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Chimney Cowl Size Guide Pot Sizes & Cowl Sizing: Measure Once, Buy Right
chimney cowl size guide: measure once, buy right
Two numbers matter: your chimney pot outside diameter (OD) and, if you have a stove liner, the liner diameter. Get these right and your cowl will fit perfectly and perform as it should.
Strap-&-band cowls size to the pot OD. Pot-hanger cowls size to the liner diameter + strap range for the pot.
How to measure your chimney pot (OD)
- Use a tape to measure the outside diameter across the very top rim of the pot.
- If the pot is out of round or has a flared lip, take two measurements at right angles and use the larger.
- No tape? Measure the circumference with string, then compute diameter: D = C / π. Example: C = 785 mm → D ≈ 785/3.142 = 250 mm.
- Note any unusual profiles (roll-top, square, crown). Select cowls with suitable bases/straps.
Pot size → choose the right strap & band range
| Common pot OD (mm) | Approx inches | Typical universal strap range* | Recommended cowl types |
|---|---|---|---|
| 200 | ≈ 8" | ~180–230 mm | Rain Cap, Bird Guard |
| 225 | ≈ 9" | ~200–260 mm | Anti-Downdraught, Rain Cap |
| 250 | ≈ 10" | ~230–300 mm | Anti-Downdraught, Spinning |
| 285 | ≈ 11¼" | ~260–320 mm | Spinning, custom/large strap sets |
*Ranges vary by model—always check the product page. If your measurement sits on a boundary, choose the range that provides mid-band clamping, not the extreme.
Have a stove liner? Size the spigot to the liner
Standard liner sizes (UK)
| Liner size | Inches | Use a pot-hanger cowl with: |
|---|---|---|
| 125 mm | 5" | 125 mm spigot (e.g., liner pot-hanger) |
| 150 mm | 6" | 150 mm spigot (popular wood-burner size) |
| 180 mm | 7" | 180 mm spigot (open fires/large appliances) |
| 200 mm | 8" | 200 mm spigot |
Don’t forget the pot OD
Pot-hanger cowls also clamp to the pot. Match the spigot to your liner, and make sure the strap/band range suits your pot OD.
Metric ↔ imperial quick conversions
| mm | Inches (approx) | mm | Inches (approx) |
|---|---|---|---|
| 150 | ≈ 6" | 250 | ≈ 10" |
| 175 | ≈ 7" | 275 | ≈ 10.8" |
| 200 | ≈ 8" | 300 | ≈ 11.8" |
| 225 | ≈ 9" | 325 | ≈ 12.8" |
Choose by problem (and size it right)
Smoke blow-back (wind)
Pick an anti-downdraught cowl (size to pot OD). Exposed/coastal? Consider a spinning cowl.
Birds & debris
A bird guard sized to pot OD keeps wildlife out while maintaining airflow.
Rain ingress
Use a rain cap sized to pot OD, or a pot-hanger version for lined stoves.
Stove with liner
Choose a liner pot-hanger with the correct spigot for your liner and a strap range for your pot.
Recommended best-sellers
Anti-Downdraught Cowl Bird Guard Cowl Rain Cap Cowl Spinning/Rotary Cowl Liner Pot-Hanger Cowl
FAQs
Do I measure inside or outside the pot?
For strap-&-band cowls, measure the outside diameter (OD) across the top rim. For pot-hanger cowls, match the spigot to your liner size and still check the pot OD for the strap range.
What if my pot is square or crowned?
Measure the widest points. Many cowls accommodate slight irregularities, but extreme shapes may need a specific base—contact us if unsure.
Metric or imperial—does it matter?
Either works. Use the conversion table above or remember: 1" ≈ 25.4 mm. Choose the product whose range clearly covers your figure.
Is bigger always better?
No. Oversized fittings can be insecure; undersized will not clamp. Aim for a comfortable mid-range fit and keep the cowl’s free exit area ≥ flue area.
