Adjust based on the fish's condition. Lower number = Heavier fish.
Estimated Weight
0.00 lbs
Weight in Kilograms
0.00 kg
Condition Factor (K)
1.00
Approx. Age
3-4 Years
Formula Used: Weight = (Length × Girth²) / Divisor
Length vs. Weight Projection
Chart: Projected weight growth maintaining current body proportions.
Reference Data for Similar Size
Length (in)
Girth (in)
Est. Weight (lbs)
Classification
Table: Comparison of weights +/- 2 inches from your input.
Comprehensive Guide to the Chinook Weight Calculator
What is a Chinook Weight Calculator?
A Chinook Weight Calculator is an essential tool for anglers, fisheries biologists, and conservationists. It allows for the non-invasive estimation of a Chinook (King) Salmon's weight using simple linear measurements—length and girth. This method is particularly valuable for catch-and-release fishing, where weighing a large, thrashing fish on a scale can cause injury, remove protective slime, or damage internal organs.
By inputting the dimensions of the fish, this calculator applies a proven mathematical formula to determine the biomass of the specimen. While scale weights are exact, a calibrated calculation is widely accepted for records, tournaments, and personal logs, often falling within 5-10% of the actual scale weight.
Understanding the estimated weight helps anglers track the health of the fishery and decide whether a fish meets personal or legal harvest criteria without harming the animal.
Chinook Weight Calculator Formula and Mathematical Explanation
The calculation relies on the relationship between a cylinder's volume and the density of the fish. Since salmon are roughly cylindrical, their weight grows exponentially with girth and linearly with length. The industry-standard formula used by the Alaska Department of Fish and Game and other agencies is:
Weight = (Length × Girth²) / Divisor
Where:
Variable
Meaning
Unit
Typical Range
Length (L)
Measured from nose to tail fork
Inches
20″ – 60″
Girth (G)
Circumference at thickest point
Inches
10″ – 40″
Divisor
Shape/Density Factor
Constant
750 – 850
The Divisor Explained: The divisor (typically 800) accounts for the specific gravity and shape of the species. A lower divisor (750) is used for exceptionally fat, chrome-bright ocean fish. A higher divisor (850) is often used for spawning fish that have lost body mass or "kype" (hooked jaw) males where length is exaggerated relative to body mass.
Practical Examples (Real-World Use Cases)
Example 1: The Kenai River Giant
An angler lands a massive King Salmon on the Kenai River. They quickly measure the length at 50 inches and the girth at 32 inches before releasing it.
Input Length: 50 inches
Input Girth: 32 inches
Calculation: (50 × 32 × 32) / 800
Math: 51,200 / 800 = 64 lbs
Result: The estimated weight is 64 lbs. This is a trophy-class fish.
Example 2: The Average Feeder Chinook
Fishing off the coast of British Columbia, a "feeder" King is caught. It is short but football-shaped.
Input Length: 30 inches
Input Girth: 20 inches
Calculation: (30 × 20 × 20) / 800
Math: 12,000 / 800 = 15 lbs
Result: The fish weighs approximately 15 lbs, a perfect size for the table.
How to Use This Chinook Weight Calculator
Measure Length: Lay the tape measure over the curve of the body from the tip of the nose to the fork of the tail. Do not measure to the tip of the tail fin (total length) unless specified, as "fork length" is standard for biomass formulas.
Measure Girth: Wrap a soft tape measure around the thickest part of the fish, usually just ahead of the dorsal fin. Ensure the tape is snug but not compressing the flesh.
Select Condition: Choose the divisor. If the fish is "chrome" and fresh from the ocean, use standard (800) or fat (750). If it is a darker river fish, use lean (850).
Interpret Results: The primary result shows pounds. The calculator also provides kilograms and a reference table for comparison.
Key Factors That Affect Chinook Weight Calculator Results
While the length/girth formula is highly accurate, several biological and environmental factors influence the true weight of a Chinook Salmon.
1. Sexual Dimorphism: Male Chinook often develop a large kype (hooked jaw) and humped back during spawning. This adds length or height that doesn't necessarily correlate to meat density, potentially skewing standard calculations.
2. Spawning Phase: Fish entering freshwater stop feeding. They live off stored fat. A "fresh" fish is dense and oily. A "dark" or "blush" fish has burned through fat reserves, making it lighter than a fresh fish of the same dimensions.
3. Egg Mass: A female Chinook (hen) carrying mature roe can weigh significantly more than a male of identical dimensions due to the density of the egg skeins.
