Airflow Velocity Calculator LPM

What This Calculator Does

This airflow velocity calculator converts a known LPM flow rate into an estimated air velocity in metres per second (m/s) using a default 100 mm round duct. It is the quickest way to cross-check whether a supply or extract grille, a fan branch, or a laboratory fume hood duct is operating within recommended velocity limits — without running a full duct traverse.

When to Use It

Use this calculator when you have an LPM reading from a flow hood (TSI Alnor, Shortridge), a vane anemometer (Kimo, Airflow), or a mass-flow meter and need to express it as a velocity for comparison against design limits in ASHRAE Fundamentals Chapter 21, CIBSE Guide B, or EN 13779. Replace the default 100 mm duct area with the actual cross-sectional area of your duct for a precise result.

Example: 500 LPM in a 100 mm duct → 500 × 0.002122 = 1.061 m/s

Inputs Required

To calculate air velocity you need two values: volumetric flow rate (LPM, CFM, or m³/s) and duct cross-sectional area (m² or ft²). The calculator above handles the unit conversion from LPM to m³/s automatically — dividing by 60 000 — then divides by the assumed duct area of 0.007854 m² (100 mm round duct, π × 0.05²). For a different duct size, calculate the area first and apply the formula manually.

Velocity at Common Duct Sizes

At 500 LPM: 100 mm duct → 1.06 m/s | 150 mm duct (area 0.01767 m²) → 0.47 m/s | 200 mm duct (area 0.03142 m²) → 0.27 m/s. At 2 500 LPM in a 200 mm duct → 1.33 m/s. Velocity increases with the square of diameter reduction — halving the duct diameter quadruples velocity at the same flow rate.

Speed vs. Velocity in Airflow

In HVAC and ventilation engineering, airflow velocity and airflow speed refer to the same quantity — the magnitude of the air movement vector at a cross-section, expressed in m/s, ft/min, or km/h. ASHRAE, CIBSE, and EN ISO 7730 (thermal comfort) all use m/s. Instrument displays on TSI VelociCalc, Testo 425, and Fluke 925 anemometers use m/s, ft/min, and km/h interchangeably. 1 m/s = 196.85 ft/min = 3.6 km/h.

Acceptable Speed Ranges

ASHRAE Fundamentals recommends duct velocities of 5–8 m/s for main supply ducts, 4–6 m/s for return ducts, and 2–3 m/s at final outlets to control noise (NC rating) and pressure loss. EN 13779 uses similar bands. Laboratory fume hood face velocities should be 0.3–0.5 m/s per OSHA and ACGIH guidelines. Exceeding 10 m/s in HVAC ductwork significantly increases fan energy and noise.

From a Pitot-Static Traverse

The most accurate field method per ASHRAE 111 and ISO 3966 is a pitot-static traverse: measure dynamic pressure at multiple grid points across the duct cross-section using a Dwyer Series 166 or Meriam pitot tube with a digital manometer. Average the velocity pressures (or velocities, per the log-Chebyshev method), then multiply average velocity by duct area to get total flow rate in m³/s, then convert to LPM by multiplying by 60 000.

From a Hot-Wire Anemometer

A hot-wire or thermal anemometer (TSI VelociCalc, Testo 405, Fluke 925) reads velocity directly in m/s at a single point. For total flow, multiply the centre-point velocity by the duct area and a profile correction factor — typically 0.85–0.90 for turbulent duct flow per ASHRAE. Multiply the resulting m³/s by 60 000 to get LPM.

Total LPM = V‑avg (m/s) × Area (m²) × 60 000

Deriving the LPM Factor

Area of a 100 mm round duct: A = π × (0.05)² = 0.007854 m². LPM to m³/s: divide by 60 000. Combined: V = LPM ÷ (60 000 × 0.007854) = LPM ÷ 471.24 = LPM × 0.002122. To use a different diameter D (in metres): factor = 1 ÷ (60 000 × π × (D/2)²).

Dynamic Pressure Relationship

Velocity is also related to dynamic pressure by Bernoulli’s equation: V = √(2 × P‑dyn / ρ), where P‑dyn is dynamic pressure in Pa and ρ is air density (≈ 1.2 kg/m³ at 20 °C, sea level). This is the formula embedded in pitot-tube calculations per ASHRAE 111 and ISO 3966. At V = 3 m/s, P‑dyn = 0.5 × 1.2 × 9 = 5.4 Pa.

Continuity Equation

The continuity equation for incompressible flow (standard HVAC air) states that the volumetric flow rate Q is constant along a duct: Q = V‑1 × A‑1 = V‑2 × A‑2. This means that when air moves from a 200 mm duct into a 100 mm duct at the same flow rate, velocity quadruples (area ratio squared). This is the principle behind duct-sizing trade-offs in SMACNA HVAC Duct Construction Standards and ASHRAE duct design procedures.

Reynolds Number and Flow Regime

For accurate velocity profile correction, calculate the Reynolds number: Re = V × D / ν, where ν is kinematic viscosity of air (≈ 1.5 × 10⁻⁵ m²/s at 20 °C). Re > 4 000 is turbulent (typical in HVAC ducts). In turbulent flow, the log-law velocity profile used by ASHRAE 111 and ISO 3966 applies a correction factor of 0.85–0.92 to convert a centre-point reading to an average velocity.

