HVAC Duct Sizing Calculator

HVAC Duct Sizing Calculator is the professional duct sizing tool for HVAC technicians, engineers, and contractors. Calculate correct ductwork dimensions based on airflow (CFM), velocity, or friction rate using ASHRAE standard sizes and ACCA Manual D methodology — the industry standard for professional air conditioning system design.

SIZING METHODS:
CFM-Based Method — Enter airflow and get recommended standard duct sizes instantly
Velocity-Based Method — Size ducts by target air velocity (FPM) for proper sizing
Friction Rate Method — Default 0.08 in. w.g. per 100 ft or custom pressure drop values

DUCT TYPE SUPPORT:
Round Duct Sizing — Calculate diameter based on CFM and friction loss
Rectangular Equivalent — Standard rectangular dimensions per the ASHRAE (Huebscher) equivalent-diameter equation
Flex Duct Correction — Automatic 10% correction factor for flexible ductwork
All Standard Sizes — Results snap to standard round sizes from 4 to 48 inches

ADVANCED CALCULATIONS:
Velocity Check — Color-coded check against Manual D limits (optimal 400-900 FPM)
Pressure Drop Analysis — ASHRAE friction-chart equation per 100 ft of duct
Reynolds Number — Full hydraulic analysis for precision engineering
Friction Loss Display — See calculated friction for every result

OPTIONAL PRO UPGRADE — $2.99 ONE-TIME IN-APP PURCHASE
The following features require a separate one-time in-app purchase. They are not required to use the calculator:
• Unlimited saved calculation history
• PDF export of results
• Dark theme
• Ad-free experience

WHY HVAC PROS USE THIS CALCULATOR:
Faster ductwork design than manual slide rules and ACCA charts
ASHRAE-compliant and Manual D-verified sizing ensures proper system performance
Cross-check velocity and friction rate for perfectly balanced systems
Prevents over/undersized ducts that reduce efficiency and increase noise
Field calculations without internet connection needed on job sites

PERFECT FOR:
HVAC design engineers planning new installations
HVAC technicians calculating duct replacement sizing
HVAC field technicians verifying existing ductwork
AC contractors preparing accurate job estimates
HVAC students learning Manual D design methodology
Residential and commercial HVAC system design

Based on ACCA Manual D constant friction method and ASHRAE Handbook fundamentals.

Part of HVACTools Suite — the complete HVAC technician app for field calculations, refrigeration tools, and air conditioning diagnostics.

Recent changes:
Added a link to TradeTools — the one-app version of this calculator and 61 more, for HVAC, electrical, plumbing, fire protection, solar and carpentry. Nothing else changed.