Outdoor Range Hood CFM Calculator

This calculator implements the plume-mass-flow sizing procedure of RB-008: CFM Requirements for Outdoor Cooking Ventilation — Appendix A, steps 4 through 8. It computes the exhaust airflow required to ingest the whole plume at the hood from the cooking source, the mounting height, the mount type, and the wind exposure class. Hood width is not an input: RB-008 §3.4.3 and Table 3.10 show that a wider hood needs slightly less airflow, not more, while a hood narrower than about 80% of the recommended width cannot be rescued with airflow at all [RB-008 §3.10]. The calculator reports the width check separately.

Reference readings

Representative values below are computed directly from the same sizing function (physics/cfm.mjs) driving the calculator above; live values update as you move the controls. Both tables are regenerated by scripts/generate-reference-tables.mjs, never typed by hand.

The primary answer — 60,000 BTU gas grill at a 30-inch mounting height (RB-008 §3.3):

Minimum CFM by source and mounting height — wall mount, moderate exposure (RB-008 Table 3.2b):

A 60,000 BTU gas grill at a 30-inch wall mount needs a minimum of 892 CFM under moderate wind exposure and a 1,200 CFM blower — the smallest standard size that clears the minimum by RB-008’s 10% selection margin [RB-008 §3.3, App. A step 8]. Sheltered sites need 727 CFM (900 CFM blower); an exposed site with side panels needs 1,004 CFM (RB-008 v1.0 printed 1,003, a rounding difference v1.1 corrects), and an exposed site without panels 1,394 CFM. For that last case RB-008 v1.0 printed a 1,500 CFM blower, only 8% above the minimum; v1.1 corrects it to 1,800 CFM under the paper’s own 1.1× rule from Appendix A step 8, which is the rule the calculator applies — and notes that capture is limited to about 60% at that exposure regardless [RB-008 §3.3]. An island mount multiplies the minimum by 1.20 — 892 becomes 1,070 CFM, still served by a 1,200 CFM blower [RB-008 §3.9]. Along the rows of the second table, the requirement grows as z^(5/3) with mounting height — for the 60,000 BTU grill it climbs from 475 CFM at 18 inches to 1,775 CFM at 48 — but only as Q_c^(1/3) with source strength, so the step from a 40,000 to a 60,000 BTU grill adds about 19% [RB-008 §2.4].

Methodology

The calculator follows RB-008 Appendix A:

1. Source. The convective heat release Q_c and virtual origin of the chosen source are taken from RB-001 Tables 3.1 and 3.2 (eight representative sources: four gas classes, a charcoal kettle, a wood-fired grill, and two pellet-smoker settings) [RB-001 §3.1–3.2].

2. Bare plume flow at the hood is the Heskestad mass flow ṁ = 0.071 Q_c^(1/3) z^(5/3) + 0.0018 Q_c at the mounting height z, converted to volume at the plume-average density of 1.10 kg/m³ [RB-008 §2.1]. This is the physical floor: a hood exhausting less than this cannot capture the whole plume regardless of geometry [RB-008 §3.1].

3. Wind exposure multiplier. The bare flow is multiplied by K_CFM = F_inf × F_wind × F_safety: 3.0 sheltered, 3.68 moderate, 4.14 exposed with side panels, 5.75 exposed without [RB-008 §2.2]. F_inf = 2.0 is the makeup-air infiltration factor from RB-003 Appendix D; F_wind is the class-representative wind factor (1.3 / 1.6 / 1.8 / 2.5); F_safety = 1.15.

4. Mount multiplier. Wall 1.00, peninsula 1.10, island 1.20, applied to the table value [RB-008 §3.9] — the open-sides infiltration, omnidirectional wind exposure, and loss of wall attachment an island hood must make up with airflow instead of a wall [RB-005 §3.4.3].

5. Blower selection. The smallest standard size at least 1.1× the minimum for the selected exposure class [RB-008 App. A step 8, §3.3]. RB-008 §3.11 goes one step further, recommending that even a Sheltered installation take a blower sized to the Moderate-exposure value to allow for occasional breezes; the calculator does not apply that stricter rule — it sizes to the class you select — and records it here as the paper’s recommendation. RB-008 lists 600/900/1200/1500 CFM as common residential ratings; the calculator extends that ladder upward (1800/2100/2400/3000) so the rule can be applied honestly above 1,364 CFM.

6. Width check (advisory only). Hood width is compared with the RB-002 recommended width for the source and height (W_rec = 1.38 × capture diameter) and reported as an RB-008 Table 3.10 coverage band; it never changes the CFM readout [RB-008 §3.10; RB-002 §3.6].

Why the answer is lower than the familiar rules of thumb: the “1 CFM per 100 BTU” and face-velocity methods treat the hood as something that must reach out and pull a passive contaminant in. A buoyant plume delivers itself at 200–535 fpm; the hood’s job is volumetric ingestion, and RB-003 §4.1 shows a properly sized outdoor hood can run a face velocity of about 29 fpm — “grossly inadequate” by the indoor criterion — while capturing correctly [RB-003 §2.5, §4.1].

Source Papers

• RB-001: Buoyant Plume Behavior — Source heat release rates, convective fractions, virtual origins
• RB-003: Velocity Decay and Capture — Why CFM, not face velocity, is the specification (§2.5, §4.1); F_inf (App. D)
• RB-005: Hood Geometry and Capture — Island premium range behind the mount multipliers (§3.4.3)
• RB-006: Wind Interaction and Crossflow — Wind exposure classes
• RB-008: CFM Requirements — Sizing methodology (§2, App. A), primary answer (§3.3), mount adjustment (§3.9), width effect (§3.10)