Duct Velocity Calculator

Example data table

Formula used

Air velocity is the volumetric flow rate divided by cross-sectional area:

• V = Q / A
• Q = airflow (CFM or m³/s)
• A = duct area (ft² or m²)

Round duct area:

• A = π × (D/2)²

Rectangular duct area:

• A = W × H

When enabled, the tool also estimates velocity pressure from speed and air density.

How to use this calculator

1. Select duct shape: round or rectangular.
2. Enter airflow and choose its unit.
3. Select a dimension unit, then enter duct size.
4. Optional: enable velocity pressure and set air density.
5. Press Calculate to view results above the form.
6. Use Download CSV or Download PDF for records.

If the design band is high, consider checking noise and pressure drop.

Practical guidance for duct velocity in construction

1) Velocity starts with airflow and area

Duct velocity is air speed through a cross‑section. The calculator converts airflow to a common unit, finds duct area, then computes V = Q ÷ A. Example: 600 CFM in a 12‑inch round duct (0.785 ft²) is about 764 FPM.

2) Typical velocity ranges used in practice

Residential supply branches often target about 600–900 FPM, while returns commonly stay near 500–800 FPM for quieter operation. Trunks can run higher, roughly 900–1,400 FPM, when space is tight and fittings are well designed. Many commercial interiors allow higher values with added sound control.

3) Match supply and return paths

A balanced system avoids one side becoming the restriction. If returns are undersized, return velocity rises, increasing grille noise and static pressure. Provide adequate return area for bedrooms and long runs, or add additional returns to keep speeds reasonable.

4) Noise sensitivity increases with speed

Higher velocity can create “whoosh” at grilles and turbulence at sharp elbows. Quiet rooms may require lower velocities than utility spaces. If your result is high, consider upsizing the run, reducing airflow per branch, or improving fittings to smooth the flow.

5) Pressure drop and fan energy

Friction loss climbs quickly as velocity increases, which can reduce delivered airflow if the fan cannot overcome the added static pressure. Higher speeds can also magnify leakage impact and make room‑by‑room balancing harder. The optional velocity‑pressure estimate helps visualize this relationship and why low‑loss fittings matter.

6) Use velocity with sizing methods

Velocity is a strong screening metric, but it is not a full duct‑sizing method. Pair it with friction‑rate or static‑regain calculations on long or complex systems. When several sections show high velocity, overall pressure drop is likely high too.

7) Field checks during commissioning

Test and balance work often produces airflow readings from hoods or traverses. Converting those numbers to velocity helps spot crushed flex duct, tight takeoffs, or blocked dampers. High velocity in one short section frequently signals the controlling bottleneck. Document before‑and‑after adjustments for records.

8) Construction details that change results

Real ducts may have less free area than drawings. Liner thickness, deformation, internal dampers, and partially closed blades all raise velocity. Long flex runs with sharp bends also behave like smaller ducts. Keep runs straight, sealed, and transitioned gradually.

FAQs

1) What airflow unit should I use?

Use the unit you have from plans or measurements. The calculator converts CFM, L/s, m³/s, and m³/h automatically, so results are consistent regardless of input unit.

2) Why does velocity matter if airflow is correct?

Velocity influences friction loss, noise, and comfort. You can have the right airflow at the equipment but still experience loud grilles or weak room delivery if velocities are excessive in branches or fittings.

3) Round or rectangular ducts—does it change velocity?

Velocity depends on cross‑sectional area. For the same airflow, any shape with the same area gives the same average velocity. Shape still affects losses because elbows and transitions behave differently.

4) What is “velocity pressure” in the results?

Velocity pressure is the dynamic pressure associated with air speed. It is estimated from velocity and air density and helps visualize how higher speed can translate into higher fitting losses and static pressure demand.

5) How do I reduce high velocity?

Increase duct area, split airflow into more runs, reduce airflow per branch, or improve routing. Using smoother fittings, longer-radius elbows, and gradual transitions can also cut turbulence and perceived noise.

6) Should I use inside or outside duct dimensions?

Use inside dimensions when possible, because airflow moves through the internal area. If you only have nominal sizes, the results are still useful for screening, but liner or thickness can raise true velocity.

7) What velocity is “too high”?

It depends on noise criteria, occupancy, and duct type. As a practical check, branches above about 900–1,000 FPM and trunks above about 1,400–1,600 FPM often deserve review for friction and sound control.