Tippet Size Calculator
Match tippet size, diameter and typical breaking strength to the fly you are about to tie on.
Open tool : Tippet Size CalculatorTrout Weight Calculator
Estimate the weight of a trout from its length and girth so you can measure it, log it and let it go.
Enter a length to see an estimate.
How to use it
- Measure lengthNose to the fork of the tail, with the fish in the water and the tape or net ruler alongside it.
- Measure girthAround the thickest part of the body, just ahead of the dorsal fin. This is the measurement that turns a guess into an estimate.
- Read the estimateThe tool returns a weight range rather than a single number, because body condition varies more than any formula can account for.
- Release the fishGet it back in the water. The measurement is worth less than the fish.
The math behind the number
With a girth measurement the tool uses the long standing angling formula girth squared × length ÷ 800, with all measurements in inches and the result in pounds. Without girth it falls back to length cubed ÷ 3500, a trout specific rule of thumb.
Both are geometric approximations. They assume an average bodied fish. A thick pre spawn hen, a lean fish late in the season and a heavy bodied lake trout will all sit outside the estimate, which is why the tool shows a range instead of pretending to a single decimal.
If you are recording a fish for a club, a tournament or a state record, the estimate has no standing. Those require a certified scale and a defined procedure set by the body keeping the record.
Common questions
How accurate is a length and girth weight estimate
It is an estimate, not a measurement. The standard angling formula assumes an average body shape. A heavy pre spawn fish or a lean post spawn fish can sit well outside it. Treat the number as a reasonable ballpark and say so when you log it.
Do I need the girth measurement
Girth is what makes the estimate worth anything. Length only formulas guess at body condition. If you can get a girth reading quickly and safely with the fish in the water, use it. If you cannot, use the length only estimate and accept a wider margin.
How do I measure a trout without hurting it
Keep the fish in the water, wet your hands, and measure fast. Many anglers use a net with a measure printed on the mesh or a soft tape rather than lifting the fish onto a hard surface. Local regulations and best practice guidance for the water you are fishing come first.