Wet Bulb Temperature Calculator: Measure Humidity & Cooling Potential (Free Tool)
You are an HVAC technician. You need to know the wet bulb temperature. It tells you how much moisture is in the air. It helps you size cooling coils. It predicts evaporative cooling performance.
Wet bulb temperature is measured with a thermometer that has a wet wick. It is always lower than or equal to dry bulb temperature.
Today, I give you a free Wet Bulb Temperature Calculator.
You enter dry bulb temperature and relative humidity (or dew point). The calculator shows:
- Wet bulb temperature (°F or °C)
- Humidity ratio (grains per pound)
- Enthalpy (BTU/lb)
- Dew point
Let me explain what wet bulb temperature is and why it matters.
What is Wet Bulb Temperature? (Simple Explanation)
Wet bulb temperature is the lowest temperature that can be reached by evaporative cooling. It is measured with a thermometer covered by a wet wick.
How it works:
Water evaporates from the wick. Evaporation cools the thermometer. The drier the air, the more evaporation, and the lower the wet bulb.
Dry bulb vs. wet bulb:
- Dry bulb = regular air temperature
- Wet bulb = cooling limit from evaporation
- Difference = humidity indicator
Example: 75°F dry bulb, 50% RH → wet bulb ≈ 62.5°F
Why This Wet Bulb Temperature Calculator Matters
Here is why you need to calculate wet bulb temperature.
Reason 1: Evaporative cooling performance
Swamp coolers can only cool air to the wet bulb temperature. The lower the wet bulb, the better the cooling.
Reason 2: HVAC load calculations
Wet bulb is needed for psychrometric analysis and coil sizing. It accounts for latent heat (moisture) in the air.
Reason 3: Human comfort and heat stress
Wet bulb above 86°F is dangerous. The body cannot cool itself by sweating. The OSHA WBGT calculator uses wet bulb to assess heat stress.
Reason 4: System capacity calculations
Wet bulb temperature and enthalpy are directly related on the psychrometric chart. A particular wet-bulb temperature equals a particular enthalpy (at 14.7 PSIA).
Reason 5: Charging by superheat
Superheat charging charts require indoor wet bulb temperature and outdoor dry bulb temperature to find target superheat.
The Wet Bulb Formula
Wet bulb is calculated from dry bulb and humidity using psychrometric equations.
The relationship is derived from the energy balance between sensible and latent heat at the wet bulb surface:
e = Esat(Tw) − Le^0.67 × γ × (Tair − Tw)
Where:
- Tw = wet-bulb temperature
- Tair = dry-bulb temperature
- e = actual vapor pressure
- Esat(Tw) = saturation vapor pressure at Tw
- γ = psychrometric constant
The equation is solved numerically.
For quick estimate:
WBT ≈ T – ((100 – RH) ÷ 5)
Example: 75°F, 50% RH → 75 – (50 ÷ 5) = 75 – 10 = 65°F (close to actual 62.5°F).
Stull approximation (valid RH 5-99%, temp -20°C to 50°C):
The empirical equation has mean absolute error less than 0.3°C over the valid range.
LIVE Wet Bulb Temperature Calculator
Enter dry bulb temperature and relative humidity. The calculator shows wet bulb and other properties instantly.
💧 Wet Bulb Temperature Calculator
🌡️ Dry Bulb Temperature
💧 Relative Humidity (%)
💧 Wet Bulb Temperature Calculator
Calculate wet bulb from dry bulb and humidity
🌡️ Dry Bulb Temperature
°F °C
💧 Relative Humidity (%)
🌡️ WET BULB TEMPERATURE
0°F
📊 ENTHALPY
0 BTU/lb
0%
💡 Wet bulb is the lowest temperature achievable by evaporative cooling. Wet bulb > 86°F is dangerous.
📐 For HVAC load calculations, use wet bulb for psychrometric analysis.
How to Use This Wet Bulb Calculator
Follow these 3 simple steps.
Step 1: Enter dry bulb temperature (air temperature)
Step 2: Enter relative humidity percentage
Step 3: Select temperature units (°F or °C)
The calculator shows:
- Wet bulb temperature
- Enthalpy (total heat content)
Real Examples: Different Conditions
Example 1: Comfortable room
- Dry bulb: 75°F
- RH: 50%
- Wet bulb: 62.5°F
Example 2: Dry desert air
- Dry bulb: 95°F
- RH: 20%
- Wet bulb: 66°F
Result: Wet bulb much lower than dry bulb. Good evaporative cooling potential.
Example 3: Humid summer
- Dry bulb: 90°F
- RH: 70%
- Wet bulb: 81°F
Result: Wet bulb close to dry bulb. Poor evaporative cooling.
Example 4: Dangerous condition
- Dry bulb: 100°F
- RH: 80%
- Wet bulb: 94°F
Result: Above 86°F. Life-threatening.
Wet Bulb vs. Dry Bulb by Humidity
| Dry Bulb | RH | Wet Bulb | Difference |
|---|---|---|---|
| 80°F | 20% | 56°F | 24°F |
| 80°F | 40% | 63°F | 17°F |
| 80°F | 60% | 69°F | 11°F |
| 80°F | 80% | 74°F | 6°F |
| 80°F | 100% | 80°F | 0°F |
Key point: The higher the humidity, the closer wet bulb is to dry bulb.
Wet Bulb and Enthalpy
On the psychrometric chart, wet-bulb temperature and enthalpy are equivalent. A particular wet-bulb temperature is also equal to a particular enthalpy (at 14.7 PSIA).
