8/12 pitch, the 7-ft contour sits 10.5 ft in from each eave. Steeper pitches buy standing area fast; shallow pitches (4/12 and under) often yield no 7-ft zone at all on a typical span.
The other calculators on this page cover the rest of an attic conversion: the egress window a sleeping room needs (5.7 sq ft net clear), the 22×30 in access opening, 1/150 vent sizing, and DOE insulation R-values by climate zone. Numbers follow the International Residential Code and U.S. Department of Energy tables; always confirm with your local building department, since amendments vary.
Attic sizing questions, answered
What is the minimum ceiling height for a habitable attic?
The International Residential Code (IRC R305.1) requires at least 7 feet (2.13 m) of ceiling height in habitable space. For rooms with sloped ceilings - the classic attic case - the exception is: every part of the required floor area must be at least 5 feet high, and at least 50% of that area must reach 7 feet or higher.
How do I calculate the standing area of an attic from the roof pitch?
Use the pitch triangle: setback from each eave = height / (pitch/12). On an 8/12 pitch over a 28-ft span, the ridge is 9.33 ft high, the 7-ft contour sits 10.5 ft in from each eave, and the standing strip (7 ft+) is 7.0 ft wide. On a 40-ft-long attic that is 280 sq ft of standing area - 25% of the 1,120 sq ft footprint. Steeper pitches widen the strip fast; shallow 4/12 roofs often have no 7-ft zone at all on a normal span.
What size egress window does an attic bedroom need?
Per IRC R310, an emergency escape window in a sleeping room needs a net clear opening of at least 5.7 sq ft (820.8 sq in) - 5.0 sq ft is allowed at grade-floor openings - with a minimum clear height of 24 in, minimum clear width of 20 in, and a sill no more than 44 in above the finished floor. Measure with the window fully open; glass size does not count.
How much attic ventilation do I need?
IRC R806.2 requires net free vent area of 1/150 of the vented attic floor area. A 1,000 sq ft attic needs 6.67 sq ft (960 sq in) at 1/150. The code allows 1/300 - 3.33 sq ft (480 sq in) for that attic - only when a Class I/II vapor retarder is installed (climate zones 6-8 per the 2021 IRC) and 40-50% of the vent area is in upper vents within 3 ft of the ridge. Split intake and exhaust 50/50.
What R-value should my attic insulation be?
The U.S. Department of Energy recommends, for an uninsulated attic: R-30 to R-49 in zone 1, R-30 to R-60 in zones 2-3, R-38 to R-60 in zone 4, and R-49 to R-60 in zones 5-8. Depth in inches = target R / R-per-inch of your material. Example: zone 5 targets R-49, and blown cellulose at R-3.5 per inch needs exactly 14 in.
What is the minimum attic access opening size?
IRC R807.1 requires a rough-framed opening at least 22 x 30 in (55.88 x 76.2 cm) in a readily accessible location, for any attic of 30 sq ft or more with at least 30 in of vertical height. Above a ceiling-mounted access you need 30 in of clear headroom at some point over the opening.
How many square feet of my attic are actually usable?
Three numbers: the full footprint, the storage area (ceiling 5 ft+), and the habitable-eligible area (ceiling 7 ft+). A 40 x 28 ft attic on an 8/12 pitch has 1,120 sq ft total, 520 sq ft of storage-grade area, and 280 sq ft eligible toward habitable space. The low strips under the eaves still hold boxes - they just cannot count toward a bedroom or office.
Can I count attic space under 5-ft ceilings as living space?
No. Under IRC R304.3, portions of a room with a sloping ceiling under 5 feet do not count toward the minimum required habitable area. The 5-ft contour is where floor area starts counting; the 7-ft contour is where standing room starts.
What size pull-down attic stair do I need?
Most off-the-shelf pull-down stairs use a 22.5 x 54 in rough opening, which fits between standard 24-in on-center framing and clears the IRC 22 x 30 in minimum. Check the stair manufacturer's rough-opening spec and keep 30 in of clear headroom above the opening.