Inputs
Example data
| Scenario | Area | Height | ACH | Hood | Heat input | Recommended exhaust | Suggested duct |
|---|---|---|---|---|---|---|---|
| Small residential | 120 ft² | 9 ft | 15 | 30 in wall, medium | 35,000 BTU/h | ~450 CFM | ~7.0 in round |
| Medium prep kitchen | 250 ft² | 10 ft | 25 | 60 in wall, heavy | 90,000 BTU/h | ~1,150 CFM | ~10.5 in round |
| Compact metric example | 18 m² | 2.7 m | 20 | 90 cm island, medium | 12 kW | ~850 CFM (≈1,440 m³/h) | ~9.0 in round |
Formula used
- ACH method: CFM = (Area × Height × ACH) ÷ 60
- Hood method: CFM = HoodWidth(ft) × Rate × CaptureFactor
- Heat method: CFM ≈ (BTU/h ÷ 100) × CaptureFactor
- Recommendation: max(methods) × (1 + SafetyFactor%)
- Duct sizing: DuctArea(ft²) = CFM ÷ Velocity(fpm)
How to use this calculator
- Enter kitchen area and ceiling height to enable the ACH method.
- Select hood type and intensity, then add hood width for capture sizing.
- Optionally add total heat input to compare the heat-based estimate.
- Set capture factor and safety factor to reflect real site conditions.
- Review the recommended exhaust, make-up air, and duct suggestion.
- Download CSV or PDF to share the result with your team.
Airflow targets tied to room volume
Start with air changes per hour (ACH) because it links airflow to kitchen volume. Typical planning ranges are 10–20 ACH for residential cooking areas, 20–40 ACH for light commercial prep, and 40–60 ACH for heavier frying, grilling, or wok lines. This calculator converts area and ceiling height into volume, then reports the CFM needed to reach your selected ACH.
Hood capture rate and linear width
Capture is usually driven by hood geometry and plume intensity, so the tool applies a rate per linear foot of hood width. Practical defaults used here are 150/200/300 CFM per ft for wall canopies (light/medium/heavy) and 200/250/350 CFM per ft for island canopies. Increase the capture factor (0.8–2.0) when overhang is limited; aim for at least 6 in (150 mm) overhang where feasible.
Heat input cross-check for equipment-heavy kitchens
When appliance totals are known, the heat method provides a quick reasonableness check: about 1 CFM per 100 BTU/h, then adjusted by capture factor. In metric terms, 1 kW ≈ 3,412 BTU/h, so the rule-of-thumb is roughly 34 CFM per kW (≈58 m³/h per kW). If heat-based airflow dominates, confirm hood coverage and duct losses.
Duct sizing from velocity and flow
Duct area is calculated from CFM ÷ target velocity. Many exhaust systems operate around 1,200–2,000 fpm (about 6–10 m/s), balancing noise and pressure loss. Higher velocity reduces duct size but can increase static pressure and grease carryover. Use the suggested round or rectangular size as a starting point, then verify fan performance against estimated system pressure.
Make-up air and pressure balance
Exhaust must be paired with make-up air to avoid excessive negative pressure, door slam, and combustion backdraft. Many designs target 70–90% tempered make-up air, with the remainder coming from transfer paths. As a planning flag, residential exhaust above roughly 400 CFM often triggers make-up air considerations. This calculator reports both exhaust and make-up volumes so coordination can begin early. Record assumptions and confirm them during final field testing.
FAQs
1) Which method should I trust most?
Use the highest of ACH, hood, and heat estimates, then add a safety factor. This reduces under-sizing when one input is incomplete or when cooking intensity varies across shifts.
2) What capture factor should I pick?
Use 1.0–1.2 for good hood coverage and calm cross-drafts. Use 1.3–1.6 for island hoods, short overhangs, or strong cross-flow. Values above 1.6 are for difficult plumes.
3) Why does duct velocity matter?
Velocity controls duct size and pressure loss. Higher velocity can increase noise and static pressure, while lower velocity increases duct size. The target helps you choose a practical starting size.
4) How do I estimate make-up air?
Many projects plan 70–90% of exhaust as tempered make-up air, with the balance from transfer air. Adjust up when the building envelope is tight or when doors must remain easy to operate.
5) Do I need to enter hood depth?
Not for this sizing approach. Width typically drives linear capture rates. Depth and overhang still matter in real installations, so use them when selecting hood models and verifying capture tests.
6) Are the results code-compliant?
They are planning estimates, not approvals. Always confirm local requirements, hood listing data, grease duct rules, fire suppression integration, and fan curves before procurement and commissioning.