Field calculator
Dew point calculator (temperature and humidity)
The dew point is the temperature at which air becomes saturated and water condenses out, and it decides a surprising number of field problems. Enter the air temperature in degrees Fahrenheit and the relative humidity as a percent, and the tool returns the dew point using the Magnus approximation. Any surface at or below the dew point will grow condensation, which is the root of several jobsite headaches: painters and coatings crews keep steel at least 5 degrees Fahrenheit above the dew point before they coat, because moisture under a coating makes it fail; cold water pipe and chilled-water duct sweat and need insulation and a vapor barrier; and a freezer or cold-storage box drives a relentless inward vapor problem for the same reason. Higher humidity and higher air temperature both push the dew point up. Treat the result as a field estimate, and confirm critical coating or condensation decisions with a calibrated psychrometer or sling hygrometer and the coating manufacturer's surface-temperature rule.
Result
Dew point: the temperature at which the air becomes saturated and water condenses out. Enter the air temperature in degrees Fahrenheit and the relative humidity as a percent, and the tool returns the dew point using the Magnus approximation. Any surface at or below the dew point grows condensation, which is why painters keep steel at least 5 degrees Fahrenheit above the dew point before coating, why cold pipe and duct sweat and need insulation, and why a freezer drives an inward vapor problem. Higher humidity and higher temperature both raise the dew point. Treat this as a field estimate and verify critical coating or condensation decisions with a calibrated psychrometer or sling hygrometer, and follow the coating manufacturer and the spec for the surface-temperature rule.
anvilfield.com/calculators/dew-point-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Dew point FAQ
What is a Manual J load calculation?
It is ACCA's room-by-room accounting of a home's heat gain and loss, used to size heating and cooling equipment from the building itself. It tallies the envelope, the windows by orientation, infiltration, ventilation, and the people and appliances inside, then splits the result into sensible and latent load for equipment selection.
Why is oversizing an AC bad?
An oversized AC cools the air fast and shuts off before it runs long enough to remove moisture, leaving the house cold and clammy above comfortable humidity. The short cycling also wastes energy and wears the compressor early, because the start is the least efficient part of every cycle. Right-sizing fixes all of it.
What design temperature do you use for a load calc?
Use the 1 percent cooling and 99 percent heating design conditions from ASHRAE, tabulated in Manual J, not the record high or low. The 1 percent dry-bulb is exceeded only about 88 hours a year. Sizing to the once-a-decade extreme oversizes the equipment for the thousands of normal hours.
What is the difference between sensible and latent load?
Sensible load is heat that changes air temperature, what a thermometer reads. Latent load is the heat in moisture, the energy to condense water vapor out of the air. Manual J figures them separately because equipment removes them separately. A high-sensible unit on a humid house cools but never dries it, which is the usual humidity complaint.
How many square feet per ton should I use to size an AC?
None as a sizing method. The square-feet-per-ton habit, often quoted around 400 to 600 square feet per ton, ignores the windows, insulation, orientation, and air leakage that actually set the load, and it almost always oversizes. Run a Manual J. Use a per-ton figure only as a rough sanity check against the calculated answer.