Gera Tools

Bass Length-to-Weight Calculator

Estimate largemouth and smallmouth bass weight from a measurement

Estimate bass weight using the classic Weight = Length × Girth² / 1200 formula plus length-only regression tables for largemouth and smallmouth bass. Built for tournament and catch-and-release bass anglers. It runs free in your browser on Gera Tools, with nothing uploaded.

Last updated Source: Gera Tools

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.

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.

Largemouth versus smallmouth: body shape differences

The two species have noticeably different builds at the same length, which is why they use separate regression constants:

Largemouth bass (Micropterus salmoides): Deeper-bodied and heavier for a given length, especially pre-spawn females. A 20-inch largemouth in good condition typically carries more weight than a 20-inch smallmouth because of the broader belly.

Smallmouth bass (Micropterus dolomieu): More streamlined and torpedo-shaped. For an equal length they tend to run lighter than largemouth. The trade-off is fight strength — smallmouth are famous for aggressive, acrobatic runs that feel disproportionate to their weight.

The girth formula divisor of 1200 applies to both because girth itself captures the body shape: a wider-bodied largemouth will naturally measure a larger girth than a slim smallmouth of the same length.

How the girth formula compares to a scale

In practice:

  • A fish in average condition will come within about 10–15% of a certified scale weight using either method
  • A heavily gravid pre-spawn female (late winter to early spring) will often weigh noticeably more than the length-only estimate predicts, and the girth formula will catch more of that extra bulk
  • A post-spawn fish that has recently released eggs may weigh less than average for its length

For tournament catch-photo-release (CPR) formats — where weigh-in at the boat is done with a handheld scale — the formula is a useful sanity check to confirm your measurement before submitting. For personal records, length-and-girth is a fair and fish-friendly method.

Quick measurement guide for catch-and-release

  1. Wet your hands before handling the fish
  2. Lay the fish on a bump board or tape measure with the mouth closed
  3. Squeeze the tail lobes together and read to the tip — this is total length
  4. Measure girth at the widest point, typically just behind the pectoral fins
  5. Note both numbers, then release immediately
  6. Enter the figures here for the weight estimate