Duct Calc: HVAC Duct Sizer
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About this app
Size ductwork in seconds — round, rectangular, and yes, flex.
Duct Calc is an offline HVAC duct calculator built for techs and designers who need answers on the job, not another subscription.
FREE, FULLY WORKING
• Duct sizing (equal friction, round) — enter CFM and friction rate (in. w.c./100 ft), get the exact diameter, the stock size to grab, and the velocity in FPM
• Unit converter — airflow, velocity, and duct area. CFM · L/s · m³/h at one tap
PRO (ONE-TIME PURCHASE)
• Flex duct sizing — friction based on published research data (fully stretched to 30% compressed). Compressed flex loses far more pressure than the charts admit; Duct Calc corrects for it
• Rectangular conversion — equivalent diameter to W×H (1-inch increments), with aspect-ratio warning
• Airflow back-calc — know the duct, get the CFM: from round diameter or rectangular W×H, by velocity or friction rate
• Register / diffuser selection — neck size, free area, and throw estimate from CFM and neck velocity
• Static pressure — straight-run loss plus fittings (elbows, tees, transitions), totaled in in. w.c.
BUILT FOR THE FIELD
• 100% offline — works in the attic, the crawlspace, the mechanical room
• Big touch targets, screen stays awake, no account, no sign-in
• Your inputs never leave the device
ONE-TIME PURCHASE
The free version is fully functional — round sizing and conversion are never crippled. One purchase unlocks everything else, forever.
No subscription. No ads. No features taken away later.
Calculations follow standard practice (Darcy–Weisbach / Colebrook friction, ASHRAE equivalent-diameter equation, published flex-duct compression data). Reference only — verify final designs against applicable codes and manufacturer data.
Duct Calc is an offline HVAC duct calculator built for techs and designers who need answers on the job, not another subscription.
FREE, FULLY WORKING
• Duct sizing (equal friction, round) — enter CFM and friction rate (in. w.c./100 ft), get the exact diameter, the stock size to grab, and the velocity in FPM
• Unit converter — airflow, velocity, and duct area. CFM · L/s · m³/h at one tap
PRO (ONE-TIME PURCHASE)
• Flex duct sizing — friction based on published research data (fully stretched to 30% compressed). Compressed flex loses far more pressure than the charts admit; Duct Calc corrects for it
• Rectangular conversion — equivalent diameter to W×H (1-inch increments), with aspect-ratio warning
• Airflow back-calc — know the duct, get the CFM: from round diameter or rectangular W×H, by velocity or friction rate
• Register / diffuser selection — neck size, free area, and throw estimate from CFM and neck velocity
• Static pressure — straight-run loss plus fittings (elbows, tees, transitions), totaled in in. w.c.
BUILT FOR THE FIELD
• 100% offline — works in the attic, the crawlspace, the mechanical room
• Big touch targets, screen stays awake, no account, no sign-in
• Your inputs never leave the device
ONE-TIME PURCHASE
The free version is fully functional — round sizing and conversion are never crippled. One purchase unlocks everything else, forever.
No subscription. No ads. No features taken away later.
Calculations follow standard practice (Darcy–Weisbach / Colebrook friction, ASHRAE equivalent-diameter equation, published flex-duct compression data). Reference only — verify final designs against applicable codes and manufacturer data.
HVAC duct sizing calculator — round, rectangular & flex. CFM, friction, offline
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Data is encrypted in transit
Data can’t be deleted
What’s new
Stability and reliability improvements.
App support
About the developer
김건우
cleverlywittyapps@gmail.com
동소문로3길 23
성북구, 서울특별시 02833
South Korea
