Duct Sizing & Airflow Calculator (Manual D Style)
Enter airflow and system details to estimate proper duct sizes. Use Simple for quick diameter suggestions or Advanced for friction-rate based Manual D approximations.
Duct Sizing Calculator
Estimates are based on ASHRAE/Manual D style methods. Use for planning; confirm with full design.
Duct Sizing – At a Glance
These values reflect your latest inputs above (Simple or Advanced mode). TEL = Total Equivalent Length.
How to Use This Calculator
Simple mode (fast)
- Enter CFM and a reasonable max velocity (e.g., 600–900 FPM supply, 600–800 FPM return).
- Get a round diameter and a 1:2 rectangular suggestion.
- Use for quick branch runs or sanity checks.
Advanced mode (Manual D style)
- Enter CFM, duct length (or TEL), friction rate (in. w.c./100 ft), and preferred velocity.
- Use FR = (ASP / TEL) × 100 to estimate friction rate (see guide below).
- Better for main trunks and long runs where pressure matters.
Quick Reference
Recommended Velocities (FPM)
- Supply trunk: 700–900
- Supply branches: 500–700
- Return trunk: 600–800
- Return grilles (face): 300–500
- Bedrooms / quiet areas: aim lower end
Friction Rate (FR) Guide
Use FR = (ASP / TEL) × 100
- ASP = Available Static Pressure (in. w.c.) after coils/filters.
- TEL = Total Equivalent Length (ft) supply + return + fittings.
- Typical FR: 0.06–0.12 in. w.c./100 ft (start at 0.08 if unsure).
Example: ASP 0.20, TEL 250 → FR ≈ (0.20/250)×100 = 0.08.
Equivalent Length (typical)
- 90° radius elbow (round): ~15–20 ft
- 90° mitered elbow (no vanes): ~35–50 ft
- Wye 45°: ~10–15 ft
- Takeoff to branch: ~5–10 ft
- Flex duct: installed tight ≈ actual; saggy can act like +25–50% extra length
Noise, Comfort & Layout Tips
Keep it quiet
- Avoid velocities >900 FPM near grilles/boots in living spaces.
- Use radius elbows or turning vanes; avoid sharp mitered turns.
- Limit sudden transitions; keep expansion angles ≤ 15° when possible.
Balance & mix air
- Size branches for similar velocities so rooms balance easier.
- Place returns to pull from hot spots and long throws to mix air.
- Seal ducts (mastic/tape) and support flex every 4–5 ft to prevent sag.
Round vs. Rectangular
Choosing rectangular sizes
For the same area, a 1:2 aspect ratio (e.g., 8×16) keeps friction reasonable. The calculator suggests a 1:2 size for quick planning.
Core relations used
- Area (ft²) = CFM / Velocity (FPM)
- Round diameter (in) ≈ √(4 × Area / π) × 12
- Friction/length considered in Advanced mode via your FR & TEL inputs
Full Manual D uses detailed loss coefficients for every fitting; use this tool for planning and verify final design professionally.
Frequently Asked Questions
What is a duct sizing calculator?
A planning tool that estimates duct diameters or rectangular sizes from your CFM, velocity, and friction rate targets. It helps balance airflow, pressure, and noise before installation.
How do I pick a friction rate?
Start around 0.08 in. w.c./100 ft, or compute with FR = (ASP / TEL) × 100. Lower FR means larger ducts (quieter, less drop); higher FR means smaller ducts (more drop).
Why is my system noisy?
Excessive velocity near grilles, sharp elbows, undersized branches, or high static pressure can cause noise. Drop velocity, use radius fittings, and check filter/coil pressure drops.
Can I mix flex and metal ducts?
Yes, but keep flex runs short, pulled tight, and well supported. Poorly installed flex greatly increases resistance and hurts airflow.
Do I size supply and return the same?
Not always. Returns usually target similar CFM but slightly lower velocities to reduce noise. Ensure total return CFM matches supply to avoid pressure issues.
Is this a replacement for Manual D?
No—this is a quick estimator to guide planning. A full Manual D includes room-by-room loads (Manual J), detailed fitting losses, and equipment curves.