The fish weight calculator estimates a fish's weight from its length and girth using species-specific formulas. Useful for catch-and-release anglers who want to document a fish's weight without a scale, and for estimating tournament weights in the field.
Estimate Fish Weight
Tip to tail fork
Around widest point
Formula Reference
How to Estimate Fish Weight
The fish weight calculator uses standard angling formulas to estimate weight from measurements. The most accurate formula uses both length and girth; a length-only estimate is less precise but useful when you cannot wrap a tape around the fish.
Measuring Correctly
For length, measure from the tip of the mouth to the fork of the tail (fork length), or the tip of the tail (total length). Most record-keeping uses fork length for bass and total length for trout. For girth, measure the circumference at the widest part of the body — usually just behind the pectoral fins. A flexible tape measure or a piece of string works well.
Why Formulas Are Estimates
Fish condition varies significantly by season. A bass in peak summer feeding condition might be 10-20% heavier than the formula prediction for the same length. Late winter or early spring fish are typically leaner. For tournament documentation or personal records, use a certified scale. These formulas are best for field estimates and catch-and-release records.
Frequently Asked Questions
Is this fish weight calculator free?
Yes, completely free with no signup required.
What formula is used to estimate fish weight?
For bass and most elongated fish, the standard formula is: Weight (lbs) = (Length × Girth²) / 800. For pike and muskie: (Length × Girth²) / 700. For trout: (Length × Girth²) / 900. These formulas give an estimate within 5-15% of actual weight.
How do I measure fish girth?
Girth is measured at the widest point of the fish, typically just behind the pectoral fins. Wrap a flexible tape measure around the fish at its thickest point. If you don't have a tape, use a string and measure it against a ruler.
What if I only have the length?
Without girth, use the simplified length-only formula: Weight = (Length³) / Species_Factor. For largemouth bass, the factor is approximately 1,600; for trout, approximately 1,900. Length-only estimates are less accurate than length + girth calculations.
Why are fish weight formulas estimates only?
Fish body condition varies significantly by season, feeding patterns, and individual health. A fish in peak summer condition will weigh more than the same length fish in early spring. Competition record fish are often much heavier than formula predictions due to exceptional condition.