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.