Calculate the Dew Point

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Dew Point Calculator

Calculate the atmospheric dew point based on temperature and relative humidity.

Celsius (°C) Fahrenheit (°F)
Calculated Dew Point:

Understanding Dew Point

The dew point is the temperature to which air must be cooled to become saturated with water vapor. When the air temperature drops below the dew point, water vapor condenses to form liquid water (dew), fog, or clouds. Unlike relative humidity, which changes as the temperature fluctuates, the dew point is an absolute measure of the amount of moisture in the air.

How to Interpret the Results

The dew point is often used by meteorologists and HVAC professionals to determine "comfort" levels:

  • Below 50°F (10°C): Very comfortable, dry air.
  • 50°F to 60°F (10°C to 15.5°C): Comfortable.
  • 60°F to 65°F (15.5°C to 18°C): Sticky; humidity is noticeable.
  • 65°F to 70°F (18°C to 21°C): Uncomfortable; oppressive humidity.
  • Above 75°F (24°C): Extremely uncomfortable; potentially hazardous.

The Calculation Method

This calculator utilizes the Magnus-Tetens Formula, which provides a highly accurate approximation for ambient earth conditions:

γ(T, RH) = ln(RH/100) + [bT / (c + T)]
Tdp = [c × γ(T, RH)] / [b – γ(T, RH)]

Where constants b = 17.625 and c = 243.04°C are used for precise meteorological tracking.

Example Calculation

If the Air Temperature is 80°F and the Relative Humidity is 60%:

  1. Convert 80°F to Celsius: (80 – 32) × 5/9 = 26.67°C.
  2. Apply Magnus Formula with RH = 60.
  3. The resulting Dew Point is 64.9°F (18.3°C).
  4. At this level, the air begins to feel quite "sticky" or humid.
function calculateDewPoint() { var T = parseFloat(document.getElementById('airTemp').value); var RH = parseFloat(document.getElementById('relHumidity').value); var unit = document.getElementById('tempUnit').value; var resultDiv = document.getElementById('dp-result-container'); var valueDiv = document.getElementById('dp-value'); var comfortDiv = document.getElementById('dp-comfort'); if (isNaN(T) || isNaN(RH)) { alert('Please enter valid numeric values.'); return; } if (RH 100) { alert('Relative humidity must be between 1 and 100%.'); return; } // Convert to Celsius if needed for formula var Tc = (unit === 'F') ? (T – 32) * 5 / 9 : T; // Magnus-Tetens Constants var b = 17.625; var c = 243.04; // Intermediate gamma calculation var gamma = Math.log(RH / 100) + (b * Tc) / (c + Tc); // Dew point in Celsius var TdpC = (c * gamma) / (b – gamma); // Final Output values var finalResult; var dewPointF; if (unit === 'F') { finalResult = (TdpC * 9 / 5) + 32; dewPointF = finalResult; valueDiv.innerHTML = finalResult.toFixed(1) + ' °F'; } else { finalResult = TdpC; dewPointF = (TdpC * 9 / 5) + 32; valueDiv.innerHTML = finalResult.toFixed(1) + ' °C'; } // Comfort Level Logic based on Fahrenheit dew point var comfortMsg = ""; var comfortColor = ""; if (dewPointF = 50 && dewPointF = 60 && dewPointF = 65 && dewPointF = 70 && dewPointF < 75) { comfortMsg = "Very Humid / Oppressive"; comfortColor = "#d35400"; } else { comfortMsg = "Extremely Uncomfortable"; comfortColor = "#c0392b"; } comfortDiv.innerHTML = comfortMsg; comfortDiv.style.color = comfortColor; resultDiv.style.display = 'block'; }

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