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HVAC calculators

Capacity, airflow and temperature converters, airflow from capacity, round and rectangular duct sizing, superheat and subcooling arithmetic, and a rough room load. Rules of thumb are labelled as such.

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Planning aid; verify against local code and the manufacturer's data; the qualified person on site is responsible. Nothing leaves this page.

Unit converters

Type in any box in a row; the others follow. 1 ton of cooling = 12,000 BTU/h = 3.517 kW.

Capacity, BTU/h

Capacity, kW

Capacity, tons of refrigeration

Airflow, CFM (ft³/min)

Airflow, m³/h

Airflow, L/s

Temperature, °F

Temperature, °C

Temperature difference, °F

Temperature difference, K (= °C)

Airflow from capacity

Sensible capacity, BTU/h

Air temperature difference ΔT, °F (supply vs return)

CFM = BTU/h ÷ (1.08 × ΔT °F). The 1.08 factor assumes standard air (about sea-level density, 0.075 lb/ft³); at altitude or high temperature the true flow for the same capacity is higher.

Tons of cooling

Rule-of-thumb CFM per ton (editable)

400 CFM per ton is a rule of thumb for typical comfort cooling, not a design value. Humid climates often use less, dry climates more. Use the equipment maker's data for the actual unit.

Round duct sizing

Airflow, CFM

Size by

Velocity, ft/min

Friction rate, inches of water per 100 ft

By velocity: area = CFM ÷ velocity. By friction rate: the ASHRAE round-duct equation for galvanised steel and standard air, Δp = 0.109136 · CFM^1.9 ÷ D^5.02 (Δp in in. w.g. per 100 ft, D in inches), solved for D. Real ducts differ with material, fittings and air density, so round up to a stocked size and check with your own duct calculator or the equipment's static-pressure data.

Rectangular duct: equivalent round diameter

Side a, inches

Side b, inches

Or: target round diameter, inches (finds side b for side a)

De = 1.3 · (a·b)^0.625 ÷ (a+b)^0.25 (ASHRAE, equal friction and flow). It gives the round duct with the same friction loss at the same airflow, not the same area.

Superheat and subcooling

This page contains no refrigerant pressure-temperature data. Read the saturation (dew or bubble point) temperature for your refrigerant from your gauge set's scale, a digital manifold or the refrigerant's own chart, then enter it here together with the measured line temperatures.

Suction line temperature at the evaporator outlet, °F or °C

Evaporator saturation temperature (from the suction pressure), same unit

Condenser saturation temperature (from the discharge pressure), same unit

Liquid line temperature at the condenser outlet, same unit

Superheat = suction line temperature − evaporator saturation temperature. Subcooling = condenser saturation temperature − liquid line temperature. Differences are the same number in °F or K, so use one unit throughout. Target values come from the equipment's data plate or manufacturer's charging chart, not from this page.

Rough room cooling load

Rule of thumb, not a Manual J load calculation. It cannot see insulation, glazing, orientation, infiltration or climate. Use it for a first idea only; size equipment from a proper load calculation.

Floor area

Area unit

Base allowance, BTU/h per ft² (editable)

People in the room

Allowance per person, watts (editable)

Equipment and lighting heat, watts

Extra for sun, glazing or top floor, %

Load = (area × base + people × W + equipment W) × 3.412 BTU/h per W where watts apply, then × (1 + extra %). The defaults are common rough figures, not standards; change them to match your own experience.