Cabo Sportfishing Magazine · Measurement guide
How to Measure a Saltwater Fish and Estimate Its Weight
By Cabo Sportfishing Magazine
The right length depends on the species and its equation. A snout-to-fork measurement, lower-jaw-to-fork measurement, eye-to-fork measurement and total length are not interchangeable. Match the landmarks before estimating a fish’s weight.
Open the free Baja fish weight calculator — centimeters or inches in; pounds and kilograms out. No signup required.
Release first; measure only if it is quick and safe
Release first. Estimate the weight afterward. If a quick measurement would delay the release, skip it.
Do not delay a release to obtain a number. Record the species, the correct straight-line length and its unit only when a quick measurement is possible. Estimate the weight afterward; never add handling just to complete this guide.
1. Snout to tail fork
Start at the tip of the snout and end at the inside of the tail fork—the notch between the two tail lobes. Do not measure to the longest tail tip. Use this for dorado, wahoo, roosterfish and the three enabled tuna species.
Snout to tail fork
Measure straight between the dots, not along the body curve. Reference schematic, not to scale.
Release first. Estimate the weight afterward. If a quick measurement would delay the release, skip it.
2. Tip of lower jaw to tail fork
For the enabled striped marlin model, start at the tip of the lower jaw, not the end of the long upper bill. End at the inside of the tail fork. A bill-to-tail measurement cannot be entered as lower-jaw fork length.
Tip of lower jaw to tail fork
Measure straight between the dots, not along the body curve. Reference schematic, not to scale.
Release first. Estimate the weight afterward. If a quick measurement would delay the release, skip it.
3. Rear edge of left eye orbit to tail fork
Blue and black marlin use eye-orbit-to-fork length. IOTC Table 2 identifies this measurement for their equations; the cited original NOAA report uses EFL. NOAA’s observer field manual defines the start precisely as the posterior margin of the left eye orbit: the rear edge of the eye socket, toward the tail. End at the inside of the tail fork. Do not start at the pupil, eye center, front edge or bill.
Rear edge of left eye orbit to tail fork
Measure straight between the dots, not along the body curve. Reference schematic, not to scale.
NOAA observer field manual, p. 55: posterior margin of the left eye orbit; not the eye center.
Sailfish: the same eye-orbit landmarks, its own equation
The enabled sailfish model uses 124–196 cm eye-fork length from the Cabo San Lucas study. That paper specifies eye-fork length without identifying an orbit edge; NOAA’s field manual explicitly includes sailfish in its posterior-left-eye-orbit definition. Use the rear edge of the left eye socket to the inside of the tail fork, not the bill or eye center. Its kilogram interpretation is provisional because the source’s methods and weight table disagree on units.
Sailfish: Rear edge of left eye orbit to tail fork
Measure straight between the dots, not along the body curve. Reference schematic, not to scale.
NOAA observer field manual, p. 55: posterior margin of the left eye orbit; not the eye center.
Release first. Estimate the weight afterward. If a quick measurement would delay the release, skip it.
4. Total length, snout to tail tip
The Pacific red snapper and spotted rose snapper models use total length: from the tip of the snout to the tail tip, not to the tail fork. Keep this distinct from fork length. Do not substitute the length of another snapper species.
Total length, snout to tail tip
Measure straight between the dots, not along the body curve. Reference schematic, not to scale.
Release first. Estimate the weight afterward. If a quick measurement would delay the release, skip it.
Species currently enabled in the calculator
Choose the exact species, not a similar fish. The list below comes from the same enabled dataset as the calculator and Explorer.
Roosterfish (Nematistius pectoralis)
Snout to tail fork · 5.2–133 cm
Baja California Sur. Sexes-combined coefficients for Baja California Sur. Specimens above 100 cm were scarce in the study.
Dorado (mahi-mahi) (Coryphaena hippurus)
Snout to tail fork · 33–149 cm
Los Cabos, Baja California Sur. Pooled-sex model. The methods label the output in grams, but the table’s coefficients are consistent with its reported kilogram weights; this implementation uses the kilogram interpretation.
Available but provisional: kilogram interpretation, pending author confirmation.
Striped marlin (Kajikia audax)
Tip of lower jaw to tail fork · 150–290 cm
Taiwanese offshore and coastal fisheries. IOTC round-weight model; this is not a Cabo-calibrated equation.
Pacific blue marlin (Makaira nigricans)
Rear edge of left eye orbit to tail fork · 109–269 cm
Central North Pacific research. IOTC round-weight model; this is not a Cabo-calibrated equation.
Black marlin (Istiompax indica)
Rear edge of left eye orbit to tail fork · 95–279 cm
Central North Pacific research. IOTC round-weight model; this is not a Cabo-calibrated equation.
Wahoo (Acanthocybium solandri)
Snout to tail fork · 59–169 cm
Western and Central Pacific. Pooled-sex model. The source coefficient of 0.003 for grams is already converted to kilograms here. The 59–169 cm span describes the overall study sample, not the confirmed range of the 442 fish used for the weight fit.
