In short: condensation forms whenever air is cooled below its dew point — the temperature at which it can no longer hold all its water vapor. A typical AC coil running at 400 CFM in warm, humid air can pull over 60 kg (132 lb) of water a day out of the air; a single cold window pane in the same room forms only a thin film, because almost no air moves across it. Use the calculator above with your own temperature, humidity and airflow.
These figures are planning aids based on standard psychrometric approximations. For coil, drain-pan or dehumidifier sizing on a real project, confirm with a licensed HVAC engineer.
How to calculate condensation rate
Condensation is water vapor leaving the air because the air can no longer hold it. Warm air can carry far more water vapor than cold air, so when a parcel of humid air touches a surface colder than its dew point, the air right at that surface cools, becomes saturated, and dumps its excess moisture as liquid. The calculator compares how much water the air is actually carrying to how much saturated air could hold at the surface's temperature, then multiplies that difference by how much air is flowing past:
Worked example. Take air at 24°C and 60% relative humidity moving across a 10°C coil at 100 CFM (169.9 m³/h). The saturation vapor pressure at 24°C is about 29.8 hPa, giving an absolute humidity of about 13.0 g/m³. At the coil's 10°C surface, saturated air holds only about 9.4 g/m³. The difference — 3.6 g of water per cubic meter — condenses out of every cubic meter of air that touches the coil. Multiply by the airflow: 169.9 m³/h × 3.6 g/m³ ≈ 618.6 g/hr, or about 14.8 kg (32.7 lb) per day.
Why airflow matters as much as temperature. A puddle can only form as fast as new humid air arrives to replace the air that already gave up its moisture. That is why a draft across a cold pipe sweats far more than a still pipe in a closed cupboard, even at the same temperature and humidity — this calculator's airflow input captures exactly that effect.
Why dew point is the number that matters
Relative humidity on its own does not tell you whether something will condense — it only says how close the air is to saturation at its current temperature. Dew point translates that into a single, comparable number: the temperature at which the air would become saturated if you cooled it, with nothing else changing. Any surface at or below the dew point will collect condensation; any surface above it will not, no matter how "humid" the room feels.
This calculator finds dew point with the Magnus–Tetens approximation, the standard fast-converging formula meteorologists and HVAC engineers use for everyday temperature and humidity ranges. You can read more on our methodology page. For the full science of how humidity is measured and reported, see our humidity-to-water-content calculator and dew point calculator.
Dew point by air temperature and humidity
Dew point in °C for common indoor and outdoor conditions. Any surface at or below these temperatures will start to sweat.
| Air temp | 30% RH | 50% RH | 70% RH | 90% RH |
|---|---|---|---|---|
| 15°C / 59°F | −2.5 | 4.7 | 9.6 | 13.4 |
| 20°C / 68°F | 1.9 | 9.3 | 14.4 | 18.3 |
| 25°C / 77°F | 6.2 | 13.9 | 19.1 | 23.2 |
| 30°C / 86°F | 10.5 | 18.4 | 23.9 | 28.2 |
| 35°C / 95°F | 14.8 | 23.0 | 28.7 | 33.1 |
Maximum water content of saturated air
This is the "surface, saturated" figure the calculator subtracts — the most water vapor a cubic meter of air can hold once it has cooled to each temperature.
| Temperature | Max water content (g/m³) |
|---|---|
| 0°C / 32°F | 4.85 |
| 5°C / 41°F (cold window) | 6.79 |
| 7°C / 45°F (AC coil) | 7.74 |
| 10°C / 50°F (sweating pipe) | 9.38 |
| 15°C / 59°F (basement wall) | 12.80 |
| 20°C / 68°F | 17.24 |
| 25°C / 77°F | 22.97 |
| 30°C / 86°F | 30.26 |
Worked examples
Office AC coil, 400 CFM
60.0 kg/day
24°C, 55% RH air across an 8°C coil at 400 CFM condenses about 2.5 kg (5.5 lb) an hour — roughly 60 kg (132 lb) a day — which is why coils need a proper condensate drain.
