Free tool · catch and release
Fish weight calculator
Work out how much your catch weighs from its length and girth, without taking it out of the water or hanging it on a scale. Measure, write it down, release it, and get the weight here.
How to measure a fish without harming it
- Fork length. From the tip of the lower jaw to the fork of the tail.
- Girth. Wrap the tape around the thickest part of the body. If you can only measure one side, double it.
- Pick the species. It changes the body shape and, with it, the result.
Your measurements
in
in
Length and girth
lb
≈ 2.22 kg · usual margin ±10%
14.0² × 20.0 in ÷ 800 = 4.90 lb
Want the exact weight next time? A hand scale fits in any vest pocket.
See fish scales
How it works
The formula biologists and wardens use
With girth, the calculator treats the fish as a cylinder: length times the cross-section of the body. It is the formula published by the South Carolina Department of Natural Resources, with measurements in inches and the result in pounds:
weight (lb) = girth² × length ÷ 800
800 works for fish with a normal body. For long, slim fish such as pike or muskie, 900 is used, and for heavy-bodied fish such as catfish, 700 (Angler Reviews). The calculator picks the divisor when you change species and converts to centimeters and kilograms for you.
Without girth it uses length only: length³ ÷ species factor. It is less accurate, because it assumes a fish in average condition, neither thin after spawning nor full before it.
No formula replaces a scale, but the result usually lands within 10% of the real weight (MyFishMaps).
| Species | Length only | With girth |
|---|---|---|
| Largemouth bass | 1,700 | 800 |
| Smallmouth bass | 1,850 | 800 |
| Striped bass | 2,225 | 800 |
| Walleye | 2,700 | 800 |
| Rainbow trout | 2,800 | 800 |
| Brown trout | 2,600 | 800 |
| Northern pike | 3,900 | 900 |
| Muskie | 3,325 | 900 |
| Channel catfish | 2,600 | 700 |
| Bluegill / sunfish | 1,100 | 800 |
| Crappie | 1,550 | 800 |
| Yellow perch | 1,800 | 800 |
Frequently asked questions
How do I measure girth without hurting the fish?
Use a flexible sewing tape and wet your hands before touching it. Measure at the widest part, just behind the pectoral fins, and never slide the tape under the gill cover or into the gills.
Why does it differ from my scale?
The formula assumes an average shape. A thin fish after spawning weighs less than it says; one full of eggs or that has just fed, more. Measuring girth cuts the error a lot compared with using length only.
Can I use it for records or tournaments?
No. Records and most tournaments require weighing on a certified scale. The calculator is for your fishing log and for releasing the fish quickly.
Flexible measuring tapeFor length and girth without bending the fish.See on Amazon →
Fish lip gripperHold it by the mouth while you measure.See on Amazon →
Digital hand scaleFor when you want the exact weight.See on Amazon →
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