What is the standard bass weight formula?

The classic angler formula is Weight in pounds equals Length times Girth squared, divided by 1200, with both measurements in inches. It works because a fish's weight scales with its length and the square of its girth, which together approximate its volume.

Why does the length-only estimate differ from the girth estimate?

The length-only regression is a species average for a fish in normal condition, while the girth formula measures the actual thickness of the fish in front of you. A fat, pre-spawn female will weigh more than the length-only number suggests, and the girth formula will catch that.

Do largemouth and smallmouth use different numbers?

Yes. Smallmouth tend to be more streamlined than largemouth at the same length, so their length-weight regression has slightly different constants. The girth formula divisor of 1200 is the same for both because girth already accounts for body shape.

How should I measure a bass for the best estimate?

Measure total length from the snout to the tip of the squeezed tail, and girth at the widest point of the body just behind the gills. Lay the fish flat and measure quickly so you can release it. Wet hands and minimal air time keep the fish healthy.

Are these estimates tournament-legal?

Most tournaments weigh fish on certified scales, but many catch-photo-release formats and personal-best logs accept length-and-girth estimates. Check your specific event rules; this tool is ideal for quick, fish-friendly weight records.

Bass Length-to-Weight Calculator

Bass anglers often want a weight without unhooking a scale. This calculator uses the well-known Weight = Length × Girth² / 1200 formula together with species-specific length-only regressions for largemouth and smallmouth bass, so you get a fast, fish-friendly estimate.

How it works

With a girth measurement, the tool uses the classic shape formula:

weight (lb) = (length × girth²) / 1200      (inches)

Without girth, it falls back to a length-only regression for the chosen species:

weight (lb) = a × length^b

The girth formula is generally more accurate for an individual fish because it measures actual bulk, while the length-only formula reflects a species average. Comparing the two tells you whether your fish is fat or lean for its length.

Example and tips

A 20-inch largemouth with a 14-inch girth gives (20 × 196) / 1200 ≈ 3.3 lb. The length-only largemouth regression for 20 inches lands near 4 lb, so a 3.3 lb girth result means this particular fish is on the slim side. Always squeeze the tail flat when measuring length, take girth at the thickest point, and treat the result as a close estimate rather than a certified weight.