Example: A 62.8-degree wet-bulb mass of air contains approximately 28.4 BTUs per lb. This is why accurate wet bulb measurement is critical for HVAC load calculations.
Wet Bulb and Evaporative Cooling
Evaporative coolers (swamp coolers) work by evaporating water into the air. The cooling limit is the wet bulb temperature.
Cooling potential = Dry bulb – Wet bulb
Example: 95°F dry bulb, 65°F wet bulb → potential cooling = 30°F
In humid climates (WB > 70°F): Evaporative cooling is ineffective.
Wet Bulb Globe Temperature (WBGT)
WBGT is a heat stress index used for sports and military. It combines wet bulb, globe temperature (radiant heat), and dry bulb.
WBGT = 0.7 × WB + 0.2 × GT + 0.1 × DB
Risk levels from OSHA:
| WBGT (°F) | Risk | Activity guidelines |
|---|---|---|
| < 77 | Low | Normal activity |
| 77-82 | Moderate | Caution |
| 82-86 | High | Limit exertion |
| 86-90 | Very high | Reduce activity |
| > 90 | Extreme | Cancel outdoor activity |
The OSHA calculator uses wet bulb as a key input for WBGT estimation.
Wet Bulb and Human Survival
The human body cools itself by sweating. Sweat evaporates, removing heat.
If wet bulb exceeds skin temperature (about 95°F): Evaporation stops. Body cannot cool. Heat stroke occurs within hours.
Climate change concern: Models predict wet bulb temperatures exceeding 95°F in some regions by 2100. This would be lethal without air conditioning.
Wet Bulb in HVAC
Coil leaving air temperature: Must be below room dew point to dehumidify. Wet bulb helps determine coil performance.
Superheat charging: You need to take the wet bulb temperature at the evaporator to find superheat targets.
Cooling capacity formula: Q_total = 4.5 × CFM × (h_enter – h_leave)
Where h is enthalpy (from wet bulb).
Measuring Wet Bulb
Sling psychrometer:
- Wet the wick with distilled water
- Spin the psychrometer for 30-60 seconds
- Read the wet bulb temperature
- Repeat until stable
Accuracy tips:
- Use distilled water (mineral deposits affect readings)
- Spin fast enough (10-15 ft/sec)
- Avoid radiant heat sources
Electronic psychrometer: Uses sensors to measure wet bulb directly.
Frequently Asked Questions (FAQs)
1. What is wet bulb temperature?
The lowest temperature achievable by evaporative cooling. Measured with a wet wick thermometer.
2. How do you calculate wet bulb from dry bulb and humidity?
Use psychrometric equations or the calculator above.
3. Why is wet bulb lower than dry bulb?
Evaporation cools the thermometer. The drier the air, the more cooling.
4. What is a dangerous wet bulb temperature?
Above 86°F. The body cannot cool itself effectively.
5. What is the highest possible wet bulb temperature?
95°F (at 100°F dry bulb, 100% RH). Beyond that, the human body cannot survive.
6. How does wet bulb affect evaporative cooling?
The cooling limit is the wet bulb temperature. The difference between dry bulb and wet bulb is the cooling potential.
7. What is the difference between wet bulb and dew point?
Wet bulb is measured with a wet wick. Dew point is the temperature where condensation forms. At 100% RH, they are the same.
8. Do I need a psychrometer to measure wet bulb?
You can calculate it from dry bulb and humidity, but direct measurement is more accurate.
9. What is the wet bulb temperature of saturated air?
Wet bulb equals dry bulb when RH = 100%.
10. How does wet bulb relate to enthalpy in HVAC?
On the psychrometric chart, wet bulb temperature and enthalpy are equivalent. A specific wet bulb temperature corresponds to a specific enthalpy value.
Common Mistakes
Mistake #1: Confusing wet bulb with dew point
Wet bulb is measured. Dew point is calculated. They are different.
Mistake #2: Using tap water in a sling psychrometer
Mineral deposits clog the wick. Use distilled water.
Mistake #3: Not spinning the psychrometer long enough
Spin for at least 30 seconds to reach equilibrium.
Mistake #4: Assuming wet bulb equals dry bulb in humid conditions
Only at 100% RH. At 80% RH, wet bulb is still lower.
Mistake #5: Ignoring wet bulb in heat stress calculations
Wet bulb is the most important factor for heat-related illness risk.
Wet Bulb and Climate Change
Current hottest wet bulb: 95°F (recorded in some regions).
Projected future: By 2100, some regions may exceed 95°F wet bulb for hours at a time. Without air conditioning, these areas would become uninhabitable.
Final Thoughts
Wet bulb temperature is essential for HVAC, evaporative cooling, and heat stress assessment.
My Wet Bulb Temperature Calculator gives you:
- Wet bulb from dry bulb and humidity
- Enthalpy for load calculations
- Safety guidance for high wet bulb conditions
Bookmark this page. Use it for HVAC design, sports safety, and understanding humidity.
The next time someone asks “how humid is it?”, wet bulb tells the real story.
Disclaimer: This is an educational tool. For heat stress decisions, use professional guidelines.
External Links (Authority Backlinks):
- Wikipedia – Wet-bulb temperature{:target=”_blank” rel=”noopener noreferrer”}
- Wikipedia – Evaporative cooler{:target=”_blank” rel=”noopener noreferrer”}
- Wikipedia – Wet-bulb globe temperature{:target=”_blank” rel=”noopener noreferrer”}
- OSHA – WBGT Calculator{:target=”_blank” rel=”noopener noreferrer”}
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