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Airflow CFM Calculator - Online HVAC Duct Sizing Tool

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Airflow CFM Calculator

Online HVAC Duct Sizing Tool — Calculate CFM, Duct Dimensions & Friction Loss Instantly

CFM → Duct Size
Duct Size → CFM
Find Duct Size from Airflow
Quick Presets:
🛏 Bedroom (100 CFM) 🛋 Living Room (200 CFM) 🏠 Family Room (350 CFM) 🍳 Kitchen (500 CFM) 🚿 Bathroom (80 CFM) 💼 Office (300 CFM)
CFM
Typical range: 50 – 5,000 CFM (residential/light commercial)
FPM
Recommended: 600–900 FPM (residential), 1,000–1,500 (commercial)
Round Duct
Rectangular Duct
Common ceiling joist space: 8"–12"
Find CFM from Duct Size
Round Duct
Rectangular Duct
Aspect ratio should ideally be ≤ 4:1
Calculation Results

Enter values and click Calculate to see results

Round Duct CFM Reference

CFM at various velocities for standard round duct diameters

Duct Dia. 500 FPM 600 FPM 700 FPM 800 FPM 900 FPM 1000 FPM 1200 FPM
Typical CFM Requirements

General guidelines for common spaces (may vary by climate and code)

Space Type Typical CFM Recommended
Duct Size (Round)		 Notes
Small Bathroom50–804"–5"Exhaust focus
Bedroom (small)100–1505"–6"Supply air
Bedroom (large)150–2506"–7"Supply air
Living Room200–4007"–9"High comfort need
Kitchen300–6008"–11"Plus exhaust hood
Dining Room200–3507"–9"Moderate load
Home Office150–3006"–8"Equipment heat
Conference Room400–8009"–13"High occupancy
Retail Space500–150010"–16"Per 1000 sq ft
Classroom500–100010"–14"30 occupants

HVAC Duct Sizing Guide & FAQ

Essential knowledge for proper ductwork design and airflow calculation

What is CFM and why is it important?

CFM (Cubic Feet per Minute) measures the volume of air moving through your ductwork. Proper CFM ensures adequate heating, cooling, and ventilation. Undersized ducts restrict airflow, causing equipment strain, higher energy bills, and poor comfort. Oversized ducts waste space and material without improving performance.

What's the ideal duct velocity for residential HVAC?

For residential systems, main trunk ducts should operate at 700–900 FPM, while branch ducts work best at 600–800 FPM. Velocities above 1,000 FPM can cause noticeable noise (air rushing sounds). Below 400 FPM may result in poor air distribution and temperature stratification.

Round vs. Rectangular ducts — which is better?

Round ducts are more efficient — they have less surface area per unit of cross-section, resulting in lower friction loss and less material use. Rectangular ducts are often used when space is limited (e.g., between joists or above ceilings). The trade-off is higher friction and potentially more noise if not properly sized.

How does friction loss affect duct design?

Friction loss is the pressure drop as air moves through ducts due to surface roughness. It's measured in inches of water column per 100 feet (in.wg/100ft). Designers typically target 0.05–0.10 in.wg/100ft. Higher friction means the fan must work harder, increasing energy consumption. Flexible ducts have ~30% more friction than smooth metal.

What happens if my ducts are undersized?

Undersized ducts lead to: ① High air velocity → noisy operation; ② Increased static pressure → blower motor strain; ③ Reduced equipment efficiency → higher utility bills; ④ Uneven temperatures between rooms; ⑤ Potential frozen evaporator coils in AC systems due to insufficient airflow.

Can I use this calculator for commercial HVAC design?

Yes! This tool works for both residential and light commercial applications. For commercial systems, use higher velocities (1,000–1,500 FPM) and consider factors like VAV systems, fire dampers, and acoustic requirements. For critical designs, always consult a licensed mechanical engineer and refer to ASHRAE standards and SMACNA guidelines.

What is the Huebscher formula?

The Huebscher formula calculates the equivalent round diameter for rectangular ducts: De = 1.3 × (W×H)0.625 / (W+H)0.25. This is used to determine friction loss in rectangular ducts by converting them to an equivalent round size. Our calculator applies this formula automatically when you select rectangular ducts.

How often should ductwork be inspected?

HVAC professionals recommend inspecting ductwork every 3–5 years. Look for leaks, disconnected joints, insulation damage, and dust buildup. Leaky ducts can waste 20–30% of conditioned air. Sealing with mastic or metal tape (not cloth duct tape) significantly improves system efficiency and indoor air quality.

Pro Tip: For optimal HVAC performance, maintain duct velocities between 600–900 FPM (residential) or 1,000–1,500 FPM (commercial). Always size ducts based on Manual D (ACCA) or ASHRAE standards for critical applications. This tool provides estimates — consult a licensed HVAC professional for final design approval.

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