Dormer Volume Calculator
This dormer volume is a stated model: a rectangular box plus a triangular roof on the same plan. Floor area is width times projection. The box is that area times the front wall height. The roof is that area times the roof rise, divided by two. It is an enclosed volume for planning materials roughly. It is not a rafter design, not a snow load, and not a cost.
Last reviewed: 2026-10-28 · How we calculate
How we calculate
Floor area = width × projection. Box volume = floor area × front wall height. Roof volume = floor area × roof rise ÷ 2. Total volume = box + roof. The roof is a prism with a triangular section, not a hipped roof and not a flat roof.
Worked example
A 8 × 4 ft dormer with a 4 ft wall and a 2 ft rise is 32 sq ft of floor and 5.93 yd³.
A box dormer on an existing roof
A box dormer on a rear roof is a floor, a front wall, and a roof volume you can price as timber and sheathing elsewhere. This page only reports the floor area and the enclosed volume. A cheek wall that is mostly glass should not be treated as a solid front wall of the same height. Measure the projection from the existing roof face to the new front, not from the ridge.
Measuring a box dormer
A dormer volume calculator has to say which dormer it means. This one means a box stuck out of the roof with a triangular roof on top, like a doghouse dormer with a vertical front and a simple gable whose ridge runs back to the main roof. Width is cheek to cheek. Projection is the horizontal throw from the main roof face to the front wall, not the length of the cheek measured along the slope. Front wall height is only the vertical face. Roof rise is how far the ridge sits above that front wall. The box volume is width times projection times front wall height. The roof volume is the same plan times the rise, halved, which is the volume of a triangular prism. Add them. The worked example is 2 times 1.5 times 1.2, which is 3.6, plus 2 times 1.5 times 0.8 divided by 2, which is 1.2, and the total is 4.8 cubic metres. If your roof rise is 0, the roof term is 0 and you have a flat-topped box.
How the dormer volume is built
Do not put the ridge height into the front wall field. If the front wall is 1.2 m and the ridge is 0.8 m above it, the front wall field is 1.2 and the rise is 0.8. Typing 2.0 as the wall and 0.8 as the rise counts the triangle twice and invents a taller box. A shed dormer with a single sloping roof is not this triangle. You can approximate a shed by a box at the average height, and you should say you did that, because this page will not switch models for you. Cheeks are the triangular or trapezoid side walls. Their area matters for cladding and for sheathing, and they are not a separate output. A cheek that is a right triangle of projection by front-wall-plus-rise can be measured for the siding or roofing pages. This volume already includes the space beside the cheek. It does not tell you the cheek's surface area.
Ordering from a dormer volume
Use the floor area line when you want boards, insulation area, or paint on the new floor and front wall. A 3 square metre floor is small, and it is still the number to take to the square-footage calculator if you are adding several dormers: run this page once per dormer and add the floor areas. Insulation bags need a coverage you type on the insulation page. The R-value label there does not change the bag count. Drywall sheets need the board area, which for the front wall is width times front wall height, plus the ceiling if you board the underside of the dormer roof. That underside is a slope. Its area is larger than the plan area. The roofing calculator's pitch factor is the right tool for the sloping surface. Do not divide this cubic-metre result by a sheet size.
Checking the dormer figure
Nothing here is a cost, a beam size, or a planning approval. Headroom rules, stair access to the new space, and the structure of the existing rafters are outside a four-field volume. If you are lining the dormer, take measured areas to drywall, paint, and insulation after you have opened the roof and confirmed the projection. A planned 1.5 m projection that lands at 1.4 m because of a purlin is a different volume, and the formula will not know. Re-enter the taped sizes. If you only wanted the air volume for a rough sense of the space, 4.8 cubic metres in the example is the whole model: box plus triangle, with no waste percentage because a volume of air is not ordered with waste.
Dormer fields and starting values
Width starts at 8 ft. Dormer width in feet. Switch to m / cm for metres. Projection starts at 4 ft. Projection in feet. Switch to m / cm for metres. Front wall height starts at 4 ft. Front wall height in feet. Switch to m / cm for metres. Roof rise starts at 2 ft. Roof rise in feet. Switch to m / cm for metres.
Box dormer floor area and volume
Volume is the front-wall box plus a triangular roof of the same plan. It is the space, not the timber list.
| Plan | Front wall | Floor | Volume |
|---|---|---|---|
| 2.4×1.2 m | 1.2 m | 2.88 m² | 4.32 m³ |
| 3×1.5 m | 1.4 m | 4.50 m² | 8.10 m³ |
| 3.6×1.8 m | 1.5 m | 6.48 m² | 12.64 m³ |
A shed dormer and a gable dormer do not share this roof triangle. Measure the one you are framing.
Common mistakes
- Reading the volume as a rafter-length or a roofing-square count. Roof area from pitch is the roofing calculator.
- Putting the whole height, wall plus roof, into the front wall field and also typing a rise. That double-counts the triangle.
- Using the model for a hipped dormer or a curved roof. The roof term is a simple triangular prism.
- Treating the volume as heated-floor area. Floor area is the separate square-metre line.
- Expecting studs, slates, or a price. None of those are in the formula.
- Measuring projection along the slope instead of horizontally out from the roof.
Frequently asked questions
How do you calculate dormer volume?
Multiply width by projection by the front wall height, then add width by projection by the roof rise divided by two. A 8 × 4 ft dormer with a 4 ft wall and a 2 ft rise is 32 sq ft of floor and 5.93 yd³.
What does the triangular term mean?
It is a prism whose cross-section is a triangle of the same width and projection as the floor, and whose height is the roof rise. A hip, a barrel, or a flat roof with a parapet is a different shape.
Is this the floor area of the dormer?
Floor area is width times projection and is shown on its own line. Volume is that area times the heights described above.
Can I use this for insulation?
You can take the areas to the insulation calculator, which needs a coverage per bag you type. The cubic metres here are not a batt count.
Does this meet building rules?
No. It does not check headroom, fire separation, or structure. It is a volume model only.
Where do I count shingles for the dormer roof?
On the roofing calculator, using the actual roof surface. This triangular volume is not the sloping area.