Wet Bulb Interactive Calculator

If you’re dealing with evaporative cooling, cooling tower sizing, or heat stress prediction, wet bulb temperature is the number you’ll want to focus on. Most engineers brush past it, but that’s usually a mistake. Use the Wet Bulb Interactive Calculator here to work out wet bulb temperature, relative humidity, dew point, heat index, and enthalpy change; you’ll need dry bulb temperature, relative humidity, and atmospheric pressure as input. Wet bulb temperature sets real limits in HVAC design, worker heat exposure rules, and farm weather monitoring. On this page you’ll also find the main psychrometric formulas, an example based on a real cooling tower, details on extreme wet bulb and human physiology, plus an engineering FAQ that goes straight to common calculation errors.

What is Wet Bulb Temperature?

Wet bulb temperature is simply the lowest air temperature you’ll get by evaporating water into it. It gives you a direct handle on how much cooling is realistically possible—if the air is dry, the wet bulb will be much lower than the dry bulb, meaning you have good evaporative capacity. If the air is humid, that gap closes and cooling potential drops fast.

Simple Explanation

The classic example: wrap a thermometer in a wet cloth and blow air over it. Evaporation pulls heat off, driving the temperature lower than the surrounding air. However, as that water evaporates and the surface cools, evaporation slows, and the temperature steadies out. That reading is your wet bulb temperature—a measure of how effective evaporation will be under these exact conditions. As the air gets more humid, water won’t evaporate as quickly, so the wet bulb rises toward the dry bulb. In dry conditions, evaporation is fast and the wet bulb temperature reads much lower.

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Psychrometric Diagram

How to Use This Calculator

1. Pick the mode you need—wet bulb temperature, relative humidity, dew point, heat index, or enthalpy change—from the dropdown.
2. Enter the dry bulb temperature (°C) and the actual atmospheric pressure in kPa (default is sea level: 101.325 kPa).
3. Add the other required values for your chosen mode—relative humidity, wet bulb temperature, or both.
4. Click Calculate to see the result.

Wet Bulb Temperature Calculator

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Simple Example

Dry bulb temperature: 30°C. Relative humidity: 50%. Atmospheric pressure: 101.325 kPa.
Wet bulb temperature result: ≈ 21.8°C.
Dew point result: ≈ 18.4°C.
Vapor pressure: ≈ 2.125 kPa.
That 8.2°C gap shows evaporative cooling can do meaningful work here—if you’re designing a cooler or tower, this is the kind of spread that gives you options.

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Governing Equations

Theory & Practical Applications

Fundamental Psychrometric Principles

Wet bulb temperature is basically a physical limit set by whatever combination of air temperature, humidity, and pressure you have. Forget dew point and dry bulb if you’re interested in evaporation—you need wet bulb for that. The reading you get depends on the point where evaporation (which absorbs heat) and heat coming in from the air balance each other. That’s why you see the classic wet cloth/thermometer method—the physics behind it comes from simultaneous heat and mass transfer between the thermometer, water, and air. The size of the gap between dry and wet bulb tells you directly how effective evaporation will be for cooling systems or climate control—when that gap is big, evaporation does real work. When it’s small, forget relying on evaporation.

When you get down to practical measurements, keep in mind the depression (the difference between dry and wet bulb) is much more influenced by relative humidity at the dry end of the scale than at the humid end. For example, at 30°C dry bulb and 20% relative humidity, each 1% RH shift changes wet bulb about 0.15°C. When you’re up at 80% RH, 1% makes under 0.05°C difference. So, the same instrument that’s good enough for dry climates can be too blunt in a greenhouse or tropical setting where precise readings matter.

Wet bulb temperature is never greater than the dry bulb by definition—you only get equal readings at 100% RH when evaporation completely stalls out.

