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.