Whether you’re chasing a personal best, fishing a catch-and-release tournament, or just settling a friendly bet at the boat ramp, knowing what your fish weighs matters. But carrying a scale isn’t always practical, and hanging a fish on one long enough to get a reading is hard on a fish you plan to release. The good news: you can get a reliable weight estimate with nothing but a tape measure and a little math.
The formula that actually works
For most fish, weight is closely tied to two measurements: length and girth. The formula anglers have trusted for decades is:
Weight (lb) = (Length × Girth × Girth) ÷ 800 — with length and girth both measured in inches.
Girth is the key. Two fish of the same length can weigh very differently — a fat, well-fed bass carries far more than a long, skinny one. Because girth captures how heavy-bodied a fish is, this method is usually accurate to within 5–10% of the true weight for a fish in average condition.
How to measure your fish
Length: Lay the fish flat and measure from the tip of the nose to the end of the tail. Keep the tape straight along the body.
Girth: This is the distance all the way around the fish at its fattest point — usually just behind the gills and pectoral fins. Wrap a flexible tape (or a piece of fishing line you measure afterward) snugly around the body. Don’t cinch it tight; just bring it flush against the fish.
Match the formula to your species
The 800 divisor works well for thicker-bodied fish. Leaner, more streamlined species carry less weight for their length, so they use a slightly larger divisor of about 900:
- Divide by 800: largemouth and smallmouth bass, catfish, panfish
- Divide by 900: trout, walleye, northern pike, muskie
Using the right divisor keeps slim fish from being overestimated and heavy-bodied fish from being shortchanged.
A worked example
Say you land a largemouth bass that’s 18 inches long with a 12-inch girth. Plug it in: (18 × 12 × 12) ÷ 800 = 2,592 ÷ 800 = 3.24 lb. A quick, repeatable number — no scale required.
No way to measure girth? Estimate from length
If all you have is a length, you can still ballpark the weight, but expect it to be rougher — a length-only estimate has to assume an average body shape, and real fish are fatter or thinner than average. Treat it as a guide and add girth whenever you can for a number you can actually trust.
Tips for a more accurate estimate
- Measure quickly and keep the fish wet and calm, especially if you’re releasing it.
- Measure girth at the widest point, not back toward the tail.
- Pre-spawn females run heavy — a fish full of eggs can weigh 10–20% more than the formula suggests.
- For an official record, an estimate never counts; only a certified scale does. The formula is for everyday use and release fishing.
Skip the math — use the free calculator
Don’t want to do arithmetic with cold, wet hands? Our free fish weight calculator does it for you: choose your species, enter length and girth, and it instantly returns the weight in pounds, ounces, and kilograms — using the exact formula above, with the right divisor already built in for each species.