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ConverterDew Point & Relative Humidity Calculator
Free online calculator to convert between temperature, relative humidity, and dew point. Calculate psychrometric properties for HVAC applications.
The Dew Point & Relative Humidity Calculator converts between temperature (T), relative humidity (RH), and dew point (DP). Essential for HVAC design, indoor air quality analysis, and understanding condensation risk.
Condensation occurs when surface temperature drops below dew point. Comfortable indoor RH is typically 30-60%.
Have feedback? Report bugs, suggest features, or share your thoughts — we read them allWhat is Dew Point and Relative Humidity?
Dew Point is the temperature at which air becomes saturated with moisture and water vapor begins to condense into liquid (dew). Relative Humidity (RH) is the ratio of current moisture content to the maximum moisture the air can hold at that temperature, expressed as a percentage. Understanding the relationship between temperature, RH, and dew point is crucial for HVAC design, mold prevention, and indoor comfort. High dew points (above 65°F/18°C) feel muggy, while low dew points (below 50°F/10°C) feel dry.
Psychrometric Properties
- Dew Point: Temperature at which condensation begins (100% RH)
- Relative Humidity: Current moisture as % of saturation moisture
- Absolute Humidity: Actual mass of water vapor per unit volume of air (g/m³)
- Wet Bulb Temperature: Lowest temperature achievable by evaporative cooling
- Vapor Pressure: Partial pressure of water vapor in air mixture (kPa)
- Saturation Pressure: Vapor pressure at 100% RH for given temperature
How to Use This Calculator
- Select your preferred temperature unit: Celsius, Fahrenheit, or Kelvin
- Choose calculation mode: Calculate DP from T/RH or calculate RH from T/DP
- Enter dry bulb temperature (ambient air temperature)
- For DP calculation: Enter relative humidity percentage (0-100%)
- For RH calculation: Enter dew point temperature
- Click Calculate to see all psychrometric properties
- Results include dew point, RH, wet bulb, absolute humidity, and vapor pressure
Psychrometric Calculation Formulas
1. Magnus-Tetens Formula (Saturation Vapor Pressure)
e_s(T) = 6.112 × exp[(17.67 × T) / (T + 243.5)] kPa (for T in °C)
2. Actual Vapor Pressure
e = (RH / 100) × e_s(T)
3. Dew Point from RH
T_d = [243.5 × ln(e/6.112)] / [17.67 - ln(e/6.112)]
4. Relative Humidity from Dew Point
RH = 100 × [e_s(T_d) / e_s(T)]
5. Absolute Humidity
AH = (2165 × e) / (T + 273.15) g/m³
HVAC Applications
- Indoor Comfort: Maintain 30-60% RH and DP below 60°F (15°C) for comfort
- Mold Prevention: Keep surface temperatures above dew point to prevent condensation
- HVAC Design: Size dehumidification equipment based on latent load and target DP
- Condensation Control: Ensure cold surfaces (windows, pipes) stay above DP
- Process Control: Monitor DP for drying, painting, coating applications
- Energy Efficiency: Lower DP reduces cooling energy and improves comfort
Comfort & Health Guidelines
- Below 30% RH: Too dry - dry skin, static electricity, respiratory irritation
- 30-60% RH: Ideal range - comfortable, healthy, prevents mold
- Above 60% RH: Too humid - feels muggy, mold risk, dust mites thrive
- DP below 50°F (10°C): Very dry, uncomfortable for extended periods
- DP 50-60°F (10-15°C): Comfortable, ideal for most people
- DP above 65°F (18°C): Muggy, uncomfortable, high latent cooling load
Practical Tips
- Monitor dew point, not just RH - DP is absolute measure of moisture
- RH changes with temperature, but DP remains constant (for constant moisture)
- Condensation forms on surfaces below dew point temperature
- High dew points indicate high moisture content regardless of temperature
- Dehumidification is most efficient when cooling air below its dew point
- Use psychrometric charts to visualize T/RH/DP relationships
- Indoor DP should typically be 10-15°F below indoor temperature
- Window condensation indicates indoor DP exceeds window surface temperature
Common HVAC Scenarios
- Summer cooling: 75°F, 50% RH → DP = 55°F (good comfort)
- Winter heating: 70°F, 30% RH → DP = 38°F (acceptable but dry)
- Muggy conditions: 80°F, 70% RH → DP = 69°F (very uncomfortable)
- Dry winter: 68°F, 20% RH → DP = 24°F (too dry, add humidity)
- Condensation risk: Indoor 70°F DP 50°F, window surface 45°F → condensation occurs
Frequently Asked Questions
Use the Magnus-Tetens approximation: first compute the saturation vapor pressure es(T) = 6.112 × exp[(17.67 × T) / (T + 243.5)] in kPa (T in °C), then the actual vapor pressure e = (RH/100) × es(T), and finally dew point Td = [243.5 × ln(e/6.112)] / [17.67 − ln(e/6.112)]. The calculator does this automatically — just enter dry-bulb temperature and RH, choose your unit (°C, °F, or K), and click Calculate. Quick mental shortcut: for every 1°C drop in dew point below ambient, RH falls by about 5% in the typical 50-70% range. So 25°C with 50% RH gives a dew point around 14°C; 25°C with 60% RH gives around 17°C.
