HVAC math that holds up in the field
Free calculators and decoders for technicians, contractors, and serious DIYers — sizing, airflow, and refrigerant numbers you can act on, with the formula shown for every result.
AC Tonnage Calculator by Model Number
Type the model number off the rating plate and get the unit’s tonnage and BTU/hr — Carrier, Trane, Goodman, Lennox, Rheem, York and more.
Decode a model number →Superheat & Subcooling Calculator
Enter gauge pressure and line temperature for R-410A or R-22. Built-in PT chart, charging targets, and a plain-English diagnosis of the reading.
Check a charge →Duct Size & CFM Calculator
Size a round or rectangular duct from CFM and velocity, or convert to equivalent round diameter — with a built-in CFM reference chart.
Size a duct →Every calculator ships only after its math passes a published test suite against real model numbers and standard charts.
Loads fast on a phone in a hot attic. No sign-ups, no downloads, no ten-step wizards between you and the number.
Each tool explains the exact formula and a worked example, so you can verify the result instead of trusting a black box.
Built for people who fix and install, not just browse
HVAC numbers live in scattered places — a PT chart in the truck, a sizing table in a 40-page PDF, a model-number code buried in a manufacturer spec sheet. DuctMath pulls the everyday calculations into fast, single-purpose tools: decode what a unit is from its model number, verify a refrigerant charge with superheat and subcooling, and — as the toolbox grows — size ducts by CFM, check static pressure, and run quick load estimates.
Every tool follows the same rules. The answer comes first, the formula is written out in plain text, a worked example shows the math with real numbers, and the reference chart the tool reads from is printed on the same page. If a result ever depends on a judgment call — like nominal capacity versus a proper Manual J load calculation — the tool says so instead of pretending the shortcut is the whole story.
How DuctMath tests its math
Calculators only earn trust once. Before a tool goes live, its engine runs a written test suite: known model numbers from the six major North American brands, published pressure–temperature points, and deliberate trap cases like SEER digits that masquerade as size codes. When a tool has limits — altitude effects on PT values, or refrigerants we haven’t verified yet — those limits are stated on the page, not discovered by you at the worst moment.