Standard Ale (0.75M cells/mL/°P)
High Gravity Ale (1.0M cells/mL/°P)
Standard Lager (1.5M cells/mL/°P)
High Gravity Lager (2.0M cells/mL/°P)
Low Gravity / Fresh Yeast (0.5M cells/mL/°P)
Pitching Requirements
Total Cells Needed:
–
Gravity (Plato approx.):
–
Wyeast Pack Viability
Age of Pack:
–
Estimated Viability:
–
Viable Cells per Pack:
–
Recommendation:
–
function calculateWyeastPitch() {
// Inputs
var batchSizeGal = parseFloat(document.getElementById('batchSize').value);
var ogStr = document.getElementById('originalGravity').value;
var og = parseFloat(ogStr);
var pitchRate = parseFloat(document.getElementById('targetPitchRate').value);
var mfgDateInput = document.getElementById('mfgDate').value;
// Validation
if (isNaN(batchSizeGal) || batchSizeGal <= 0) {
alert("Please enter a valid batch size greater than 0.");
return;
}
if (isNaN(og) || og <= 1.0) {
alert("Please enter a valid Original Gravity (SG) greater than 1.000.");
return;
}
if (!mfgDateInput) {
alert("Please select the manufacture date found on your Wyeast pack.");
return;
}
// Constants
var LITERS_PER_GALLON = 3.78541;
var INITIAL_CELLS_BILLIONS = 100; // Wyeast standard pack assumes 100B cells initially
var VIABILITY_LOSS_PER_DAY = 0.007; // Approx 0.7% loss per day (linear approximation)
// Step 1: Calculate Fermentation Requirements
// Convert SG to Plato approximation: °P ≈ (SG – 1) * 1000 / 4
var plato = (og – 1.0) * 1000 / 4;
var batchSizeLiters = batchSizeGal * LITERS_PER_GALLON;
// Formula: Cells (Billions) = Pitch Rate * Liters * Plato
var cellsNeededBillions = pitchRate * batchSizeLiters * plato;
// Step 2: Calculate Viability based on Date
var mfgDate = new Date(mfgDateInput);
var today = new Date();
// Reset time portion for accurate day calculation
mfgDate.setHours(0,0,0,0);
today.setHours(0,0,0,0);
var timeDiff = today.getTime() – mfgDate.getTime();
var daysOld = Math.ceil(timeDiff / (1000 * 3600 * 24));
if (daysOld < 0) {
alert("Manufacture date cannot be in the future.");
return;
}
var viabilityDecimal = 1.0 – (daysOld * VIABILITY_LOSS_PER_DAY);
if (viabilityDecimal 2.5 || viableCellsPerPackBillions < 30) {
starterDiv.innerHTML = "Strong Recommendation: Due to low viability or high cell requirements, making a yeast starter is highly recommended rather than buying multiple packs. Direct pitching this many packs is expensive and may still result in underpitching.";
recBox.style.borderLeftColor = "#d9534f"; // Red warning color
recBox.style.backgroundColor = "#f8d7da";
} else if (packsNeeded > 1.2) {
starterDiv.innerHTML = "Note: You need more than one fresh pack. Consider a small starter or pitching two packs to ensure a healthy fermentation.";
recBox.style.borderLeftColor = "#f0ad4e"; // Orange warning color
recBox.style.backgroundColor = "#fcf8e3";
} else {
starterDiv.innerHTML = "A single direct pitch should be sufficient for this batch.";
recBox.style.borderLeftColor = "#4caf50"; // Green success color
recBox.style.backgroundColor = "#e7f4e4";
}
}
Understanding Yeast Pitch Rates and Viability for Homebrewing
Achieving a consistent, high-quality fermentation is arguably the most critical step in brewing great beer. A primary cause of stalled fermentations, off-flavors (like esters and fusel alcohols), and poor attenuation is "underpitching"—not supplying enough healthy yeast cells to handle the sugar load of your wort. This Wyeast Pitch Rate Calculator helps you determine exactly how much yeast you need based on your specific batch parameters and the age of your yeast pack.
Why Pitch Rate Matters
The "pitch rate" is the number of yeast cells added per milliliter of wort, per degree Plato (a measure of sugar density). Different beer styles require different pitch rates to achieve their characteristic flavor profiles. Ales generally require lower rates (around 0.75 million cells/mL/°P), while lagers, which ferment colder and slower, require significantly higher rates (around 1.5 million cells/mL/°P) to ensure a clean finish.
High-gravity beers impose more osmotic stress on yeast, also necessitating higher pitch rates to ensure the yeast can complete fermentation before exhausting themselves.
Wyeast Activator™ Pack Viability
Wyeast laboratories guarantee roughly 100 billion viable cells in their standard "Activator" smack packs at the time of manufacture. However, yeast is a living organism, and its viability decreases over time as stored glycogen reserves are depleted. Refrigeration slows this decline, but it does not stop it completely.
A common rule of thumb in homebrewing is that liquid yeast loses approximately 20% viability per month (or roughly 0.7% per day) from the date of manufacture. This calculator uses the manufacture date stamped on your Wyeast pack to estimate current viability. A pack that is 3 months old may only have 40% viability, meaning you only have roughly 40 billion healthy cells available to pitch, rather than the initial 100 billion.
Using the Results: Direct Pitch vs. Yeast Starter
Once the calculator determines the total "Cells Needed" versus the "Viable Cells per Pack," it will recommend how many packs are required.
Direct Pitching: If the calculator suggests 1 or perhaps 2 packs, you may be able to "direct pitch" them right into your fermenter, provided they are relatively fresh.
Yeast Starter: If the calculator indicates you need 3+ packs, or if your yeast is old (low viability), making a yeast starter is strongly recommended. A starter is essentially a mini-batch of wort used to propagate yeast cells over 24-48 hours before your main brew day. This is far more economical than buying multiple packs and ensures the yeast is highly active when pitched.
By accurately calculating your pitch rate and accounting for yeast age, you take control of the fermentation process, leading to cleaner, better-tasting homebrew.