4. Region/Stock: Kenai River Kings are genetically predisposed to be heavier and thicker than Sacramento River Kings. Regional genetics play a role in the body shape factor.
5. Measurement Error: Measuring a slimy, moving fish is difficult. A 1-inch error in girth impacts the result squared (exponentially), whereas a 1-inch error in length is linear. Accuracy in girth is critical.
6. Stomach Content: An actively feeding ocean Chinook may have several pounds of herring or baitfish in its stomach, adding temporary weight that measurements won't account for fully.
Frequently Asked Questions (FAQ)
1. How accurate is the Chinook weight calculator formula?
For most ocean-bright fish, the formula is accurate within +/- 5-8%. It tends to slightly overestimate weight for post-spawn fish and underestimate for exceptionally girthy females full of roe.
2. Should I measure the total length or fork length?
Always use fork length (nose to the V-notch in the tail) for weight formulas. Total length includes the fin tips, which can vary due to wear and tear and do not represent body mass.
3. Why is girth squared in the formula?
The fish is treated geometrically as a cylinder. The volume of a cylinder is $\pi \times r^2 \times h$. Girth represents the circumference (related to radius), so mathematically, the cross-sectional area (and thus volume/weight) increases with the square of the girth.
4. Can I use this calculator for Coho or Sockeye salmon?
Yes, but with caution. Coho and Sockeye generally have different body shapes. While the math works roughly, species-specific divisors are better (e.g., Coho often use a slightly higher divisor due to being more streamlined).
5. What is the world record Chinook Salmon weight?
The IGFA all-tackle world record is 97 lbs 4 oz, caught by Les Anderson on the Kenai River in 1985. Commercial catches have reported fish over 120 lbs historically.
6. Does this calculator work for metric measurements?
This calculator is built for inches and pounds (Imperial). However, the results section automatically converts the final weight to Kilograms for metric users.
7. Why does the "Condition" option change the weight?
The "Condition" alters the divisor constant. A lower divisor (750) assumes a denser, fatter fish. A higher divisor (850) assumes a leaner fish. This allows experienced anglers to fine-tune the estimate based on visual inspection.
8. Is it legal to keep a fish based on estimated weight?
Regulations are usually based on length, not weight. Always check local Department of Fish and Game regulations regarding slot limits or minimum size restrictions, which strictly refer to tape measurements.
// Main Logic
function calculateWeight() {
var lengthInput = document.getElementById('fishLength');
var girthInput = document.getElementById('fishGirth');
var divisorInput = document.getElementById('divisorConst');
var length = parseFloat(lengthInput.value);
var girth = parseFloat(girthInput.value);
var divisor = parseFloat(divisorInput.value);
// Validation logic
var valid = true;
if (isNaN(length) || length 80) {
document.getElementById('errorLength').style.display = 'block';
valid = false;
} else {
document.getElementById('errorLength').style.display = 'none';
}
if (isNaN(girth) || girth 60) {
document.getElementById('errorGirth').style.display = 'block';
valid = false;
} else {
document.getElementById('errorGirth').style.display = 'none';
}
if (!valid) return;
// Calculation Formula: W = (L * G^2) / Divisor
var weightLbs = (length * Math.pow(girth, 2)) / divisor;
var weightKg = weightLbs * 0.453592;
// Fulton's K-Factor approximation (Metric units usually used: 100000 * W(g) / L(mm)^3)
// Simplified relative K for this context
// A standard healthy salmon has K ~ 1.0.
// We calculate based on inputs relative to "standard" growth.
// This is a derived metric for the UI display.
