Fish Weight Calculator

How the length and girth formula works

A fish's weight depends on its volume, and volume depends on how long and how thick the fish is. Girth squared captures the cross-section of the body; multiplying by length gives a number proportional to volume, and dividing by 800 converts it to pounds for a typical fish. Texas Parks and Wildlife uses this formula in its fisheries advice and gives the example of a 26-inch largemouth bass with a 20.25-inch girth: 20.25² × 26 ÷ 800 ≈ 13.3 lb.

In metric

With centimetres, weight in kilograms ≈ length × girth² ÷ 28,900. A 60 cm fish with a 38 cm girth weighs about 60 × 38² ÷ 28,900 ≈ 3.0 kg.

Fish weight chart

Two fish of the same length can differ a lot in weight – a fat pre-spawn bass can outweigh a lean post-spawn fish of the same length by a pound or more – which is why girth matters so much. Slender, streamlined species such as trout often come out slightly heavy with ÷ 800; some anglers use ÷ 900 for them.

Length-only estimates

Without a girth, any estimate is rough because body condition varies so much. The calculator assumes a typical girth of about 55–60% of length and shows the resulting range rather than a single number. If you want a weight you can record, take a girth measurement – it only takes a few seconds with a soft tape or a piece of string.

How to measure a fish

1. Length: lay the fish flat on a measuring board or tape and measure in a straight line from the tip of the snout (mouth closed) to the tip of the tail. Many tournaments use total length with the tail lobes pinched together – check your rules.
2. Girth: wrap a soft tape or string around the thickest part of the body, usually just in front of the dorsal fin. Pull it snug but do not squeeze.
3. Record both, then release the fish quickly.

Catch and release: handling tips

• Wet your hands before touching the fish to protect its slime coat.
• Support the body horizontally – do not hang heavy fish by the jaw.
• Keep it in or close to the water while you measure; have the tape ready before you land it.
• Use a measuring mat or a tape fixed to your boat or net handle to speed things up.
• Revive tired fish facing into the current until they swim away strongly.

How accurate is a fish weight calculator?

Accuracy depends mostly on the girth measurement and on how typical the fish's shape is. The formula is an approximation designed for typical body shapes, so unusually fat, thin, spawning or very large fish can differ noticeably from their scale weight. For records and tournament weigh-ins, only a certified scale counts.

Recording your catch

If you keep a fishing log, note the date, place, species, length, girth and estimated weight, plus a photo of the fish on a measuring board. Over a season this builds a picture of how fish condition changes – before and after spawning, or between waters.

Sources

1. Texas Parks and Wildlife – Pond management FAQ: estimating fish weight from length and girth — tpwd.texas.gov/landwater/land/habitats/faq/fisheries/f4.phtml