What problem this solves
Generic fan selectors use blade span as a proxy for cooling power: bigger blade = bigger room. But the actual metric is air flow, measured in CFM (cubic feet per minute), and the right CFM depends on room volume and target ACH (air changes per hour). A 12 m² room with a 2.4 m ceiling needs less CFM than the same room with a 3.6 m ceiling. The Energy Star blade-span ranges are useful but not definitive; ACH-based sizing is more accurate.
Sloped ceilings add another layer: a fan mounted on a slope sits closer to one side of the room than the other, which changes effective coverage. A downrod (or angled mount) compensates, and the right length depends on the slope. The calculator handles this.
Who it’s for
- Designers specifying ceiling fans for new builds
- Homeowners replacing an under-sized fan in a bedroom
- Kitchen designers adding a fan over an island or cooktop
- Workshop designers picking an industrial fan
Why ACH matters
ACH is the standard HVAC metric for ventilation: how many times the room’s air volume is moved per hour. A residential living room at ACH 4 feels comfortable; a workshop at ACH 4 feels stuffy because the air carries more heat, moisture, and fumes. The calculator picks the right ACH for the room use and computes the CFM accordingly.
Limitations
- ACH is a guideline, not a measured standard; real cooling depends on insulation, sun exposure, occupant count.
- Blade span recommendations are Energy Star ranges; high-performance fans may cover more area per span.
- Sloped ceiling math assumes the slope is uniform; vaulted ceilings with multiple slopes need separate analysis.
- Existing fan CFM is approximate; manufacturer specs vary.
- No smart-fan or DC-motor adjustment — these move more air per watt than AC motors.
Practical rules of thumb
For a comfortable bedroom, target 50–80 CFM per m² of room area at ACH 4–5. For a kitchen with a cooktop, target ACH 6 to clear heat and smells. For a workshop, ACH 8 or higher. Always mount the fan so blades are at least 2.1 m above the floor and 60 cm below the ceiling (or higher). For sloped ceilings, use a downrod long enough to keep the blades level.