All three. Pick from the unit dropdown and all inputs/outputs use that scale consistently — including dew point, wet bulb, and the absolute humidity value (which is always in g/m³ since it is a density). Conversion formulas: °F = °C × 9/5 + 32, K = °C + 273.15. The Magnus equation works in °C internally; we convert your input and convert results back automatically. RH is always in percent (0-100%) regardless of temperature unit. Vapor pressure outputs are in kPa (kilopascals), the SI psychrometric unit; multiply by 10 for mbar, or by 0.145 for psi. Most HVAC equipment specs in the US use °F for dry-bulb and dew point, so switching to °F makes equipment matching easier.
Dew point is an absolute measure of moisture content; RH is a relative measure that changes with temperature even when actual moisture stays constant. If outdoor air is 30°C with 70% RH (dew point 24°C, very humid), and you cool it to 20°C without dehumidifying, RH jumps to 100% (still 24°C dew point, now saturated). The same air mass has the same moisture, but the RH number is misleading. For comfort, design, mold prevention, and equipment selection, target dew point: below 13°C (55°F) feels dry but comfortable, 13-16°C feels neutral, above 18°C (65°F) feels muggy regardless of temperature. ASHRAE 55 thermal comfort standard specifies a maximum humidity ratio (related to dew point), not a maximum RH, for this reason.
Wet bulb temperature is what a thermometer wrapped in wet cloth reads in moving air — it represents the lowest temperature you can reach by evaporative cooling alone. It is always between the dry bulb (highest) and the dew point (lowest); the gap narrows as RH rises. At 100% RH all three are equal. Practical uses: cooling tower performance is limited by wet bulb (you cannot cool process water below the wet bulb of intake air); evaporative coolers (swamp coolers) work only when wet bulb is much lower than dry bulb (dry climates); and OSHA heat-stress limits use wet bulb globe temperature (WBGT) as the danger index. The calculator computes wet bulb iteratively from dry bulb and RH using the psychrometric Carrier equation.
Condensation forms whenever a surface temperature drops below the dew point of surrounding air. To prevent it, either raise the surface temperature (insulate the pipe/wall/window) or lower the indoor dew point (dehumidify). Example: indoor air at 22°C, 60% RH has dew point 14°C. Single-pane window in winter with outdoor temp −5°C might have inner surface temperature of 8°C — below dew point, so condensation forms. Fix: double or triple glazing raises inner surface to 16°C (above dew point, no condensation), or run a dehumidifier to drop dew point below 8°C. For chilled-water pipes, calculate the minimum acceptable insulation thickness from the worst-case dew point in the mechanical room; insufficient insulation causes constant dripping and corrosion.
Absolute humidity (AH) is the actual mass of water vapor per unit volume of moist air, in g/m³. Formula: AH = (2165 × e) / (T + 273.15) where e is vapor pressure in kPa and T is in °C. Unlike RH, AH stays constant when you heat or cool air without adding/removing moisture — making it ideal for moisture-balance calculations across HVAC systems. Use AH when sizing dehumidifiers (you need to remove X g/m³ × volume × ACH = total g/hr), for industrial drying processes (target product moisture vs ambient), for greenhouse design (vapor pressure deficit drives transpiration), and for indoor pool dehumidification where evaporation rates depend on absolute moisture gradients, not RH alone.
Dew point spread = dry bulb temperature − dew point temperature. It indicates how 'dry' the air feels regardless of RH. A spread of 10°C+ feels comfortably dry, 5-10°C is neutral, 2-5°C feels humid/muggy, and 0-2°C is oppressive (near saturation, sticky, sweat does not evaporate). At 30°C/90% RH and 30°C/40% RH, the spread is 2°C vs 14°C — the second feels noticeably more comfortable even at the same temperature. ASHRAE 55 comfort zone roughly translates to dew point spread of 8-15°C indoors. Aviation METAR reports always include both T and Td so pilots can spot fog risk: when spread drops below 2°C with calm winds and cooling temperatures, fog formation is imminent.
The Magnus-Tetens formula (with coefficients 17.67 and 243.5 °C, sometimes called the August-Roche-Magnus equation) gives accuracy better than 0.4% over the −40°C to +50°C range — adequate for virtually all HVAC, meteorology, and building science work. More precise formulations (Hyland-Wexler 1983 used by ASHRAE; Sonntag 1990; Wagner-Pruss IAPWS-95) differ by less than 0.1°C in computed dew point across the practical range, at the cost of much greater complexity. For high-temperature industrial applications (>50°C drying, sterilization) use the ASHRAE Hyland-Wexler form. For temperatures below freezing where ice is involved, use the Magnus formula with different coefficients (22.46, 272.62 over ice). For everyday building HVAC, Magnus-Tetens is the industry standard and sufficient.
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