Attic Volume Calculator
An attic looks like empty space until you need a number for it: how much insulation to order, what size fan will actually clear the hot air, whether a storage plan fits. Most attics are triangular prisms, not boxes, and the math has one step people forget. Forget it and the answer doubles. This attic volume calculator gets the geometry right for a gable roof and then sizes the attic fan the same space needs, with the inlet vent area to match.
The tool is pre-filled with a typical gable attic: 30 ft long, 20 ft wide, 10 ft from floor to peak. That is a floor area of 600.00 sq ft and a volume of 3,000.00 cubic feet, not the 6,000 a box calculation would claim. On the fan tab, the same floor needs a 420.00 CFM fan on a standard roof and 1.40 sq ft of inlet vent area. Every number in the sections below matches those outputs.
Rule-of-thumb sizing used across the fan industry. Confirm final selection with a ventilation professional for powered installations.
Volume assumes a gable roof with a symmetric triangular cross-section. Hip, gambrel, and mansard roofs need the adjustments described in the roof-shape section below.
Quick Answer
Key takeaways
- Gable attic volume is half the box volume: 1/2 x length x width x peak height. The half is the whole point.
- Measure peak height straight up from the floor. A tape run along the rafter is longer and inflates every downstream number.
- Fan sizing runs off floor area, not volume: area x 0.7 CFM, plus 20% for steep roofs and 15% for dark roofs.
- A fan is only as good as its inlet. Budget 1 sq ft of net free inlet area for every 300 CFM, or the fan pulls conditioned air out of the house instead.
How to measure your attic in four steps
Measure the floor and the peak, in that order, and the tool does the rest. Attic measurements go wrong in predictable places, so each step below names its trap.
Length. Gable end to gable end along the attic floor. On most houses this is simply the long side of the house footprint.
Width. Eave to eave across the floor. If the house has an L-shape or an attached garage roof, measure each rectangular section separately and add the volumes.
Peak height. Stand at the center of the attic and measure straight up to the ridge. Not to the collar ties, not along a rafter. For the worked attic that vertical is 10 ft, while the rafter slope itself runs close to 14 ft: using the slope figure would push the volume from 3,000 to roughly 4,200 cubic feet.
Run both tabs. Volume first for insulation and storage questions, then the fan tab with the same floor dimensions for ventilation questions.
The gable volume formula and why the half matters
The formula is triangle geometry, nothing more. Slice a gable attic across its width and you see a triangle sitting on the attic floor. That triangle extruded along the length is the attic.
The doubling error deserves its own warning because it is the most common attic mistake on the internet. Generic room-volume tools multiply length by width by height and stop. Applied to the worked attic they return 6,000 cubic feet. Every purchase built on that figure is roughly twice too big: twice the loose-fill insulation, a fan selected for a space that does not exist, storage plans drawn for air that is actually roof slope. If a number for a gable attic ever comes from a plain box formula, halve it before you spend money on it.
Worked example: the 30 by 20 attic, both tabs
| Figure | Calculation | Result |
|---|---|---|
| Floor area | 30 x 20 | 600.00 sq ft |
| Attic volume | 1/2 x 30 x 20 x 10 | 3,000.00 cubic feet |
| Box formula (wrong for gable) | 30 x 20 x 10 | 6,000 cubic feet |
| Fan, standard roof | 600 x 0.7 | 420.00 CFM |
| Fan, steep roof | 420 x 1.20 | 504.00 CFM |
| Fan, dark roof | 420 x 1.15 | 483.00 CFM |
| Fan, steep and dark | 420 x 1.20 x 1.15 | 579.60 CFM |
| Inlet vent area (standard) | 420 / 300 | 1.40 sq ft |
Notice the fan numbers never touch the 10 ft peak height. Powered fan sizing in the industry runs off floor area because the 0.7 factor already bakes in a typical attic height and a target of roughly ten air changes per hour. Volume still matters for passive questions: insulation coverage is sold by the cubic and square foot, and the same floor-area logic drives material estimates on projects costed with a concrete volume calculator or earthwork planned with a berm volume calculator, where shape factors decide the answer the same way the triangle factor does here.
Roof shapes this tool does not pretend to cover
The tool models one shape honestly: a symmetric gable. Other common shapes need a hand adjustment, and pretending otherwise would sell false precision.
Adjust it yourself
- Hip roof: Broan's published guidance uses length x width x height x 1/3 in place of 1/2, because a hip roof slopes on all four sides. Run the volume tab, then multiply the result by two-thirds for a close estimate.
- Gambrel or mansard: split the cross-section into a rectangle (floor width times wall height) plus the triangle above it, compute both volumes, and add them.
- Uneven or L-shaped plans: compute each rectangular section as its own attic and add the volumes.
Leave it to the gable formula
- Standard two-slope gable roofs, symmetric or close to it.
- Attics where the peak runs the full length of the house.
- Quick fan sizing, which needs floor area only and works for any shape with a measurable floor.