Cooling Tower Performance and HVAC Applications

Every cooling tower is built with local wet bulb temperature as the key design input, because you simply can’t cool water below it by evaporation alone. Mechanical draft towers often get water within 2-3°C of ambient wet bulb; natural draft towers do a bit worse, maybe 4-6°C. That’s why a plant with a 35°C water inlet and 28°C wet bulb can only get outlet water down to about 30°C, maybe a little less if the tower is oversized or the weather is favorable. That 4–5°C gap adds up in fuel and operating cost when scaled up to a big industrial site.

For HVAC, you’ll see wet bulb temperature used for sizing evaporative coolers or figuring out how hard chillers need to work. Direct evaporative coolers—like swamp coolers—usually can get within 2-4°C of wet bulb, which is a huge swing in dry places. In Phoenix with summer design numbers of 43°C dry bulb, 21°C wet bulb, a swamp cooler can supply air around 23-25°C; in Houston with 35°C dry bulb, 27°C wet bulb, there’s no practical benefit. The effectiveness of indirect evaporative coolers is a little better, as they use more clever cycles (like the Maisotsenko process), and they occasionally push supply air a couple degrees below wet bulb, but this is only achievable in certain climates with dry air.

Heat Stress Assessment and Occupational Safety

All practical heat stress rules are based on wet bulb or its weighted cousin, Wet Bulb Globe Temperature (WBGT). The idea is simple: humans can’t cool themselves if sweat can’t evaporate, and that’s set by wet bulb, not just air temperature. Work-rest cycles, military training protocols, and major sports events all use WBGT levels as their trigger for when to rest, limit exposure, or stop activity, because the risk spirals when wet bulb climbs above about 29–31°C. The theoretical limit of human survival sits close to 35°C wet bulb—above this, sweating doesn’t keep up with internal heat production even at rest. These limits aren’t just theory—regions in the Persian Gulf and Indus Valley already see wet bulbs near 32°C during severe humid heatwaves. That’s why air temperature alone is a poor measure of heat stress risk—what matters is evaporation potential, and that’s wet bulb, period.

Agricultural and Meteorological Applications

In agriculture, wet bulb tells you about both frost risk and how much water crops lose to evaporation. When wet bulb drops below freezing while dry bulb is still above, you have the right conditions for radiative frost—moisture from the air can freeze on crops even before the air reaches 0°C. Growers use the wet bulb/dry bulb spread to optimize frost protection—smaller spreads mean less evaporative cooling, which can help prevent crop icing during cold, humid nights. For irrigation, wet bulb is part of the vapor pressure deficit calculation: it’s the driving force behind evapotranspiration. The higher the vapor pressure deficit, the more water is drawn out of plants, regardless of air temperature. That means two fields with the same temperature but different humidity can need very different irrigation schedules.

Worked Engineering Example: Cooling Tower Capacity Analysis

Problem: An industrial site in Atlanta runs a cooling tower for a 500-ton chiller. The hottest design day is 33.7°C dry bulb, 24.8°C wet bulb, pressure 96.8 kPa (about 312m elevation). The tower needs to cool 252 L/min (4.2 kg/s) of condenser water from 37.2°C to 30.6°C. Check if this tower can do the job, figure needed air flow, and see what happens if the wet bulb rises to 26.5°C in a heat wave.

Given:

• Dry bulb temperature: T_db = 33.7°C
• Wet bulb temperature: T_wb = 24.8°C (design), 26.5°C (extreme)
• Atmospheric pressure: P = 96.8 kPa
• Water flow rate: ṁ_w = 252 L/min = 4.20 kg/s (ρ = 1000 kg/m³)
• Inlet water temperature: T_w,in = 37.2°C
• Outlet water temperature: T_w,out = 30.6°C
• Specific heat of water: c_p,w = 4.187 kJ/kg·K

Solution — Part 1: Heat Rejection Requirement

First, calculate heat to reject:
Q = ṁ_w × c_p,w × (T_w,in - T_w,out)
Q = 4.20 kg/s × 4.187 kJ/kg·K × (37.2 - 30.6) K
Q = 4.20 × 4.187 × 6.6
Q = 116.1 kJ/s = 116.1 kW

Convert that to tons of refrigeration (1 ton = 3.517 kW):
Q = 116.1 / 3.517 = 33.0 tons

This is the actual heat the cooling tower has to dump, combining the chiller’s load and compressor work.

