π Fish Weight Calculator
Measure length and girth, pick the body type, and get the estimated weight.
Estimated Fish Weight
How to Use This Fish Weight Calculator
Catch-and-release anglers often want the weight without a scale. Measure the fish length from nose to tail fork and the girth around the deepest part of the body, pick the body type closest to your species, and the calculator estimates the weight. It is accurate enough for bragging rights and fishing logs.
Measure length in inches: nose to the fork of the tail, fish lying flat on a bump board or tape.
Measure girth in inches: wrap the tape around the body at its deepest point, just ahead of the dorsal fin.
Pick fat-bodied for bass, pike, muskie, and catfish; slender for walleye, trout, and salmon.
Read the weight in pounds, pounds-and-ounces, and kilograms. Photograph and release.
The Formula
The formula treats the fish as roughly a tapered cylinder: weight scales with length times the square of girth, since girth squared approximates cross-sectional area. The divisor adjusts for body shape: 800 for deep, heavy-bodied species and 900 for slimmer ones. A 36-inch pike with a 19-inch girth gives 36 x 361 / 800 = 16.245 lb. The same measurements on the slender formula give 14.44 lb. Real fish vary with season, stomach contents, and spawning condition, so treat the result as an estimate within about 10 percent, not a certified scale weight.
Worked Example
Enter length 36 inches, girth 19 inches, fat-bodied. Girth squared = 361. Times length = 12,996. Divided by 800 = 16.245 lb, shown as 16 lb 4 oz (0.245 x 16 = 3.9 oz) and 7.37 kg. Switch to slender: 12,996 / 900 = 14.44 lb. Try a 20-inch bass with a 12-inch girth: 20 x 144 / 800 = 3.6 lb, a very respectable largemouth. Run both through the calculator and check the arithmetic by hand.
Measuring Tips for Accuracy
Keep the fish in the water or on a wet surface while you measure; speed matters more than precision for the fish health. Lay the tape flat without stretching the fish straight, since over-straightening adds phantom inches. Measure girth snugly but not cinched: compressing the belly understates weight. For toothy species like pike and muskie, use a bump board for length and estimate girth visually rather than wrestling the fish. Photograph the fish on the measuring board; the photo settles every later debate about the numbers.
When You Need a Real Scale
Formula estimates do not count for records, tournaments, or state certifications, which require certified scales and witnesses. They also struggle with unusual fish: a spawned-out female or a gorged pre-spawn male can miss the formula by 15 percent or more. Species-specific length-weight charts from your state fisheries agency beat the general formula when they exist. But for everyday catch-and-release fishing, where the goal is a quick number and a healthy release, the length-girth formula is the right tool: fast, harmless, and upfront about being an estimate.
A Second Worked Example: The Walleye
A 28-inch walleye with a 15-inch girth, slender formula. Girth squared: 225. Times length: 6,300. Divided by 900: 7.0 pounds exactly. The same fish on the fat-bodied divisor would read 7.88 pounds, nearly a pound heavier; picking the right body type matters more than measuring to the quarter inch. Walleye, trout, and salmon take the 900; bass, pike, and muskie take the 800.
Metric anglers: 28 inches is 71 cm, 7 pounds is 3.18 kg. The formula wants inches, so convert first or the answer is nonsense. The calculator handles the unit display; the formula's 800 and 900 are inch-calibrated constants.
Species Shape Guide
Body type is the formula's biggest judgment call. Deep-bodied: largemouth and smallmouth bass, northern pike, muskie, catfish, carp. Slender: walleye, trout, salmon, snook. In-between species like redfish can go either way; when unsure, run both and split the difference, or pick by the individual fish's condition. A spawned-out muskie is slimmer than a pre-spawn one, and the formula cannot see that.
Season and stomach move real weight around the estimate. A bass stuffed with shad weighs more than the formula says; a post-spawn female weighs less. The 10 percent accuracy band covers most of this, but the exceptional fish, the record chasers, deserve a real scale.
Common Fish-Weighing Mistakes
First: girth at the wrong spot. Measure at the deepest point, just ahead of the dorsal fin; measuring at the tail or the head shrinks the reading and the weight. Second: the dry-land delay. Every minute out of water stresses the fish; measure fast, photograph fast, release fast.
Third: length to the wrong point. Nose to fork of tail is the standard; total length to the tail tip reads longer and inflates the estimate. Fourth: the boga-grip hang. Hanging a big fish vertically by the jaw can injure it; support the body horizontally for the photo. Fifth: the record claim. Formulas estimate; records require certified scales and witnesses. Log the estimate proudly, claim records accurately.
