Fish length to weight Fishery tools
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Estimate fish weight from length (W = aL^b)

Estimate weight from fish length and coefficients a and b, then keep the result, error range, saved coefficient sets, and shareable JSON in one workflow.

The page makes TL / FL / SL assumptions explicit and supports quick estimates, coefficient fitting, and batch processing without leaving the browser.

Enter length in cm or inches and show weight in g, kg, oz, or lb Use reference presets, browser saving, and JSON sharing Solve for a from one sample or fit a and b from multiple samples Calculate batches and export a CSV for many fish

What this tool covers

Enter the species, length, length type, and coefficients a and b to calculate an estimated weight and practical error range immediately.
Start with FishBase reference presets, then replace them with your own field or plant coefficients and reuse them later.
Download coefficient sets as JSON, import them again, or create a shareable URL for the current state.
When coefficients are unknown, derive them from one measured fish or fit them from multiple length–weight pairs.
Switch units for Japanese, English, and other international workflows without leaving the page.

How to use

  1. Enter the species name, length, unit, and length type, then choose a reference preset or enter coefficients a and b directly.
  2. Adjust the error percentage to update the estimated weight and range on the right in real time.
  3. Once the coefficients are settled, save them with a name and notes or export them as JSON.
  4. Use the coefficient fitting tab when you do not know the coefficients yet, and the batch tab when you need results for many measurements.

Examples

Estimate from a reference preset

Input

Species: Japanese jack mackerel, length: 35 cm, length type: TL, a=0.01096, b=2.97, error: ±20%

Output

Estimated weight: about 421 g, range: about 337 g to 505 g

Solve for a from one sample

Input

Length: 35 cm, observed weight: 432 g, assumed b: 3.0

Output

a = 0.01008, formula: W(g) = 0.01008 × L(cm)^3.0

Batch calculation

Input

Run 25 cm, 30 cm, and 35 cm with the same coefficient set

Output

List estimated weight, low range, and high range for each row and export the table as CSV

Reference presets

Built-in presets are reference values only. Check the source label and length-type assumption in the result panel, and replace them with measured operating values whenever possible.

Japanese jack mackerel: a=0.01096, b=2.97, TL, based on cm/g. FishBase reference value.
Chub mackerel: a=0.00741, b=3.05, TL, based on cm/g. FishBase reference value.
Olive flounder: a=0.00794, b=3.07, TL, based on cm/g. FishBase reference value.
Japanese seabass: a=0.01259, b=3.02, TL, based on cm/g. FishBase reference value.
Red seabream: a=0.01288, b=3.03, TL, based on cm/g. FishBase reference value.
Blackhead seabream: a=0.02138, b=2.99, TL, based on cm/g. FishBase reference value.

Glossary

TL / FL / SL

Total length, fork length, and standard length. The coefficient set is only valid when the same length definition is used.

Coefficients a and b

The values that define the relationship W = aL^b. They vary by species, region, season, and stock condition.

Error range

The working range around the estimate. Adjust it to reflect regional differences, maturity, and body condition.

Reference coefficient

A coefficient set taken from published or public reference data. Validate it before operational use.

Fitted coefficient

A coefficient set derived from your own measured fish, either by back-solving or by regression.

Formulas

  • Estimated weight W(g) = a × L(cm)^b
  • Lower bound = W × (1 - error% / 100)
  • Upper bound = W × (1 + error% / 100)
  • Single-sample solve: a = W / L^b
  • Multi-sample fit: log(W) = log(a) + b log(L)

Frequently asked questions

I do not know a and b. What should I do?

Use the coefficient-fitting tab. Solve for a from one measured sample, or fit both a and b from multiple length–weight pairs.

Can I calculate many fish at once?

Yes. The batch tab accepts table entry and CSV-style paste, and you can download the results as CSV.

Can I switch between cm and inches, or g and lb?

Yes. Change input and output units at any time. Internal calculations always use cm and g.

Why can the estimate be far from the actual weight?

Fish of one species still vary by region, season, maturity, body condition, and measurement method. Treat the estimate as a range, not a guaranteed exact value.

Where are saved coefficients stored?

Without login support, saved sets live in this browser's local storage. Keep a JSON export if you need a portable copy.

Is any data sent to a server?

No. Calculation, saving, and JSON generation all stay inside the browser.

Important notes

  • This estimate is a guide only. Region, season, maturity, body condition, and measurement method can change the result substantially.
  • Always use coefficients with the same length type and unit assumption. The internal base is cm and g.
  • Reference presets are not operating values. Use measured data to build your own sets whenever possible.