Estimate how much water is lost to evaporation based on temperature, humidity, and airflow.
Evaporation Rate (Volume):—
Evaporation Rate (Mass):—
Alternative Volume:—
Based on humidity ratio differential and mass transfer coefficients.
Understanding Water Evaporation Rate
Water evaporation is a fundamental physical process where liquid water turns into water vapor. Understanding the rate at which this happens is crucial for managing swimming pools, aquariums, industrial cooling towers, and agricultural reservoirs. This calculator helps you estimate the water loss over time based on environmental conditions.
Key Factors Affecting Evaporation
Several dynamic variables influence how quickly water evaporates from a surface:
Water Temperature: Warmer water has higher kinetic energy, making it easier for molecules to escape the surface. Higher water temperature generally increases evaporation.
Air Temperature & Humidity: These determine the "thirst" of the air. Dry, warm air can hold more moisture than cool, saturated air. The difference between the water's vapor pressure and the air's vapor pressure is the driving force.
Air Velocity (Wind Speed): Wind blows away the layer of humid air sitting immediately above the water surface. Higher wind speeds maintain a steeper concentration gradient, significantly speeding up evaporation.
Surface Area: Evaporation occurs at the interface between water and air. The larger the surface area, the more space there is for molecules to escape.
The Mathematics Behind the Calculation
This calculator utilizes standard engineering formulas derived from Dalton's Law of Partial Pressures and mass transfer coefficients. The core logic involves calculating the difference in Humidity Ratio (kg of water vapor per kg of dry air) between the saturated air at the water's surface and the ambient air.
Formula Concept: Rate = Θ × Area × (xsurface – xair)
Where x represents the humidity ratio derived from vapor pressure calculations (using the Magnus formula), and Θ is an evaporation coefficient that increases with wind speed.
Practical Applications
Swimming Pools: Pool owners use these calculations to size heaters and dehumidifiers for indoor pools, or to estimate water top-up requirements for outdoor pools.
Aquariums: Hobbyists need to know evaporation rates to manage salinity in saltwater tanks (since only fresh water evaporates, leaving salt behind) and to maintain water levels.
Industrial Processing: Open tanks and cooling ponds rely on evaporation for heat rejection. Accurate estimates are vital for energy and water balance calculations.
How to Reduce Evaporation?
If your calculation shows a high rate of water loss, consider these mitigation strategies:
Use a Cover: A physical pool cover or floating blanket is the most effective way to stop evaporation, reducing it by up to 95%.
Reduce Wind Exposure: Installing windbreaks or fences around a body of water can lower air velocity across the surface.
Lower Water Temperature: Keep the water as cool as practically possible when not in use.
var currentUnit = 'metric';
function toggleUnits() {
var radios = document.getElementsByName('units');
for (var i = 0; i < radios.length; i++) {
if (radios[i].checked) {
currentUnit = radios[i].value;
break;
}
}
if (currentUnit === 'metric') {
document.getElementById('label-water-temp').innerText = '(°C)';
document.getElementById('label-air-temp').innerText = '(°C)';
document.getElementById('label-velocity').innerText = '(m/s)';
document.getElementById('label-area').innerText = '(m²)';
document.getElementById('waterTemp').placeholder = 'e.g. 25';
document.getElementById('airTemp').placeholder = 'e.g. 22';
document.getElementById('airVelocity').placeholder = 'e.g. 0.5';
document.getElementById('surfaceArea').placeholder = 'e.g. 10';
} else {
document.getElementById('label-water-temp').innerText = '(°F)';
document.getElementById('label-air-temp').innerText = '(°F)';
document.getElementById('label-velocity').innerText = '(mph)';
document.getElementById('label-area').innerText = '(ft²)';
document.getElementById('waterTemp').placeholder = 'e.g. 77';
document.getElementById('airTemp').placeholder = 'e.g. 72';
