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.