Ventilation & Fresh Air Calculator
Work out the outdoor air for each room with the ASHRAE 62.1 Ventilation Rate Procedure, the minimum exhaust for toilets, kitchens and car parks, or the airflow from air changes per hour. Results are in L/s, m³/h and cfm, ready for duct sizing. Modelling the installation too? Browse our MEP Revit families.
ASHRAE 62.1-2022 ratesFresh air, exhaust or ACHMetric and imperialPer-room tableFormula with your numbers
Method
Outdoor air per ASHRAE 62.1-2022 Ventilation Rate Procedure (people + floor area).
Metric: m², m, L/s. Imperial: ft², ft, cfm. Switching converts what you typed.
Zones
One row per room or zone.
Rates from ASHRAE 62.1-2022 Table 6-1. Leave People empty to use the table's default occupant density. Dwelling units are outside 62.1: use ASHRAE 62.2 for homes and apartments. Healthcare spaces use ASHRAE 170.
Minimum exhaust rates from ASHRAE 62.1-2022 Table 6-2. Per-fixture rows have a continuous and an intermittent rate; pick how the fan runs.
Preset air change rates are typical design values from common references, not code minimums. Edit them to match your code or brief.
Outdoor air required
Enter your rooms to see the result.
Next: size the duct.
Duct size calculatorPreliminary sizing only. Fresh air follows the ASHRAE 62.1-2022 Ventilation Rate Procedure for a 100 % outdoor-air system; exhaust uses its minimum rates. Air change presets are typical values, not code minimums. Your local code (IMC, UK Approved Document F, local regulations) may require more. It does not cover infiltration, heat or moisture loads, kitchen hoods or smoke control design.
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Request a projectVentilation airflow in three steps
Pick a method
Fresh air uses people and floor area (ASHRAE 62.1). Exhaust uses the code rates per fixture or per m². Air changes uses room volume × ACH.
Add your rooms
Add a row per room or zone and choose its type. Leave people empty to use the standard’s default density, and pick how air is supplied for Ez.
Read and size the duct
See the total in L/s, m³/h and cfm, a row-by-row table and the formula with your numbers, then take the airflow to the duct size calculator.
How the calculator works
Fresh air (ASHRAE 62.1 Ventilation Rate Procedure)Vbz = Rp × Pz + Ra × Az
Rp is the outdoor air per person, Pz the number of people, Ra the outdoor air per floor area and Az the floor area, all from Table 6-1. Then Voz = Vbz ÷ Ez, where Ez (Table 6-4) depends on how air is supplied and returned. For a 100 % outdoor-air system, Vot = ΣVoz.
Example: an office of 100 m² with the default 5 people per 100 m². Vbz = 2.5 × 5 + 0.3 × 100 = 12.5 + 30 = 42.5 L/s. With ceiling supply of cool air, Ez = 1.0, so Voz = 42.5 L/s = 153 m³/h = 90 cfm.
Exhaust: flow = rate per fixture × number of fixtures, or rate per m² × floor area (Table 6-2).
Air changes: Q (L/s) = V (m³) × ACH ÷ 3.6, or Q (cfm) = V (ft³) × ACH ÷ 60. A 5 × 4 × 3 m room at 10 ACH: 60 × 10 ÷ 3.6 = 167 L/s.
Units: 1 L/s = 3.6 m³/h = 2.11888 cfm.
One air handler that serves several zones with recirculated air needs Vot = Vou ÷ Ev (Section 6.2.4). The calculator does not work out Ev; it gives ΣVoz for a 100 % outdoor-air system.
ASHRAE 62.1-2022 outdoor air rates (selected spaces)
| Space | Rp per person L/s (cfm) | Ra per area L/s·m² (cfm/ft²) | People per 100 m² |
|---|---|---|---|
| Office space | 2.5 (5) | 0.3 (0.06) | 5 |
| Reception areas | 2.5 (5) | 0.3 (0.06) | 30 |
| Conference, meeting | 2.5 (5) | 0.3 (0.06) | 50 |
| Classrooms (age 9 plus) | 5 (10) | 0.6 (0.12) | 35 |
| Lecture hall (fixed seats) | 3.8 (7.5) | 0.3 (0.06) | 150 |
| Restaurant dining rooms | 3.8 (7.5) | 0.9 (0.18) | 70 |
| Retail sales | 3.8 (7.5) | 0.6 (0.12) | 15 |
| Hotel bedroom, living room | 2.5 (5) | 0.3 (0.06) | 10 |
| Places of religious worship | 2.5 (5) | 0.3 (0.06) | 120 |
| Gym, sports arena (play area) | 10 (20) | 0.9 (0.18) | 7 |
| Libraries | 2.5 (5) | 0.6 (0.12) | 10 |
| Corridors | 0 (0) | 0.3 (0.06) | – |
The SI rates are the standard’s own rounded values (7.5 cfm becomes 3.8 L/s, not 3.54). People per 100 m² is the same number as per 1,000 ft², so metric and imperial results differ slightly. Addendum w (2025) lowers some default densities, for example lecture halls to 143 and restaurant dining to 67. Minimum exhaust (Table 6-2): public toilets 25 L/s (50 cfm) per WC or urinal for continuous operation, 35 L/s (70 cfm) intermittent; private toilets 12.5 L/s (25 cfm) or 25 L/s (50 cfm); commercial kitchens 3.5 L/s·m² (0.70 cfm/ft²); parking garages 3.7 L/s·m² (0.75 cfm/ft²).
Ventilation questions
ASHRAE 62.1-2022 asks for 2.5 L/s (5 cfm) per person plus 0.3 L/s per m² (0.06 cfm/ft²) of floor area. At the default density of 5 people per 100 m², that works out to about 8.5 L/s (17 cfm) per person before Ez.
Vbz is the outdoor air needed in the breathing zone (people part plus area part). Voz is the outdoor air the zone must receive after allowing for how well the diffusers deliver it (Voz = Vbz ÷ Ez). Vot is the outdoor air intake at the air handler. For a 100 % outdoor-air system Vot is the sum of Voz; for a recirculating multi-zone system it also depends on the system ventilation efficiency Ev.
Multiply the room volume by the air changes per hour. In metric, Q (L/s) = V (m³) × ACH ÷ 3.6, so a 60 m³ room at 10 ACH needs 167 L/s (600 m³/h). In imperial, Q (cfm) = V (ft³) × ACH ÷ 60.
ASHRAE 62.1-2022 Table 6-2 gives 25 L/s (50 cfm) per water closet or urinal when the fan runs continuously during hours of use, and 35 L/s (70 cfm) per fixture otherwise or where heavy use is expected, such as schools and theatres.
A common design basis is 6 air changes per hour for normal ventilation, 10 where cars queue with engines running, and 10 for smoke clearance; confirm the smoke rate with the fire strategy. ASHRAE 62.1 instead sets 3.7 L/s per m² (0.75 cfm/ft²) of floor area, and no exhaust where two or more sides are at least 50 % open.
No. Dwelling units are covered by ASHRAE 62.2. ASHRAE 62.1 still covers common areas of residential buildings, such as corridors. Healthcare spaces follow ASHRAE 170.
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