Ventilation code is a separate check from fan sizing. The International Residential Code asks for net free ventilating area of at least 1/150 of the vented space, with a 1/300 exception when vapor retarder and balanced vent placement conditions are met. For the 600 sq ft worked floor that is 4.0 sq ft of total net free area at 1/150, split between low intake and high exhaust. A powered fan does not replace that passive requirement, and ENERGY STAR warns that a fan in a leaky, poorly vented attic can pull cooled air out of the living space and raise summer bills. Sizing air movement for a room or duct run uses similar area thinking in an air volume calculator, and whole-project budgeting often pairs these figures with a build cost per square foot calculator.
What competing calculators miss
Search results for this topic split into two camps that never meet. Fan manufacturers and installation guides (Broan documentation, EngineerFix, solar fan sellers) explain CFM sizing well but offer no interactive volume tool, leaving readers to hand-run a formula printed in a PDF. Generic volume calculators offer the tool but run the box formula, which is simply wrong for a gable attic by a factor of two. Almost none of them mention inlet vent area, which decides whether an installed fan helps or backfires.
This page closes those three gaps in one place: correct triangle geometry for volume, the industry 0.7 CFM rule with the steep and dark roof adjustments the better guides describe, and the CFM/300 inlet figure printed next to every fan result. It is also honest about its boundary: hip and gambrel roofs get a documented hand adjustment in the open, not a silent gable assumption.
Frequently asked questions
For a gable roof: volume = 1/2 x length x width x peak height. The cross-section is a triangle, and the triangle area (half of width times height) runs the full length of the house. For the worked attic, 1/2 x 30 x 20 x 10 = 3,000.00 cubic feet, with a floor area of 600.00 sq ft.
Because the attic cross-section is a triangle, and a triangle has half the area of the rectangle around it. Skip the half and you get the box volume, 6,000 cubic feet for the worked attic, which is double the real air volume. That single error doubles insulation estimates and oversizes any volume-based equipment choice.
Measure straight up from the attic floor to the highest point of the roof, perpendicular to the floor. Do not measure along the sloped rafter. The rafter is the hypotenuse of the triangle and is always longer than the true height, so a rafter measurement inflates the volume in proportion.
The calculator models a gable roof. For a hip roof, manufacturer guidance from Broan uses roughly one-third of length x width x height instead of one-half. For gambrel and mansard shapes, split the cross-section into a rectangle below and a triangle on top, compute each part, and add them. State which method you used when you hand the number to a contractor.
Multiply floor area by 0.7 for base CFM, add 20% for a steep roof and 15% for a dark-colored roof, then provide 1 sq ft of inlet vent area per 300 CFM. For a 600 sq ft floor that is 420.00 CFM and 1.40 sq ft of inlet area on a standard roof. The Fan size tab runs the same steps.
No. The volume tab accepts feet or meters, but the fan tab works in feet only, because the 0.7 CFM rule and the inlet rule are imperial industry conventions. Convert your floor dimensions to feet first if your house was measured in meters.
Insulation quantities, heating and cooling load estimates, attic fan sizing, storage planning, and checking whether a conversion meets minimum ceiling-height rules. Each use tolerates a different amount of slop: a rough volume is fine for storage, while insulation and fan purchases deserve the measured version.
Yes. The formula counts pure geometry. Insulation depth, trusses, beams, and duct runs all occupy part of that space, so usable storage volume is smaller than the geometric volume, especially near the eaves where the triangle narrows to nothing.
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References
- ENERGY STAR (US Environmental Protection Agency and Department of Energy), "About Attic Ventilation" in the DIY Sealing and Insulating Guide - explains natural and fan ventilation, ice dam prevention, and why attic fans need sealed, vented attics to help rather than hurt. ENERGY STAR: About Attic Ventilation. Accessed October 2026.
- IIBEC (International Institute of Building Enclosure Consultants), "Attic Ventilation 101" - states the IRC minimum net free ventilating area of 1/150 of the vented space and the 1/300 exception conditions. IIBEC: Attic Ventilation 101 and IRC R806. Accessed October 2026.
- US Department of Energy, Building Science Education Solution Center, "Blown Insulation in Existing Attics" - installation guidance for vented attics, baffles, and code-level insulation. DOE Building Science: Blown Insulation in Existing Attics. Accessed October 2026.
- ENERGY STAR, "A Do-It-Yourself Guide to Sealing and Insulating with ENERGY STAR" (full guide PDF) - attic insulation levels, natural ventilation, and attic fan cautions. ENERGY STAR: DIY Sealing and Insulating Guide. Accessed October 2026.
- Broan-NuTone, powered attic ventilator documentation via Mans.io - gives attic volume by roof style (gable length x width x height x 1/2, hip x 1/3) and the 10 air changes per hour CFM method with a worked example. Broan: Attic Ventilation and Volume Formulas. Accessed October 2026.
- EngineerFix, "How to Choose and Install Gable Fans for Your Attic" - documents the 0.7 CFM per sq ft rule, the 15% dark roof and 20% steep roof increases, and 1 sq ft of intake per 300 CFM. EngineerFix: Choosing and Installing Gable Fans. Accessed October 2026.
- Quality Edge, "Science of Soffit Ventilation" - works the 1/150 rule by hand and explains balanced intake and exhaust using Department of Energy guidance. Quality Edge: Science of Soffit Ventilation. Accessed October 2026.