Chimney-Cap Size Calculator
A cap has to guard the flue without choking it — find the minimum screen open area so the draft stays free.
1 Enter your numbers
A cap keeps rain, animals and sparks out, but its screen must have at least as much open area as the flue or it strangles the draft. Your 8.0" flue is 50.3 in², so choose a cap whose mesh net free area meets or beats that, sized to slip over the flue’s outside dimensions.
A chimney cap is the cheapest, most useful thing on a chimney — it keeps out rain, animals and sparks. But a cap has a job it can quietly fail at: its mesh screen has to pass at least as much open area as the flue, or it throttles the very draft the chimney worked to build. A tight, clogged or undersized screen is a smoke complaint waiting to happen.
Enter the flue diameter and the tool gives the minimum net free vent area the cap has to clear — the actual open area of the mesh, after the wires. Size the cap body to the flue’s outside dimensions so it slips over the tile or liner, and pick a screen whose net free area meets or beats the flue cross-section. Same circle geometry as the rest of the flue math.
Formula
Match the open area to the flue:
- Flue cross-section
A = π × (d ÷ 2)² - Cap net free vent area
≥ A - Cap body sized to the flue’s outside dimensions (to fit over the tile/liner)
Net free area is the open area of the screen only — it is smaller than the screen’s outer size because the mesh wires take up room.
Worked example
Cap a 12-inch round flue. Its cross-section is π × 6² = 113.1 in², so the cap’s screen has to offer at least 113 in² of net free area — not the cap’s outside footprint, the actual holes. A tall cap with generous mesh clears this easily; a squat cap with a fine screen might only net 80–90 in² and would pinch the draft. Then size the body to the 12″ tile’s outside so it clamps on.
Buy by net free area, keep the mesh clear
Net free area, and keep it clean. The spec that matters is the screen’s open area, and it only counts when the mesh is clear. Watch for:
- Buying by the cap’s outside size instead of its net free area — ask for the open-area number.
- A fine spark-arrestor mesh that clogs with soot; on a wood flue it needs periodic cleaning.
- Multi-flue chimneys — a full-width cover must clear the total area of every flue under it.
Reference table
The minimum net free vent area a cap screen must clear for each standard flue. Buy a cap that meets or beats the number for your flue — a fine mesh with too little open area throttles the draft.
| Flue diameter | Min cap net free area |
|---|---|
| 6″ round | ≥ 28.3 in² |
| 7″ round | ≥ 38.5 in² |
| 8″ round | ≥ 50.3 in² |
| 10″ round | ≥ 78.5 in² |
| 12″ round | ≥ 113.1 in² |
| 14″ round | ≥ 153.9 in² |
Frequently asked questions
What size chimney cap do I need?
Size the body to your flue’s outside dimensions so it fits over the tile or liner, and pick a screen whose net free vent area is at least the flue cross-section — for an 8″ round flue that is about 50 in².
What is net free vent area?
The actual open area of the cap’s screen after the mesh wires are subtracted — not the cap’s outside footprint. It is the number that has to meet or beat the flue area so the draft stays free.
Can a chimney cap hurt the draft?
Yes, if the screen is too small or clogged. A cap that nets less open area than the flue throttles the exhaust. Choose a generous mesh and clean it, especially on a wood-burning flue.
How do I cap a chimney with several flues?
Either a separate single-flue cap on each tile or one full-width cover. A full-width cover’s screen has to clear the combined area of all the flues beneath it, not just one.