document.getElementById('airVelocity').placeholder = 'e.g. 1.1';
document.getElementById('surfaceArea').placeholder = 'e.g. 100';
}
// Clear result when switching units to avoid confusion
document.getElementById('result-container').style.display = 'none';
}
function calculateEvaporation() {
// 1. Get raw input values
var waterTempRaw = parseFloat(document.getElementById('waterTemp').value);
var airTempRaw = parseFloat(document.getElementById('airTemp').value);
var humidity = parseFloat(document.getElementById('relHumidity').value);
var velocityRaw = parseFloat(document.getElementById('airVelocity').value);
var areaRaw = parseFloat(document.getElementById('surfaceArea').value);
// 2. Validate inputs
if (isNaN(waterTempRaw) || isNaN(airTempRaw) || isNaN(humidity) || isNaN(velocityRaw) || isNaN(areaRaw)) {
alert("Please fill in all fields with valid numbers.");
return;
}
if (humidity 100) {
alert("Relative Humidity must be between 0 and 100%.");
return;
}
if (areaRaw <= 0) {
alert("Surface Area must be greater than 0.");
return;
}
// 3. Normalize inputs to Metric for Calculation (Celsius, m/s, m²)
var T_water = waterTempRaw;
var T_air = airTempRaw;
var V = velocityRaw;
var A = areaRaw;
if (currentUnit === 'imperial') {
// Convert F to C
T_water = (waterTempRaw – 32) * 5 / 9;
T_air = (airTempRaw – 32) * 5 / 9;
// Convert mph to m/s (1 mph = 0.44704 m/s)
V = velocityRaw * 0.44704;
// Convert sq ft to m² (1 sq ft = 0.092903 m²)
A = areaRaw * 0.092903;
}
// 4. Calculate Saturation Vapor Pressures (kPa) using Magnus Formula
// Psat = 0.6112 * exp((17.67 * T) / (T + 243.5))
var P_sat_water = 0.6112 * Math.exp((17.67 * T_water) / (T_water + 243.5));
var P_sat_air = 0.6112 * Math.exp((17.67 * T_air) / (T_air + 243.5));
// 5. Calculate Actual Vapor Pressure of Air
var P_air_actual = P_sat_air * (humidity / 100);
// 6. Calculate Humidity Ratios (x) in kg_water / kg_dry_air
// x = 0.622 * P_vapor / (P_atm – P_vapor)
// Standard atmospheric pressure P_atm = 101.325 kPa
var P_atm = 101.325;
var x_surface = 0.622 * P_sat_water / (P_atm – P_sat_water);
var x_air = 0.622 * P_air_actual / (P_atm – P_air_actual);
// 7. Calculate Evaporation Rate (g_h) in kg/hr
// Formula: g = Θ * A * (x_surface – x_air)
// Θ (Theta) = 25 + 19 * V (Engineering Toolbox standard coefficient for water surfaces)
var Theta = 25 + (19 * V);
// Evaporation rate in kg per hour
var evapRateKgHr = Theta * A * (x_surface – x_air);
// Handle Condensation (negative evaporation)
var isCondensation = false;
if (evapRateKgHr < 0) {
evapRateKgHr = 0; // Or handle as condensation gain
isCondensation = true;
}
// 8. Convert Results for Display
// Water density approx 1 kg/L
var evapRateLitersHr = evapRateKgHr;
var evapRateLitersDay = evapRateLitersHr * 24;
// Imperial Conversions
// 1 kg = 2.20462 lbs
// 1 Liter = 0.264172 Gallons
var evapRateLbsHr = evapRateKgHr * 2.20462;
var evapRateGallonsDay = evapRateLitersDay * 0.264172;
// 9. Display
var resultPrimary = document.getElementById('result-volume-primary');
var resultMass = document.getElementById('result-mass');
var resultSecondary = document.getElementById('result-volume-secondary');
var formulaNote = document.getElementById('formula-note');
if (isCondensation) {
resultPrimary.innerText = "0 (Condensation)";
resultMass.innerText = "0 kg/hr";
resultSecondary.innerText = "Conditions favor condensation";
formulaNote.innerText = "The air is warmer and more humid than the water surface (Dew point reached).";
} else {
if (currentUnit === 'metric') {
resultPrimary.innerText = evapRateLitersDay.toFixed(2) + " Liters / day";
resultMass.innerText = evapRateKgHr.toFixed(3) + " kg / hour";
resultSecondary.innerText = (evapRateLitersDay / 1000).toFixed(4) + " m³ / day";
} else {
resultPrimary.innerText = evapRateGallonsDay.toFixed(2) + " Gallons / day";
resultMass.innerText = evapRateLbsHr.toFixed(3) + " lbs / hour";
resultSecondary.innerText = (evapRateGallonsDay / 24).toFixed(3) + " Gallons / hour";
}
formulaNote.innerText = "Based on humidity ratio differential and mass transfer coefficients.";
}
document.getElementById('result-container').style.display = 'block';
}