Conversion Table — 100 mm Round Duct

Reading the Table

All values assume a 100 mm internal-diameter round duct (A = 0.007854 m²). The ft/min column is included for cross-reference with US equipment datasheets from Carrier, Trane, Greenheck, and Twin City Fan that express duct velocities in FPM. Multiply m/s by 196.85 to convert to ft/min.

Reading an LPM Flow Meter

Flow hoods from TSI Alnor (model EBT731) and Shortridge Instruments output total airflow in LPM or CFM by averaging velocity across the outlet face. A hood reading of 800 LPM from a 600 × 300 mm supply grille means 800 litres of air pass through the grille every minute. Divide by 60 000 to get 0.01333 m³/s, then divide by the grille free area to get face velocity — typically 1.5–2.5 m/s for supply grilles per ASHRAE design tables.

From LPM to Air Changes Per Hour (ACH)

Airflow in LPM can be converted to room air changes per hour (ACH) — a key metric in ISO 14644 cleanroom classification, ASHRAE 170 healthcare ventilation, and EN 13779 office HVAC. ACH = (LPM × 60) ÷ Room Volume (litres). Example: 2 000 LPM into a 30 m³ (30 000 L) room = (2 000 × 60) / 30 000 = 4 ACH.

ACH = (LPM × 60) ÷ Room Volume (L)

Design Velocity Limits

ASHRAE Fundamentals (Chapter 21) and SMACNA HVAC Duct Construction Standards define recommended duct velocities by system type. Residential supply mains: 3–5 m/s. Commercial supply mains: 5–8 m/s. Return ducts: 3–5 m/s. High-velocity systems (VAV): up to 12.5 m/s. Exceeding these limits increases noise (ASHRAE NC criteria), dynamic pressure loss, and fan energy consumption. Velocities below 2 m/s can lead to dust settling in horizontal ducts.

Balancing and Commissioning

HVAC commissioning per ASHRAE 111, BSRIA BG29/2012, and CIBSE AM11 requires measuring and recording duct velocities at all main branches to verify that designed LPM flow rates are achieved. Balancing engineers use pitot traverses or hot-wire anemometers; the velocity readings are converted back to LPM (V × A × 60 000) and compared against the design schedule. Systemair, Halton, and Trox supply TAB (testing, adjusting, and balancing) reports in both m/s and LPM.

Round vs. Rectangular Ducts

Round ducts have lower friction loss per unit length than rectangular ducts at the same flow rate. The ASHRAE equivalent diameter (D‑e) for a rectangular duct is: D‑e = 1.30 × (ab)⁰⋅⁵ / (a+b)⁰⋅² where a and b are the duct side dimensions (mm). At equal velocity, a 200 × 100 mm rectangular duct (D‑e ≈ 154 mm) carries the same flow as a 154 mm round duct but has 32 % higher friction loss per metre. Rectangular duct systems from Lindab, Halton, and Trox are sized using ASHRAE duct-design charts or software such as Carrier HAP.

Velocity Measurement Points

For a full velocity traverse per ASHRAE 111, a round duct requires measurements at 6 radial positions on two perpendicular diameters (12 points). A rectangular duct uses the log-Chebyshev grid: 5 rows by 5 columns = 25 points for ducts up to 900 mm side. Each point velocity is recorded, averaged, and multiplied by duct area to yield total flow in m³/s, then converted to LPM by multiplying by 60 000.

CFM to m/s Formula

Convert CFM to m³/s by multiplying by 0.000471947. Then divide by duct area in m²: V (m/s) = CFM × 0.000471947 ÷ A (m²). Alternatively, express in ft/min directly: V (ft/min) = CFM ÷ duct area (ft²). For a 4‑inch (0.1016 m) round duct: area = 0.00811 ft², so V (ft/min) = CFM ÷ 0.00811. Carrier, Trane, and Lennox equipment schedules use ft/min; European equipment from Systemair and Halton uses m/s.

CFM to LPM First

If your instrument reads CFM, convert to LPM first (CFM × 28.3168466 = LPM) and use the calculator at the top of this page. Alternatively, apply the formula directly. Example: 18 CFM × 28.3168466 = 509.7 LPM. In a 100 mm duct: 509.7 × 0.002122 = 1.082 m/s.

V (m/s) = CFM × 0.000471947 ÷ A (m²)

Total Airflow from Velocity

To calculate total airflow in LPM from a measured velocity: LPM = V (m/s) × A (m²) × 60 000. Example: anemometer reads 3.5 m/s in a 150 mm duct (A = 0.01767 m²): LPM = 3.5 × 0.01767 × 60 000 = 3 710 LPM. This is the reverse calculation used during HVAC commissioning to verify that measured velocity matches the design LPM schedule.

Multiple Outlets and Branches

For a multi-outlet system, sum the LPM from each outlet or branch to confirm the total equals the supply fan output. This is the “flow balance” check required by ASHRAE 111, BSRIA BG29/2012, and EN 12599 (ventilation for buildings — testing and measurement). A balanced system has branch flow totals within ± 10 % of design values; a tight commissioning tolerance per ASHRAE is ± 5 %.

LPM = V (m/s) × A (m²) × 60 000