Solution — Part 2: Approach Temperature Check

Approach temperature tells if the tower can hit its water outlet set point:
Approach = T_w,out - T_wb
Approach = 30.6 - 24.8 = 5.8°C

For mechanical draft towers, 2.8–5.6°C is normal; up to 8.3°C for basic designs. At 5.8°C, this setup is typical and realistic.

Solution — Part 3: Required Air Flow Rate

First, find saturation vapor pressure at wet bulb using the Magnus formula:

e_s,wb = 0.61121 × exp[(18.678 - 24.8/234.5) × (24.8/(257.14 + 24.8))]
e_s,wb = 3.132 kPa

Calculate humidity ratio at wet bulb:

W_wb = 0.622 × 3.132 / (96.8 - 3.132)
W_wb = 0.02080 kg water/kg dry air

Enthalpy of incoming air at wet bulb (for this mass flow):

h_in = 1.006 × 24.8 + 0.02080 × (2501 + 1.86 × 24.8)
h_in = 77.93 kJ/kg dry air

Do the same at the warmer outlet water (i.e., air leaving saturated at 30.6°C):

e_s,out = 4.397 kPa

W_out = 0.02960 kg water/kg dry air

h_out = 106.49 kJ/kg dry air

Now apply energy balance to get the dry air mass flow:

Q = ṁ_a × (h_out - h_in)
116.1 kW = ṁ_a × 28.56
ṁ_a = 4.065 kg/s dry air

L/G ratio (liquid to gas mass flow):
L/G = 4.20 / 4.065 = 1.033

This L/G is typical for cooling towers, so the design works under these conditions.

Solution — Part 4: Extreme Wet Bulb (26.5°C)

If the wet bulb climbs to 26.5°C and you try to maintain the same approach:

T_w,out,new = 26.5 + 5.8 = 32.3°C

New cooling: Q_new = 4.20 × 4.187 × (37.2 - 32.3)
Q_new = 86.2 kW = 24.5 tons

Capacity drop: (116.1 - 86.2) / 116.1 = 25.8%

So, about a quarter of your cooling is lost from a 1.7°C jump in wet bulb. This is why cooling tower guarantees are always tied to wet bulb, not just air temperature. You either size bigger for rare extremes, run at reduced load sometimes, or split the tower into stages for flexibility. If you underestimate wet bulb, system performance drops off a cliff on the hottest, most humid days.

Bottom line: Sizing to wet bulb is a must, not a detail. The non-linear loss of capacity with rising wet bulb makes precise psychrometric calculation important for any application where cooling margins are tight.

Measurement Instrumentation and Error Sources

If you want accurate wet bulb measurements, don’t cut corners on the basics. The wick needs to be clean and always soaked with distilled water—tap water leaves mineral deposits that reduce evaporation and skew your readings high. Aspirated or sling types need real airflow, typically over 3 m/s for sling psychrometers or 4.5 m/s for aspirated systems. Slow airflow thickens boundary layers and leads to readings that aren’t representative—expect wet bulbs to read up to 1°C or more too high in these cases. Modern electronic sensors work out wet bulb indirectly using dry bulb and humidity, which avoids wick issues but brings its own sources of drift and inaccuracy, especially as humidity sensors age or need calibration. For very precise applications, chilled mirror dew point sensors are the reference—they’re expensive, but direct and stable for years. For many HVAC roles, a quality digital sensor with periodic calibration gets you close enough, but for critical measurements, the method and maintenance make the largest difference in reliability.

Frequently Asked Questions