Duct flow and velocity, room air changes, and actual-to-standard flow
Three airflow jobs share one identity, Q = A × V, and this page does all three honestly. Ducts: flow from velocity or velocity from flow for round and rectangular ducts, with the conversions (m³/s, m³/h, L/s, CFM), the hydraulic diameter for rectangular sections, and the velocity pressure. Rooms: air changes per hour from a flow and a room volume, or the flow a target ACH demands. Conditions: actual flow converted to standard conditions through temperature and pressure, because a cubic metre of hot thin air is not a cubic metre of standard air. What the page will not do is turn a flow into a fan pressure: that needs the system’s resistance curve and the fan’s published curve, and the brief for this page says so outright.
Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. Ventilation design for occupied and regulated spaces follows the governing standards and codes; verify designs with a qualified HVAC engineer.
What this calculator does
For ducts it runs continuity both ways and reports every unit a practitioner might be handed, plus the two quantities that connect flow to hardware: hydraulic diameter (the round-equivalent size a rectangular duct behaves like for friction purposes) and velocity pressure (the pressure the moving air carries by virtue of moving, the quantity pitot tubes measure and fan calculations start from). For rooms it converts flow into air changes per hour or works backwards from a target. For conditions it applies the ideal-gas correction between actual and standard states, the conversion fan catalogues assume when their tables say “standard air”.
How to use it
- Velocity varies across a duct. A single centre reading overstates the average; traverse readings (or a capture hood at a grille) give the mean velocity this page’s arithmetic deserves.
- Rectangular ducts are sized by both dimensions. Area alone misses friction behaviour; the hydraulic diameter (4 × area / perimeter, which for a rectangle is 2 × W × H / (W + H)) is the figure friction charts consume.
- ACH is a rate, not a guarantee of fresh air. Air changes count volume moved; how much of it is outdoor air, and how well it mixes, decide actual air quality, and standards set targets by space type rather than by this page.
- Standard versus actual matters at altitude and temperature. A fan moves actual volume; mass flow, cooling capacity and catalogue comparisons live in standard terms, which is why the converter exists and why it needs absolute pressure and absolute temperature logic internally.
The formulas
- Continuity: Q = A × V; A = π/4 × D² (round) or W × H (rectangular).
- Hydraulic diameter: D_h = 4A / perimeter = 2 × W × H / (W + H) for rectangles.
- Velocity pressure: p_v = ½ × ρ × V².
- Air changes: ACH = Q × 3,600 / room volume; required Q = ACH × volume / 3,600.
- Standard conversion: Q_standard = Q_actual × (P_actual / 101.325) × (293.15 / T_actual in kelvin).
Worked examples
The round duct. 300 mm diameter at 5 m/s: area 0.0707 m², flow 0.35 m³/s, which is 1,272.35 m³/h or 748.87 CFM, and the velocity pressure at standard density is 15.00 Pa. Three unit systems and the pitot quantity from two input numbers, which is the whole shape of duct arithmetic.
The rectangular duct and the room. A 400 × 200 mm duct carrying 0.5 m³/s runs at 6.25 m/s with a hydraulic diameter of 266.67 mm (note: smaller than either naive guess from its sides, which is why wide flat ducts pay friction penalties). Feed 0.104 m³/s of that air into a 52 m³ room and the air changes 7.20 times per hour; ask what flow a 6 ACH target needs in the same room and the answer is 0.09 m³/s, the backward direction the room block computes.
Reading your results
Read duct velocity against noise and friction: residential branches live quietly at 2 to 4 m/s, commercial mains tolerate 5 to 9, and every doubling of velocity quadruples velocity pressure and roughly octuples fan power against a fixed system, which is the economics hiding inside these innocent conversions. Read ACH against the space’s purpose (standards publish targets per occupancy type) and read the standard-flow figure whenever comparing against a catalogue, because catalogues are printed in standard air whether or not your plant room is.
Limitations
- No flow-to-pressure conversion. Pressure depends on the entire system’s resistance, which no single-duct calculation knows; the Flow Rate and Pipe Loss Calculator handles friction properly for liquid pipework, and air systems need their own duct-loss design with fan curves.
- Uniform velocity assumed. Real profiles, bends near the measurement point and swirling flow all bend readings; the page computes from the mean velocity you supply.
- Standard conditions are a convention. This page uses 20°C and 101.325 kPa; other industries anchor elsewhere (some at 0°C or 60°F), and cross-industry comparisons should state the anchor.
- Density is a single input. Humidity and gas composition move it; the field is editable for altitude and temperature estimates, and precision mass-flow work belongs with proper psychrometrics.
Frequently asked questions
Why can a flow not be converted straight into a pressure or a fan size?
Because pressure is the system’s answer to flow, not a property of the flow itself: the same 0.35 m³/s might need 50 Pa through a short fat duct or 800 Pa through a long thin one with filters and coils. Fan selection matches a published fan curve against a system resistance curve computed from every element the air passes, and any tool that prints a pressure from a flow alone has invented the system. This page computes the flow quantities exactly and stops where invention would begin, which the brief for the page insists on explicitly.
What air change rate does a room need?
It depends on the room’s use, occupancy and governing standard: quiet offices sit at the low end, commercial kitchens, laboratories, workshops and healthcare spaces run far higher, and many figures quoted online mix total air changes with outdoor-air changes, which are different quantities. Use this calculator to find what flow a stated target implies (or what a measured flow delivers), and take the target itself from the standard that governs your building type.
Why does the same fan deliver less at altitude?
A fan is a volume machine: it moves the same actual cubic metres per minute, but thinner air carries less mass, less oxygen and less heat per cubic metre, and produces proportionally less pressure. The standard-flow converter quantifies the density half of that story: 0.35 actual m³/s at altitude and temperature can be worth only about 0.30 standard m³/s, which is the shortfall processes designed on sea-level catalogue figures discover in service.
CFM, L/s, m³/h: which should I work in?
Whichever your instruments and documents already use: North American HVAC speaks CFM, most of the rest of the world works in L/s for rooms and m³/h or m³/s for plant, and conversion errors between them are a documented source of mis-sized systems. The results here print all four every time, so a figure can be checked against whichever document is on the desk without a second calculation.
Related calculators
Method: continuity with round and rectangular areas, hydraulic diameter, velocity pressure, ACH identities and the ideal-gas standard conversion at 20°C and 101.325 kPa. Last reviewed: 14 October 2026.