Enter chimney and cap details
Values are estimates; confirm manufacturer and local code.
Example data table
Use these sample rows to understand typical input ranges.
| # | Shape | Flue size | Side clearance | Overhang | Mesh | Recommended lid size | Meets target |
|---|---|---|---|---|---|---|---|
| 1 | Round | Ø 8 in | 1 in | 2 in | 1/2 in | Ø 14 in | Yes |
| 2 | Rectangular | 12 × 8 in | 1 in | 2 in | 3/4 in | 18 × 14 in | Yes |
| 3 | Square | 10 × 10 in | 1.5 in | 2 in | 1/4 in | 17 × 17 in | Varies |
Formula used
This calculator estimates cap sizing around two practical goals: (1) fit around the flue, and (2) adequate ventilation area to reduce draft restriction.
- Flue area:
- Round: A = π(D/2)²
- Rectangular/Square: A = L × W
- Oval: A = πab (a and b are semi-axes)
- Recommended net free area (NFA):
NFAmin = Aflue × (wind factor) × (spark factor)Wind factor increases targets in exposed locations. Spark factor increases targets when using an arrestor screen.
- Estimated screen NFA (conservative):
NFAest = (screen perimeter × effective screen height) × (mesh open fraction)
- Lid footprint:
- Outer screen dimension = flue dimension + 2 × side clearance
- Lid footprint = outer screen dimension + 2 × overhang
How to use this calculator
- Select your unit system, then choose the flue shape.
- Enter the inside flue dimensions measured at the top.
- Set side clearance and lid overhang for water protection.
- Choose mesh opening and spark arrestor based on fuel type.
- Adjust wind and snow exposure to add ventilation margin.
- Click Calculate cap size, then review results.
- Use the CSV and PDF buttons to save outputs.
Net free area targets and airflow margin
The calculator sizes the cap around net free area (NFA). It starts with the flue cross‑section area and increases the target for exposure and screening. Wind exposure applies a multiplier (low 1.00, moderate 1.10, high 1.25). If a spark screen is selected, the target increases again (1.40) to limit draft loss. Screen NFA is estimated from perimeter × screen height × mesh open fraction for a conservative pass/fail check.
Clearance and overhang ranges used in planning
Side clearance provides an air gap between the flue and the screen frame. A common minimum is about 1 in (25 mm), while 1.5–2 in can help on larger fireplaces. Lid overhang protects the crown and reduces rain entry; 2 in (50 mm) is typical, and 3 in may help where wind‑driven rain is frequent. For many masonry chimneys, a 6–8 in screen height balances animal exclusion, splash control, and ventilation during storms.
Mesh selection data and maintenance implications
Mesh opening changes debris control and airflow. This tool uses approximate open fractions of 60% for 1/4 in, 72% for 1/2 in, and 78% for 3/4 in screening. Fine mesh can clog faster with soot and creosote. Inspect at least twice per heating season for 1/4 in mesh, and seasonally for 1/2–3/4 in mesh.
Material choices and expected durability
Stainless steel is often selected for high moisture or coastal air because it resists corrosion and retains strength. Galvanized steel is usually lower cost but can shorten service life in humid climates. Copper weathers well and is commonly chosen for appearance, but it is typically a premium option.
Installation verification and record keeping
After measuring the liner at the top, confirm that the recommended lid footprint clears crown edges and that screen height maintains top clearance. Verify the mounting style matches the chimney construction and fasteners are rated for outdoor use. Use the CSV and PDF exports to document dimensions, assumptions, and the free‑area check for project files.
Frequently asked questions
Measure the inside dimensions at the top opening where the cap will sit. For round liners, measure inside diameter. For masonry tiles, measure both inside width and length to the nearest 1/8 in or 2 mm.
Increase screen height, choose a more open mesh, or increase side clearance. Re-run the calculation until the estimated net free area meets or exceeds the recommended target, and confirm the cap model provides similar ventilation.
Finer mesh can reduce ember size, but it also restricts flow and can clog sooner. If you select fine mesh, plan more frequent inspections and cleanings, especially for wood-burning systems that produce soot and creosote.
Top-mount caps cover the crown and can protect cracked crowns. Inside-flue caps fit within the liner and may suit tight crowns. Band or clamp mounts are common on metal chimneys. Always match the mount to the chimney construction.
Fuel affects soot and corrosion. Wood often benefits from easier-clean meshes and higher ventilation margins. Gas and oil can produce acidic condensate, so stainless material is commonly preferred. Follow the appliance manual and local requirements.
Overhang mainly affects rain protection. Draft is influenced more by net free area, screen height, and top clearance. Keep adequate clearances and ventilation openings, and avoid designs that block the exit path above the flue.