Fish Weight Calculator: Estimate Catch Weight by Length and Girth
Enter the fish species, length, and girth to get an instant weight estimate. The calculator uses the standard length-girth formula trusted by fisheries biologists, so you can log an accurate weight without a scale. Switch between inches/pounds and centimetres/kilograms as needed.
How fish weight is estimated from measurements
The standard formula for oval or round-bodied fish is: Weight (lb) = Length x Girth^2 / divisor. For slender species like walleye and pike, girth measurement is unreliable, so the simplified formula Weight (lb) = Length^3 / divisor is used instead. All measurements are in inches for the formula, then converted to your chosen unit.
Why accurate measurement matters
A scale is the gold standard, but most anglers do not carry one. The length-girth formula typically comes within 10-15% of actual weight, which is accurate enough for catch records and tournaments. Girth at the widest point is the most critical measurement for round-bodied species.
Catch and release tips
Keep the fish in the water while measuring when possible. Use a flexible measuring tape for girth. Minimize air exposure to under 60 seconds. Cradle the fish horizontally and lower it gently back into the water, allowing it to swim away on its own.
Understanding the weight range
The low and high estimates represent a plus or minus 10% band around the formula result. A fish in peak condition with a fuller belly may land at the high end, while a post-spawn or thinner specimen may fall at the low end. Actual weight depends on body condition, season, and feeding activity.
Species formula reference
| Species | Formula type | Divisor |
|---|---|---|
| Largemouth Bass | Length x Girth^2 | 1200 |
| Smallmouth Bass | Length x Girth^2 | 1225 |
| Rainbow Trout | Length x Girth^2 | 1400 |
| Walleye | Length^3 | 2700 |
| Northern Pike | Length^3 | 3500 |
Divisors from Anderson and Neumann (1996) standard weight equations.
Frequently asked questions
How accurate is the length-girth fish weight formula?
Typically within 10-15% of actual weight for well-fed fish in season. Accuracy improves with careful girth measurement at the widest point of the body.
Why does walleye use a different formula than bass?
Walleye and northern pike have slender, torpedo-shaped bodies. Measuring their girth is less consistent, so biologists use a length-cubed formula instead, which is more reliable for slender fish.
What is total length vs fork length?
Total length is from the tip of the closed mouth to the end of the tail fin. Fork length ends at the fork of the tail. This calculator uses total length, which is the most common standard for these species.
Where do I measure girth on a fish?
Wrap the tape around the widest part of the body, usually just forward of the dorsal fin. Keep the tape snug but not compressing the fish.
Can I use this formula for other species?
The formula works for similar body shapes. For a round-bodied species, use divisor 1200-1500. For slender species, use 2700-3500. Species-specific divisors give the most accurate results.