Fish Weight Calculator — Estimate Weight from Length & Girth
Measure your fish's length (tip to tail) and girth (circumference at the widest point) to estimate its weight without a scale. If you don't have a girth measurement, the calculator estimates it from the length using a standard 0.58 ratio.
Units
in
in
Girth estimated as 0.58 × length — enter actual girth for greater accuracy
- 1
Estimate girth
0.58 × 20 = 11.6Typical body ratio used when girth is not measured. - 2
Girth squared
11.6² = 134.56 - 3
Fish weight (lbs)
20 × 134.56 ÷ 800 = 3.364
How does this calculator work?
Fish weight (lbs) = (length in inches × girth in inches²) ÷ 800. If girth is unknown, estimate it as 0.58 × length. The result is a field estimate — expect ± 10–20% of true scale weight. Works best for typical game fish; slim or very round species deviate more.
Formula
How this is calculated
The standard sport-fishing weight formula W = (L × G²) / 800 was derived from physical measurements of thousands of fish and relates a fish's length and body circumference (girth) to its mass. Length is measured in inches from the tip of the closed mouth to the tail fork; girth is the circumference around the widest part of the body, typically just behind the pectoral fins. The result is in pounds.
When girth is not measured — common in catch-and-release scenarios where speed matters — most fisheries agencies use an approximation of girth ≈ 0.58 × length for typical fish body shapes. This substitution gives the simplified formula W ≈ (L × (0.58L)²) / 800 ≈ L³ / 2,370, often published in state fishing guides. The 0.58 ratio is a population average; actual ratios vary by species and body condition (a heavy, well-fed fish has a higher ratio; a slim, spent fish has a lower one).
For metric inputs the calculator converts centimetres to inches, applies the formula, then converts pounds to kilograms. This formula was developed primarily for freshwater game fish (bass, trout, walleye) and gives reasonable estimates for many species, but dedicated length–weight regression equations exist for specific species and give more accurate results. Consider this an estimate ± 10–20% of actual weight.
Frequently asked questions
The formula is a widely used approximation for sport-fishing estimates. With both length and actual girth measurements, errors are typically within 5–15% of the scale weight. Without a girth measurement (using the 0.58 estimate), errors can be 15–25% depending on the fish's body condition and species. For scientific or regulatory purposes, use a scale.
The W = (L × G²) / 800 formula was primarily validated for freshwater game fish such as largemouth bass, smallmouth bass, walleye and trout. It is a useful approximation for many species with typical cylindrical body shapes, but slim-bodied fish (pike, gar) or very round fish (bluegill, crappie) may deviate more from the estimate.
Measure the circumference around the thickest part of the body — usually just in front of or at the dorsal fin, behind the pectoral fins. Use a flexible tape measure around the full body at that point. A consistent measurement location gives more reproducible results.
Also known as
TG we-Calculate Editorial Team. (2026). Fish Weight Calculator — Estimate Weight from Length & Girth [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/fish-weight-calculator
TG we-Calculate Editorial Team. "Fish Weight Calculator — Estimate Weight from Length & Girth." TG we-Calculate. 2026. https://we-calculate.com/calculator/fish-weight-calculator.
TG we-Calculate Editorial Team, "Fish Weight Calculator — Estimate Weight from Length & Girth," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/fish-weight-calculator
@misc{wecalculate_fish_weight_calculator, title = {Fish Weight Calculator — Estimate Weight from Length & Girth}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/fish-weight-calculator}}, year = {2026}, note = {TG we-Calculate} }
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