HVAC Calculator & Ductulator

Free offline duct sizing: enter CFM and friction rate for round and rectangular sizes. R-410A charge calculations and unit conversions are free too.

Size a duct from CFM and friction rate, or find the airflow capacity of an existing round duct. The ductulator is free, works offline, and opens directly when you launch the app. No account required.

HVAC Calculator & Ductulator also includes refrigerant charge calculations, unit conversions, and Pro tools for airflow, load estimates, and PDF job reports.

DUCT SIZING (DUCTULATOR) Enter CFM and friction rate to get round and rectangular duct sizes, with velocity limits checked for supply, return, and branch runs. Reverse mode calculates the max CFM of an existing duct. Sheet metal and flex are both supported.

REFRIGERANT CHARGE Superheat and subcooling from gauge pressure and line temperature, compared against a target. R-410A is free; Pro adds R-32, R-454B, R-22, R-134a, and R-407C. Blends are handled correctly — dew line for superheat, bubble line for subcooling. PT values are computed from NIST reference equations, so you can enter any pressure instead of rounding to a chart's 25-psi steps. Target superheat for fixed-orifice systems is included (indoor wet bulb + outdoor dry bulb).

AIRFLOW Temperature-rise CFM, the three fan laws, and CFM per ton with the 350–450 range built in — confirm airflow before adjusting the charge.

LOAD & CONVERSIONS Quick BTU and tonnage estimates by climate zone (a clearly labeled rule of thumb, not a Manual J), plus BTU, kW, tons, CFM, L/s, PSI, kPa, °F and °C conversions.

JOBS & PDF REPORTS (PRO) Save calculations to named jobs and export a PDF service report with your name and company on it — ready to hand over with the invoice.

FREE AND PRO The ductulator, R-410A charge, and unit conversions are free. Pro adds the other five refrigerants, the full PT chart, airflow checks, load estimates, and PDF job reports. Pro requires a purchase. Available plans and local prices are shown in the app before you buy.

Questions, or a number that looks off? Email hello@pairmind.dev — every calculator is verified against published reference data.

For trained technicians. Verify results against manufacturer specifications. Load estimates are not a substitute for ACCA Manual J/D/S.

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