Cold pipe with a light draft
48.3 g/hr
20°C, 65% RH room air moving at just 50 CFM across a 12°C pipe still condenses about 48 g/hr — around 2.6 lb a day — enough to soak nearby insulation over time.
Bathroom exhaust vent
28.4 kg/day
Muggy 22°C, 80% RH shower air pulled across a 5°C vent grille at 80 CFM condenses about 1.18 kg/hr — about 28.4 kg (62.6 lb) a day if it ran continuously.
What else affects condensation
Air velocity and turbulence. Faster, more turbulent airflow keeps fresh humid air in contact with the cold surface, which can push real-world condensation above this calculator's estimate, which assumes the air fully equilibrates.
Surface material and insulation. A well-insulated cold pipe or duct stays warmer at its outer surface for the same internal temperature, reducing or eliminating condensation — this is the entire purpose of pipe lagging and duct wrap.
Barometric pressure and altitude. The Magnus–Tetens approximation used here is calibrated for roughly sea-level atmospheric pressure; at high altitude, actual dew points shift slightly from these estimates.
Air recirculation. In an enclosed space with limited fresh air, condensing moisture out of the air lowers the room's humidity over time, which reduces the rate — this calculator estimates the instantaneous rate at the humidity you enter, not how it decays.
Frequently asked questions
What causes condensation on a cold surface?
Condensation happens when air touching a surface is cooled below its dew point — the temperature at which it can no longer hold all its water vapor. The excess vapor turns to liquid on the surface. This is why a cold soda can sweats, why AC coils drip, and why single-pane windows fog in winter: in every case, the surface is colder than the air's dew point.
How do I find the dew point of the air?
Dew point depends on air temperature and relative humidity, using the Magnus–Tetens approximation. For example, air at 24°C and 60% relative humidity has a dew point of about 15.8°C. Any surface at or below that temperature will start collecting condensation.
Why does an air conditioner produce so much condensate water?
An AC evaporator coil is deliberately kept well below the room's dew point, and a fan continuously pushes large volumes of humid air across it. At 24°C, 55% RH air moving across an 8°C coil at 400 CFM, this calculator estimates about 2.5 kg (5.5 lb) of condensate per hour — roughly 60 kg (132 lb) per day — which is why coils need a condensate drain or pan.
Does more airflow mean more condensation?
Yes, for a fixed temperature and humidity difference, condensation rate scales directly with the volume of air moving across the cold surface — twice the airflow delivers roughly twice the moisture and, up to a point, twice the condensate. This holds as long as the surface stays cold enough to keep saturating the passing air; if the surface starts warming up, the rate falls.
What is the difference between relative humidity and condensation rate?
Relative humidity describes how close the air is to saturation at its own temperature — it says nothing about condensation by itself. Condensation rate is a physical quantity (mass of water per unit time) that only appears once that humid air contacts something colder than its dew point. High humidity makes condensation more likely, but a cold surface and moving air are what actually turn it into liquid water.
How accurate is this calculator?
The Magnus–Tetens approximation used here matches reference psychrometric tables to within about 0.1°C of dew point across typical indoor and outdoor ranges. The condensation-rate estimate assumes the air fully equilibrates with the surface, which is a good planning approximation for coils, ducts and pipes, but real hardware also depends on surface material, boundary-layer thickness and air velocity, which this tool does not model.
Sources & references
- World Meteorological Organization. Guide to Instruments and Methods of Observation — Magnus–Tetens approximation for saturation vapor pressure and dew point.
- ASHRAE Handbook — Fundamentals, Psychrometrics chapter, coil condensate load estimation practice.
- National Oceanic and Atmospheric Administration (NOAA), National Weather Service. Vapor pressure and humidity reference formulas.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)