Cupola Size Calculator
Enter the length of your roof ridge in feet, choose the building type and give the roof pitch. You get a cupola base width, the nearest stock size, the height and a weathervane length that looks in proportion.
- Proportion rules
- Stock sizes
- Roof pitch base cut
Measure the ridge without dormers or breaks. For a roof with two ridges, size for the one the cupola sits on.
Most cupolas end up too small. From the ground, a cupola looks much smaller than it is. The widely used rule is 1 to 1.5 inches of cupola width for every foot of uninterrupted ridge. A garage with a 24 foot ridge suits a cupola about 30 inches wide, not the 18 inch model that looks big in the shop.
Cupola size chart
Base width by ridge length. Search for your ridge length or a cupola size.
| Ridge (ft) | House (in) | Garage (in) | Barn (in) | Stock size |
|---|---|---|---|---|
| 12 | 12 | 15 | 18 | 18 in |
| 16 | 16 | 20 | 24 | 22 in |
| 20 | 20 | 25 | 30 | 26 in |
| 24 | 24 | 30 | 36 | 30 in |
| 30 | 30 | 38 | 45 | 36 in |
| 36 | 36 | 45 | 54 | 42 or 48 in |
| 40 | 40 | 50 | 60 | 48 in |
| 50 | 50 | 63 | 75 | Custom |
Stock cupolas are usually square and sold in widths of 18, 22, 24, 30, 36 and 42 inches. Larger sizes are often made to order.
Ridge length and cupola width
Three proportion rules side by side. Slide the ridge length.
Choose the bolder end when the building is two storeys or sits far back from the road. Choose the smaller end for single-storey houses seen up close.
How to size a cupola
Measure from the ground, then check from the street. Step through it.
Measure the ridge
Use the roof plan, or measure the gable end walls and allow for overhang.
Before buying, cut a cardboard box to the size, put it on the ridge and look from where people will see it. It is the most reliable test.
Louvered, windowed or vented
Cupolas can be decoration or ventilation. Compare the main types.
Angled slats let air out while keeping rain off. Popular on barns and garages.
Glass panels let light into the space below and look good from the street.
Purely decorative. The cheapest choice and the easiest to seal.
Louvered cupolas work as roof vents for barns and stables, but they only help if the space below is open to them. Check with your roofer about flashing and airflow.
Before you install
A cupola sits on the most exposed part of the roof. Tick each point.
Tick each item as you go.
A cupola with a weathervane can catch a lot of wind. Bolt it through the base to the roof framing, not just to the sheathing.
What the calculator gives you
All the sizes for a balanced look. Each tile explains one result.
Calculated from your ridge length.
The next standard size up.
About 1.5 times the base width.
Your roof pitch as a ratio and an angle.
A vane about 90% of the cupola width.
Height ratios are guides, not rules. Taller roofs with steep pitches can carry a taller cupola, while low-pitched modern roofs look better with a squat one.
Frequently Asked Questions
No question matches that search.
What size cupola for a 24 foot garage?
About 24 to 36 inches wide. 30 inches suits most two-car garages.
What is the rule for cupola size?
1 to 1.5 inches of cupola width per foot of uninterrupted roof ridge.
How tall should a cupola be?
About 1.5 times its base width, not counting the weathervane.
Is it better to go too big or too small?
Slightly too big. Cupolas look smaller from the ground than they do up close.
What size weathervane goes on a cupola?
An arrow or figure about 80% to 100% as long as the cupola is wide.
Do cupolas provide ventilation?
Louvered ones do, if the roof space below is open to them. Windowed and solid ones are decorative.
How do I match a cupola to my roof pitch?
Order the base cut to your rise per 12, such as 6/12. The calculator shows the angle in degrees too.
Give the roof a crowning detail
Measure the ridge, pick the look you want and round up to a stock size. Check it with a cardboard mock-up from the street, match the base to your pitch, and the cupola will look like it was always meant to be there.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.