var standardWeight = (length * Math.pow(length * 0.55, 2)) / 800; // Assuming girth is roughly 55% of length for standard
var kFactor = weightLbs / standardWeight;
// Age estimation (Rough heuristics for Chinook)
var ageText = "1-2 Years";
if (weightLbs > 10) ageText = "3 Years";
if (weightLbs > 20) ageText = "4 Years";
if (weightLbs > 40) ageText = "5-6 Years";
if (weightLbs > 60) ageText = "7+ Years (Trophy)";
// Update UI
document.getElementById('resultWeight').innerText = weightLbs.toFixed(2) + " lbs";
document.getElementById('resultKg').innerText = weightKg.toFixed(2) + " kg";
document.getElementById('resultKFactor').innerText = kFactor.toFixed(2);
document.getElementById('resultAge').innerText = ageText;
updateChart(length, weightLbs, divisor);
updateTable(length, girth, divisor);
}
function resetCalculator() {
document.getElementById('fishLength').value = 36;
document.getElementById('fishGirth').value = 20;
document.getElementById('divisorConst').value = 800;
calculateWeight();
}
function copyResults() {
var w = document.getElementById('resultWeight').innerText;
var l = document.getElementById('fishLength').value;
var g = document.getElementById('fishGirth').value;
var text = "Chinook Estimation:\nLength: " + l + "in\nGirth: " + g + "in\nEst. Weight: " + w;
// Create temporary element to copy
var tempInput = document.createElement("textarea");
tempInput.value = text;
document.body.appendChild(tempInput);
tempInput.select();
document.execCommand("copy");
document.body.removeChild(tempInput);
alert("Results copied to clipboard!");
}
// Chart Drawing Logic (Native Canvas)
function updateChart(currentLength, currentWeight, divisor) {
var canvas = document.getElementById('weightChart');
var ctx = canvas.getContext('2d');
// Handle high DPI
var dpr = window.devicePixelRatio || 1;
var rect = canvas.getBoundingClientRect();
canvas.width = rect.width * dpr;
canvas.height = rect.height * dpr;
ctx.scale(dpr, dpr);
var width = rect.width;
var height = rect.height;
var padding = 40;
// Clear
ctx.clearRect(0, 0, width, height);
// Data Generation: Curve from L=20 to L=60 assuming consistent shape (Girth ratio)
// We assume Girth grows linearly with length for the curve: G = L * (currentGirth/currentLength)
var ratio = document.getElementById('fishGirth').value / currentLength;
var dataPoints = [];
var maxWeight = 0;
var minL = 10;
var maxL = 70;
for (var l = minL; l maxWeight) maxWeight = w;
dataPoints.push({l: l, w: w});
}
// Axis scaling
var contentWidth = width – (padding * 2);
var contentHeight = height – (padding * 2);
// Draw Axes
ctx.beginPath();
ctx.strokeStyle = "#ccc";
ctx.lineWidth = 1;
// Y Axis
ctx.moveTo(padding, padding);
ctx.lineTo(padding, height – padding);
// X Axis
ctx.lineTo(width – padding, height – padding);
ctx.stroke();
// Draw Curve
ctx.beginPath();
ctx.strokeStyle = "#004a99";
ctx.lineWidth = 3;
var first = true;
for (var i = 0; i < dataPoints.length; i++) {
var p = dataPoints[i];
var x = padding + ((p.l – minL) / (maxL – minL)) * contentWidth;
var y = (height – padding) – (p.w / (maxWeight * 1.1)) * contentHeight; // *1.1 for headroom
if (first) {
ctx.moveTo(x, y);
first = false;
} else {
ctx.lineTo(x, y);
}
}
ctx.stroke();
// Draw Current Point
var cx = padding + ((currentLength – minL) / (maxL – minL)) * contentWidth;
var cy = (height – padding) – (currentWeight / (maxWeight * 1.1)) * contentHeight;
ctx.beginPath();
ctx.fillStyle = "#28a745";
ctx.arc(cx, cy, 6, 0, 2 * Math.PI);
ctx.fill();
// Labels
ctx.fillStyle = "#333";
ctx.font = "12px sans-serif";
ctx.textAlign = "center";
ctx.fillText("Length (in)", width / 2, height – 5);
ctx.save();
ctx.translate(15, height / 2);
ctx.rotate(-Math.PI / 2);
ctx.fillText("Weight (lbs)", 0, 0);
ctx.restore();
// Value Label
ctx.fillStyle = "#000";
ctx.fillText("You (" + currentLength + "\", " + currentWeight.toFixed(1) + "lbs)", cx, cy – 15);
}
function updateTable(length, girth, divisor) {
var tbody = document.getElementById('referenceTableBody');
tbody.innerHTML = "";
// Generate rows +/- 2 inches length
for (var i = -2; i <= 2; i++) {
var l = length + i;
if (l 30) classif = "Large";
if (w > 50) classif = "Trophy";
tr.innerHTML = "
" + l.toFixed(1) + "
" +
"
" + girth.toFixed(1) + "
" +
"
" + w.toFixed(2) + "
" +
"
" + classif + "
";
tbody.appendChild(tr);
}
}
// Init
window.onload = function() {
calculateWeight();
// Add resize listener for chart
window.addEventListener('resize', function() {
var l = parseFloat(document.getElementById('fishLength').value);
var w = parseFloat(document.getElementById('resultWeight').innerText); // primitive parsing
var d = parseFloat(document.getElementById('divisorConst').value);
calculateWeight(); // easiest way to redraw
});
};