The evaporation rate of a solvent is a measure of how quickly it transitions from a liquid to a vapor state under specific conditions. In the industrial and chemical sectors, this is usually expressed as a Relative Evaporation Rate (RER), where n-Butyl Acetate (BuAc) is used as the standard reference with a value of 1.0.
This calculation is critical for formulating paints, coatings, inks, and cleaning agents where the drying time must be precisely controlled to ensure surface quality and operational efficiency.
The Calculation Logic
While experimental measurement is the gold standard, the RER can be estimated using the vapor pressure and molecular weight of the solvent. The formula used in this calculator is derived from the empirical relationship:
Note: This provides an approximation. Actual rates are influenced by humidity, airflow, and temperature.
Evaporation Speed Categories
Category
RER Range (BuAc = 1.0)
Typical Examples
Fast
> 3.0
Acetone, MEK, Hexane
Medium
0.8 – 3.0
Toluene, Ethyl Acetate, n-Butyl Acetate
Slow
< 0.8
Xylene, Water, Mineral Spirits
Practical Example
If you are working with Acetone:
Vapor Pressure: 231 mmHg
Molecular Weight: 58.08 g/mol
Calculated RER: ~5.6 (Fast Evaporator)
This tells a chemist that Acetone will evaporate over 5 times faster than n-Butyl Acetate, making it suitable for rapid cleaning but potentially problematic for coatings that require a smooth leveling period.
function calculateEvaporation() {
var vp = document.getElementById("vaporPressure").value;
var mw = document.getElementById("molWeight").value;
var sName = document.getElementById("solventName").value;
var resDiv = document.getElementById("resultOutput");
if (!vp || !mw || vp <= 0 || mw 3.0) {
category = "Fast Evaporator";
} else if (rer >= 0.8) {
category = "Medium Evaporator";
} else {
category = "Slow Evaporator";
}
var ratio = Math.round(rer * 10) / 10;
if (rer > 1) {
comparison = (sName ? sName : "The solvent") + " evaporates " + ratio + " times faster than n-Butyl Acetate.";
} else if (rer < 1) {
var inverse = Math.round((1 / rer) * 10) / 10;
comparison = (sName ? sName : "The solvent") + " evaporates " + inverse + " times slower than n-Butyl Acetate.";
} else {
comparison = (sName ? sName : "The solvent") + " evaporates at the same rate as n-Butyl Acetate.";
}
document.getElementById("rerValue").innerText = rer;
document.getElementById("rerCategory").innerText = category;
document.getElementById("rerComparison").innerText = comparison;
resDiv.style.display = "block";
}