Free tool
Fish weight calculator
| Length | Weight | Rough age |
|---|---|---|
| 8 in | 0.27 lb | 1 to 2 years |
| 10 in | 0.56 lb | about 2 years |
| 12 in | 1.0 lb | 2 to 3 years |
| 14 in | 1.6 lb | about 3 years |
| 16 in | 2.5 lb | about 4 years |
| 18 in | 3.6 lb | about 5 years |
| 20 in | 5.1 lb | 6 to 7 years |
| 22 in | 6.9 lb | about 9 years |
| 24 in | 9.2 lb | 12+ years |
| 26 in | 11.8 lb | 12+ years |
Total length, nose to the tip of the pinched tail. Weights are for a largemouth bass in average shape. A fat fish, like a female before the spawn, weighs more and a thin one less, which a girth measurement picks up. Rows past 24 inches are bigger than typical for the species.
How the calculator works
Fish of the same species have a predictable shape, so their weight follows their length along a curve. Fisheries biologists write that curve as a power formula, and anglers have used simpler versions of it at the scale for decades:
Weight (lb) = Length (in) ^ b / c
b is close to 3 for every fish. That is why a fish twice as long weighs about eight times as much: it got longer, deeper and thicker all at once. Most species sit a little above 3 because they fill out as they age. c sets the shape: a long, slim pike has a big c and a deep-bodied bluegill a small one.
These are the same numbers fishline uses when it measures a fish from a photo and when it projects a released fish's growth, so the calculator and the app agree.
When you have a girth
Length alone assumes a fish in average shape. If you put a soft tape around the fattest part of the body, usually just ahead of the dorsal fin, the calculator switches to the girth formula:
Weight (lb) = Length x Girth x Girth / 800
It works across species because it measures the fish you actually caught. A fat fish before the spawn, a fish that just ate, or a thin one after a hard winter all show up in the girth.
Measuring length right
- Lay the fish flat on its side on a bump board or tape, mouth closed.
- Push the nose against the bump, then pinch the tail lobes together.
- Read total length at the tip of the tail. Do not follow the curve of the body.
More on this in how to estimate a fish's weight from its length.
Weight formulas by species
The length to weight formula for every species fishline knows, with the weight it gives at a typical keeper length. Tap a species for its full chart.
| Species | Formula | Example |
|---|---|---|
| Largemouth bass | W = L^3.2 / 2,850 | 18 in = 3.6 lb |
| Smallmouth bass | W = L^3.2 / 2,950 | 16 in = 2.4 lb |
| Spotted bass | W = L^3.2 / 2,810 | 14 in = 1.7 lb |
| White bass | W = L^3.1 / 2,340 | 14 in = 1.5 lb |
| Striped bass | W = L^3.1 / 2,840 | 33 in = 18.0 lb |
| Walleye | W = L^3.18 / 4,730 | 23 in = 4.5 lb |
| Sauger | W = L^3.1 / 3,710 | 17 in = 1.8 lb |
| Yellow perch | W = L^3.1 / 2,400 | 11 in = 0.70 lb |
| Northern pike | W = L^3.1 / 4,970 | 33 in = 10.3 lb |
| Muskellunge | W = L^3.2 / 7,970 | 41 in = 18.2 lb |
| Chain pickerel | W = L^3.1 / 4,580 | 20 in = 2.4 lb |
| Black crappie | W = L^3.1 / 2,120 | 12 in = 1.0 lb |
| White crappie | W = L^3.1 / 2,190 | 13 in = 1.3 lb |
| Bluegill | W = L^3.2 / 1,830 | 8 in = 0.42 lb |
| Redear sunfish | W = L^3.2 / 1,970 | 10 in = 0.80 lb |
| Channel catfish | W = L^3.1 / 3,200 | 27 in = 8.6 lb |
| Blue catfish | W = L^3.2 / 4,190 | 41 in = 34.6 lb |
| Flathead catfish | W = L^3.2 / 4,130 | 38 in = 27.5 lb |
| Rainbow trout | W = L^3 / 2,700 | 20 in = 3.0 lb |
| Brown trout | W = L^3 / 2,700 | 21 in = 3.4 lb |
| Brook trout | W = L^3 / 2,700 | 13 in = 0.81 lb |
| Lake trout | W = L^3.05 / 3,080 | 30 in = 10.4 lb |
| Chinook salmon | W = L^3.05 / 2,970 | 32 in = 13.1 lb |
| Coho salmon | W = L^3.05 / 2,930 | 24 in = 5.5 lb |
| Common carp | W = L^3.1 / 2,360 | 27 in = 11.6 lb |
| Freshwater drum | W = L^3.1 / 2,730 | 23 in = 6.1 lb |
| Redfish | W = L^3.05 / 2,900 | 32 in = 13.4 lb |
| Speckled trout | W = L^3 / 2,600 | 20 in = 3.1 lb |
| Flounder | W = L^3.1 / 3,200 | 18 in = 2.4 lb |
| Snook | W = L^3.1 / 4,000 | 29 in = 8.5 lb |
| Tarpon | W = L^3 / 3,200 | 54 in = 49.2 lb |
| Mahi-mahi | W = L^3 / 2,800 | 41 in = 24.6 lb |
Questions
How accurate is a length to weight calculator?
For a fish in average shape it lands close. Individual fish vary: a thin fish after the spawn or a fat one before it can sit well off the chart, by a pound or more on the bigger species. A girth measurement closes most of that gap.
What is the girth formula for fish weight?
Length times girth times girth, divided by 800, with both measured in inches. A 20 inch fish with a 12 inch girth works out to 20 x 12 x 12 / 800, or 3.6 lb. Girth captures how thick the fish is, which length alone cannot.
Should I measure total length or fork length?
Total length: nose to the tip of the tail with the tail lobes pinched together. That is how most freshwater regulations measure and what these formulas expect. Some saltwater rules use fork length instead, so check before you keep a fish.
Can I get the weight without measuring the fish?
Yes. Take a photo of the fish with fishline and it estimates the species, length and weight for you, using your hand, a bump board or other gear in the frame for scale. Free accounts get 5 measurements a month.
Want the age too? Try the fish age calculator.
Or skip the tape. Snap a photo and fishline measures it.
Species, length and weight from one photo. 5 free measurements every month.