Fish Length-Weight Relationship Calculator

The Fish Length-Weight Relationship Calculator estimates a fish's weight from its body measurements using species-specific formulas — useful for catch-and-release fishing when you want a weight estimate without a scale. Enter your fish length, optional girth, and select your fish species type (bass, trout, walleye, pike, and more) to get the estimated fish weight along with a min/max weight range and calculated girth if not provided.

If not provided, girth will be estimated as 0.58 × length

Results

Estimated Fish Weight

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Weight Range (Min)

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Weight Range (Max)

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Calculated Girth

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Frequently Asked Questions

How accurate are fish weight calculations from length?

Fish weight calculations are estimates based on allometric relationships and can vary by ±15-25% from actual weight. Factors like fish condition, season, and individual variation affect accuracy.

What if I don't know the fish's girth measurement?

If girth is not provided, the calculator estimates it using the standard formula: Girth = 0.58 × Length. This provides a reasonable approximation for most fish species.

Do different fish species have different weight formulas?

Yes, different species have unique body shapes and density, requiring species-specific formulas. For example, pike use Length³/2700 while sunfish use Length×Girth²/1200.

Should I measure total length or fork length?

Most formulas use total length (tip of nose to end of tail). For Pacific halibut and some species, fork length (nose to fork of tail) may be specified.

Why do my calculated weights seem different from actual catches?

Calculated weights are statistical averages. Individual fish can vary significantly based on feeding conditions, spawning status, age, and overall health.

Can I use this calculator for saltwater and freshwater fish?

Yes, the calculator includes formulas for both saltwater species (like Pacific halibut, snook) and freshwater species (like bass, walleye, pike).

What's the difference between net weight and round weight?

Round weight is the whole fish weight, while net weight excludes head, guts, and sometimes fins. Commercial fisheries often use net weight for processing.