Flue size by fireplace opening (the 1/10 rule)
This is our own flue-sizing matrix and the moat behind the site. For every fireplace opening it gives the required round flue area and diameter, the required rectangular area, and the nearest standard round liner or clay flue tile, all by the timeless 1/10 rule (round ≥ opening ÷ 12, rectangular ≥ opening ÷ 10) and plane geometry (A = π·(d/2)²). It is a dated snapshot of stable geometry and published planning conventions, not a live feed. See how it’s derived in the methodology, and use it with the fireplace-flue-size calculator, the round↔rectangular converter and the how-to-measure-flue-size reference.
| Opening (W × H) | Opening area (in²) | Round flue ≥ (in²) | Round diameter (in) | Rect. flue ≥ (in²) | Nearest standard flue |
|---|---|---|---|---|---|
| 24 × 24 | 576 | 48.0 | ~8 | 57.6 | 8" round / 8x12 tile |
| 30 × 29 | 870 | 72.5 | ~10 | 87.0 | 10" round / 12x12 tile |
| 36 × 30 | 1,080 | 90.0 | ~12 | 108.0 | 12" round / 12x16 tile |
| 42 × 32 | 1,344 | 112.0 | ~12 | 134.4 | 12" round / 16x16 tile |
| 48 × 36 | 1,728 | 144.0 | ~14 | 172.8 | 14–15" round / 16x20 tile |
LABELED published planning snapshots — confirm your exact flue and opening dimensions and follow NFPA 211. A round flue must be at least 1/12 of the opening, a rectangular one at least 1/10; always round up to the next standard liner or tile. Snapshot: 2026-07-14.
Sources
These are LABELED planning conventions that follow published primary sources; confirm the current text with each body and with your local code.
- NFPA — NFPA 211, Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances, the basis for flue sizing and the 3-2-10 height rule.
- ICC — International Residential Code, Chapter 10 (Chimneys and Fireplaces), which sets the minimum flue area relative to the fireplace opening.
- Brick Industry Association — Technical Notes on Brick Construction (residential fireplaces and chimneys), for the 1/10–1/12 rule and standard clay flue-tile sizes.