Free HVAC and building science calculators — duct sizing, equipment sizing, refrigerant charts, psychrometrics, condensation and pipe flow. Built by a licensed professional engineer, checked against the references the industry designs from, and honest about what each one can't tell you.
Used by contractors and technicians who need the number, and by homeowners who need to know whether the number they were given makes sense.
Each one is named for what people search, and described by what it actually does for you.
The sizing chart everyone looks for — plus why the same 2,000 sq ft house can need 2 tons or 5 tons, and why an oversized unit leaves a humid house cool and clammy. Includes a check for a system you already have.
PT charts and charge diagnostics for R-410A, R-454B, R-32, R-22 and more — with separate vapor and liquid columns, because blends have two saturation pressures. Includes the A2L refrigerants and what changes when you stop reading an R-410A chart.
An online ductulator that sizes flex, sheet metal and rectangular duct from CFM — by velocity, by friction rate, or both at once. Includes printable flex duct CFM charts, rectangular equivalents, return grille sizing, and what compressed flex really costs you.
Enter the room conditions and a surface temperature — window, duct, rim joist, slab, chilled pipe — and get a straight answer with the margin in degrees. Handles frost below freezing and the 5 °F coating spread.
Convert between relative humidity, dew point and actual moisture content — and see why RH swings with temperature even when no moisture was added or removed.
Roof pitch drives attic volume and roof area, which drive required net free vent area. Get pitch, angle, rafter length and true roof area, plus the IRC 1/150 and 1/300 requirement split between upper and lower vents.
Pressure drop, velocity and the smallest pipe size that works — with real inside diameters for copper, PEX, CPVC, PVC and steel, and fitting losses counted. Explains why one fixture is weak when the rest of the house is fine.
The full moist-air state from any two known conditions — wet-bulb, humidity ratio, enthalpy, specific volume, density — with a psychrometric chart and true elevation correction.
If you know the question but not the jargon, start here.
Most free building calculators are built by marketers rather than engineers. They tend to be close enough for a rough estimate and quietly wrong at the edges — below freezing, at altitude, on a refrigerant blend with temperature glide, or anywhere the simplifying assumption stops holding.
That is a real problem when the answer decides whether a duct sweats inside a wall, whether a compressor gets liquid, or whether a system is sized correctly for the house it's going into.
Everything here is built to the reference standards the industry actually designs from — ASHRAE psychrometrics, Darcy-Weisbach with an iterative Colebrook solution, ACCA Manual D and S, NIST-referenced refrigerant properties — and each tool publishes its own validation so you can check the work rather than trust it. Where a result depends on an assumption, the assumption is stated. Where a tool can't answer the question you're really asking, it says so.