Pacific red snapper (Lutjanus peru)
Total length, snout to tail tip · 21–55 cm
Santa Rosalía, Baja California Sur. Approximate coefficients reported in the study abstract for this species.
Approximate model from rounded study coefficients.
Spotted rose snapper (Lutjanus guttatus)
Total length, snout to tail tip · 21–59 cm
Santa Rosalía, Baja California Sur. Approximate coefficients reported in the study abstract for this species.
Approximate model from rounded study coefficients.
Yellowfin tuna (Thunnus albacares)
Snout to tail fork · 20–170 cm
IATTC whole-weight equation, Eastern Pacific fisheries. IATTC whole-weight equation. The 20–170 cm limit is the span of the NOAA reference chart, not the original sample range.
Bigeye tuna (Thunnus obesus)
Snout to tail fork · 80–190 cm
Central Pacific study; equation also used by IATTC for Eastern Pacific assessments. Nakamura & Uchiyama (1966) whole-weight equation: W (kg) = 0.00003661 × fork length (cm)^2.90182, reproduced by IATTC SAC-07-04a, p. 4. ICCAT’s Carroceda & Colmenero (2016), Table 6, p. 481, reports 9,144 Central Pacific fish and an 80–190 cm fork-length span for this exact equation. Not calibrated for Cabo. The 80–190 cm limits are documented in the ICCAT literature review, which marks this entry as a secondary citation; the original 1966 sample table has not been independently inspected. The calculator and chart use only that reported span, not a guaranteed accuracy interval.
Skipjack tuna (Katsuwonus pelamis)
Snout to tail fork · 39–71 cm
Eastern Pacific. Chatwin (1959) Eastern Pacific equation, as reproduced in FAO Table 2.
Sailfish (Istiophorus platypterus)
Rear edge of left eye orbit to tail fork · 124–196 cm
Los Cabos. Pooled-sex model from 650 sailfish sampled off Cabo San Lucas (1990–2015). The source specifies eye-fork length but not the orbit edge; NOAA’s billfish field manual supplies the posterior margin of the left eye orbit. Table 1 reports kilogram weights, while the methods call W grams; this tool uses the kilogram interpretation provisionally.
Available but provisional: kilogram interpretation, pending author confirmation.
Worked example: a 100 cm dorado
This is an illustrative calculation, not a reported catch. Select dorado, measure from snout to tail fork, choose cm and enter 100. That is inside its stated 33–149 cm range. The enabled equation is W (kg) = 0.00000455 × L (cm)^3.13.
18.3 lb / 8.3 kg
The dorado result is provisional because of the source’s gram/kilogram discrepancy. It is not a scale weight. The equivalent inch input is 100 ÷ 2.54 ≈ 39.3701 inches; any difference from typing fewer decimals is rounding, not a different model.
What an estimate can—and cannot—tell you
The tool applies W = a × L^b, converts inches to centimeters with × 2.54, and kilograms to pounds with × 2.2046226218. It keeps full coefficient precision and rounds only the displayed result. Two fish of the same length may weigh differently; these estimates do not certify tournament standings, records or payments.
For models with stated limits, the calculator suppresses numbers outside those limits. Wahoo’s limits describe the overall study sample; yellowfin’s limits describe NOAA chart coverage, not the original fit sample. Bigeye’s calculator and chart use 80–190 cm fork length, reported for the exact Nakamura & Uchiyama equation in ICCAT’s literature review (Table 6, p. 481). That range is a secondary citation, not an independently inspected original sample table or a guaranteed accuracy interval. The bigeye and marlin equations come from their disclosed study regions, not a Cabo calibration.
Sources and measurement verification
Eye-to-fork verification uses two distinct sources: IOTC’s model table and its cited NOAA original report establish EFL for blue and black marlin; NOAA’s field manual, page 55, supplies the exact posterior-left-orbit landmark. The original report does not itself specify which orbit edge. NOAA describes the original report as an informal administrative publication.
- Uchiyama & Kazama (2003), NOAA PIFSC H-03-01 — original marlin equation report, methods p. 3 and Table 4
- NOAA American Samoa Observer Program Field Manual — eye-to-fork definition, p. 55
- NOAA Fishery Bulletin, Ortega-García et al., roosterfish length–weight relationship
- Ortega-García et al. (2018), Latin American Journal of Aquatic Research 46(1), Table 1, pooled-sex sailfish model
- IOTC 2014 WPB12, Table 2, round-weight length–weight equations
- Western and Central Pacific wahoo length–weight study, pooled sexes
- Santa Rosalía, Baja California Sur snapper length–weight study, abstract coefficients
- IATTC whole-weight length–weight equations for yellowfin and bigeye tuna
- Chatwin (1959) Eastern Pacific skipjack equation, reproduced in FAO Table 2
- NOAA yellowfin length–weight reference chart
- Carroceda & Colmenero (2016), ICCAT 72(2):472–484, Table 6, p. 481 — bigeye sample range (secondary citation)
- NOAA observer field manual, p. 55, billfish eye-orbit landmark (includes sailfish)
InTheBite — Innovating the Marlin Weight Formula — further reading on a different length-and-girth method, not the source for this calculator’s length-only equations or eye-orbit landmark.