Worked Example: The 28-Inch Pike
A northern pike measuring 28 inches long with a 14-inch girth: weight = (28 Γ 14Β²) / 800 = (28 Γ 196) / 800 = 5,488 / 800 = 6.86 pounds. That is a solid pike, likely a female full of eggs if caught in spring. The same 28-inch fish with a skinny 12-inch girth: (28 Γ 144) / 800 = 5.04 pounds. Two inches of girth is nearly two pounds of fish, which is why girth matters more than most anglers think.
For catch-and-release, this estimate is the ethical choice: measure length and girth in the water or on a quick cradle, plug in the numbers, and release. A hanging scale costs the fish slime, time, and sometimes its life. The formula is within 10 percent for most species, which is plenty for bragging rights and tournament estimates alike.
Why the 800 Formula Works (and When It Does Not)
The length Γ girthΒ² / 800 formula works because a fish is roughly a cylinder: volume scales with length times cross-sectional area, and area scales with girth squared. The 800 divisor is an empirical constant tuned to typical freshwater body densities. It is not physics from first principles; it is decades of weigh-ins distilled into one number, which is why it is so reliable for average-shaped fish.
It breaks on unusual body shapes. Eels and gar are too slender (the formula overshoots), while deep-bodied fish like crappie and bluegill need a smaller divisor (around 1,200 is the common adjustment). Saltwater species vary more. The formula also assumes a healthy, feeding fish; a spawned-out salmon or a starving winter bass weighs less than its dimensions suggest. Use 800 as the default, adjust for the weird ones.
Length-Girth Formula vs. Length-Only Charts
Length-only weight charts assume an average girth for each length, which is fine for typical fish but wrong for the memorable ones. The 24-inch bass on the chart says 7 pounds; your 24-incher with a football belly is 9. Charts are quick and need one measurement; the formula needs two but captures condition. For tournament culling decisions, the formula's extra accuracy is worth the extra 30 seconds.
Length-only charts win when you cannot safely measure girth: a thrashing muskie boatside, for example. In that case use the chart and note it as approximate. The hierarchy is simple: scale beats formula beats chart, and all three beat guessing. Pick the most accurate method the situation safely allows.
Pro Tips for Catch-and-Release Weighing
Wet your hands and the measuring tape before handling the fish. Dry hands strip the protective slime coat, which is the fish's immune system. Keep the fish horizontal and supported; hanging a big fish vertically by the jaw can dislocate it.
Measure girth at the fattest point, usually just ahead of the dorsal fin, with the tape snug but not cinched. Photograph the tape on the fish for both measurements; memory of "about 14 inches" drifts upward by the time you reach the dock. And work fast: every minute out of water is stress, so rehearse the routine before the fish of a lifetime shows up.
Frequently Asked Questions
How do you estimate fish weight without a scale?
Measure length (nose to tail fork) and girth (around the deepest part) in inches, then apply weight = length x girth^2 / 800 for fat-bodied fish or / 900 for slender fish. The result is usually within 10 percent of the true weight.
What is the formula for fish weight from length and girth?
Weight in pounds = (length x girth x girth) / 800 for bass, pike, and catfish, or / 900 for walleye, trout, and salmon. All measurements in inches. The squared girth approximates the body cross-section.
How accurate are fish weight calculators?
Typically within 10 percent for normal, healthy fish. Accuracy drops for spawned-out fish, gorged fish, or unusual body shapes. They are great for personal logs and catch-and-release, but not for records or tournaments.
Where do you measure fish girth?
Around the body at its deepest point, usually just in front of the dorsal fin. Wrap the tape snugly without compressing the belly. Keep the fish horizontal; hanging a fish vertically by the jaw distorts the measurement.
How do you measure fish length correctly?
Lay the fish flat and measure from the tip of the nose (mouth closed) to the fork of the tail. Do not stretch the fish straight or pinch the tail; both add inches that are not really there.
What is considered a good bass weight?
A 3 lb largemouth is a nice fish, 5 lb is a trophy in most waters, and 8-plus is exceptional. Smallmouth run smaller: 3 lb is excellent and 5 lb is a fish of a lifetime in many regions.
Does the formula work for saltwater fish?
The general principle works, but body shapes vary so much across saltwater species that species-specific charts are better. Use the slender divisor for tuna-like fish and the fat divisor for grouper-like fish as rough starting points.
Why divide by 800 or 900?
The divisor calibrates the length-times-girth-squared volume proxy to real weighed fish. Researchers fitted these constants to thousands of measured fish; 800 fits deep-bodied species, 900 fits slimmer ones.
Can I use cm instead of inches?
The 800/900 constants assume inches. If you measure in cm, convert first (divide by 2.54) or use the calculator, which expects inches. Mixing units is the most common error with this formula.
Should I weigh or estimate for catch and release?
Estimate. Hanging a fish on a scale by the jaw or keeping it out of water for weighing harms it. Quick length and girth measurements with a fast release are the responsible choice for fish you will not keep.