HVAC Calculator: Duct, BTU

HVAC Calculator is an offline toolkit for HVAC and refrigeration technicians, installers, estimators and apprentices. Superheat, subcooling, duct sizing, static pressure, psychrometrics, load estimates and refrigerant pressure-temperature charts, all without an internet connection.

No subscription. No account. No paywall. Every calculation is free.

GUIDED FIELD CHECKS

Most calculator apps hand you a formula and leave. This one walks you through the actual check.

Charge Check — pick your refrigerant and metering device, enter suction and liquid pressures and line temperatures, and get superheat, subcooling, target superheat and a clear reading of what the numbers mean together. Fixed orifice and TXV systems are handled differently, the way they should be.

Airflow Check — supply static, return static, total external static pressure against the rated maximum, temperature split and, in cooling, estimated CFM and CFM per ton, with the likely restriction points listed.

Load Estimate — for one room or the whole house, envelope by envelope, with a suggested capacity range and an oversizing warning.

REFRIGERATION AND CHARGING

Pressure-temperature charts for R-410A, R-32, R-454B, R-22, R-134a, R-404A, R-407C, R-448A, R-449A, R-513A, R-1234yf, R-290, R-744 and more. Bubble and dew points are shown separately for blends, and the app uses the correct one for superheat and for subcooling. Superheat, subcooling, target superheat for fixed orifice systems, evaporator and condenser saturation, condenser split, compression ratio and line-set charge adjustment.

AIRFLOW AND DUCT

Ductulator for round and rectangular duct, round to rectangular equivalents, velocity, friction rate, velocity pressure, total external static pressure, CFM from temperature rise, CFM from delta T, air changes per hour and fan laws. Duct material matters: galvanized, duct board, and flexible duct both extended and compressed, because compressed flex is one of the most common reasons a system will not move air.

PSYCHROMETRICS

Pick a supported pair of air properties, get the full state: dry bulb, wet bulb, dew point, relative humidity, humidity ratio in grains or grams, enthalpy, specific volume and vapor pressure. Mixed air conditions, coil sensible and latent load, and sensible heat ratio.

ALTITUDE THAT ACTUALLY WORKS

Set your altitude once and it changes everything downstream. Air density, the 1.08, 0.68 and 4.5 constants, friction rates and absolute pressures for compression ratio are all recalculated. If you work in Denver, Calgary, Mexico City or anywhere else above sea level, your numbers reflect it.

ELECTRICAL, ENERGY AND HYDRONICS

Ohm's law, single and three phase power, voltage imbalance, run capacitor testing in microfarads, electric heat, wire size and voltage drop. Operating cost using your own electricity rate, fuel cost comparison per usable million BTU, COP, EER and kW per ton. Water flow in GPM, pipe velocity and condensate rate.

SAVED JOBS AND REPORTS

Save readings to a job with the site name, equipment details and your own notes. Every reading from the visit stays together. Share the job as text, or export it as a PDF. Jobs stay on your device.

BUILT FOR THE FIELD

Imperial and metric, switchable anytime. High contrast for sun. Large numbers, big buttons, no five-screen tutorial. Search finds any tool in one tap. Every result shows the formula, inputs and assumptions it used, so you can check the work and so an apprentice can learn from it. In 40+ languages: English, German, Polish, French, Spanish, Italian, Portuguese, Dutch, Czech, Romanian, Turkish and more.

IMPORTANT

This app provides engineering estimates and field calculations. It is not a substitute for a full load calculation, a licensed design, manufacturer charging procedures or local code requirements. Always verify against equipment manufacturer data. Refrigerant work should only be performed by qualified personnel.

Recent changes:
First release: offline HVAC calculators for superheat, subcooling, duct sizing, PT charts and BTU load estimates.