Total volume in kettle before heating starts (Gallons or Liters).
Volume remaining immediately after flame-out.
How long the wort was boiling.
Hourly Boil Off Rate:–
Total Volume Lost:–
Evaporation Percentage:–
function calculateBoilOff() {
// Get input values
var preVol = document.getElementById('preBoilVolume').value;
var postVol = document.getElementById('postBoilVolume').value;
var duration = document.getElementById('boilDuration').value;
var errorDiv = document.getElementById('errorMsg');
var resultDiv = document.getElementById('result-section');
// Reset display
errorDiv.style.display = 'none';
resultDiv.style.display = 'none';
// Validation
if (preVol === "" || postVol === "" || duration === "") {
errorDiv.innerText = "Please fill in all fields.";
errorDiv.style.display = 'block';
return;
}
var preVolNum = parseFloat(preVol);
var postVolNum = parseFloat(postVol);
var durationNum = parseFloat(duration);
if (isNaN(preVolNum) || isNaN(postVolNum) || isNaN(durationNum)) {
errorDiv.innerText = "Please enter valid numbers.";
errorDiv.style.display = 'block';
return;
}
if (durationNum = preVolNum) {
errorDiv.innerText = "Post-boil volume must be less than Pre-boil volume.";
errorDiv.style.display = 'block';
return;
}
// Calculation Logic
var totalLost = preVolNum – postVolNum;
var durationHours = durationNum / 60;
var hourlyRate = totalLost / durationHours;
var evaporationPercent = (totalLost / preVolNum) * 100; // Total percent lost
var hourlyPercent = (hourlyRate / preVolNum) * 100; // Percent per hour based on starting vol
// Update Results
document.getElementById('hourlyRateResult').innerText = hourlyRate.toFixed(2) + " Vol/Hr";
document.getElementById('totalLossResult').innerText = totalLost.toFixed(2) + " Vol";
document.getElementById('percentResult').innerText = hourlyPercent.toFixed(1) + "% per Hour";
resultDiv.style.display = 'block';
}
Understanding Your Boil Off Rate
In homebrewing and professional brewing alike, consistency is the key to creating great beer. One of the most critical variables in your equipment profile is the Boil Off Rate. This metric determines how much water evaporates from your brew kettle during the boiling process.
Accurately knowing this number allows you to calculate the exact pre-boil volume needed to hit your target batch size and Original Gravity (OG). If you underestimate your boil off, you end up with too little beer that is too strong. If you overestimate it, you end up with watered-down wort and miss your gravity targets.
How to Use This Calculator
To determine your system's boil off rate, you should perform a test run (with water) or measure carefully during your next brew day:
Pre-Boil Volume: Measure the exact volume of liquid in your kettle right when it reaches a rolling boil.
Post-Boil Volume: Measure the volume immediately after you turn off the heat (Flame-out).
Boil Duration: Enter the number of minutes the liquid was boiling (usually 60 or 90 minutes).
Factors Influencing Evaporation
Your boil off rate is generally static for your specific equipment setup, but it can be influenced by several factors:
Kettle Geometry: The wider the diameter of your kettle, the larger the surface area, and the higher the evaporation rate. Wide, shallow pots boil off more than tall, narrow keggles.
Heat Intensity: A vigorous, rolling boil will evaporate more water than a gentle simmer. Try to maintain a consistent boil intensity for every batch.
Humidity and Altitude: Environmental factors play a minor role. Lower humidity and higher altitudes (where the boiling point is lower) can increase evaporation rates.
Applying the Results
Once you have your rate (e.g., 1.2 gallons per hour), you should input this into your brewing software (like BeerSmith, Brewfather, or Brewer's Friend). This ensures that when you design a recipe, the water calculations for strike water and sparge water are perfectly tailored to your system.
Note: This calculator assumes standard atmospheric boiling. Remember that water expands when hot and contracts when cool. For the most precise results, try to measure both volumes at near-boiling temperatures, or correct for thermal expansion (roughly 4%).