Fish Weight Calculator

Estimate the weight of 14 common New Zealand fish species from a single measurement — fork length for inshore species, or length and girth for billfish and sharks.

Weight estimates use length-weight constants published by NIWA and FishBase for New Zealand fish populations. Results are accurate to within 10–15% for a healthy fish. For billfish, the girth-based formula follows the IGFA / MBGLAC standard used by NZ big game clubs. Learn about the methodology →

Bag limit: per day
Formula source: — verified . Legal sizes and bag limits reflect NZ MPI rules — always verify at mpi.govt.nz.
Snapper on a measure mat showing fork length measurement

How to Measure

Measure fork length — from the tip of the snout to the fork (the notch) in the tail, with the fish lying flat and the tail in its natural closed position.

For billfish and sharks, also measure girth at the widest point of the body (usually just behind the pectoral fins). All measurements in centimetres.

NZ Legal Size & Bag Limit Reference

Minimum legal sizes and daily bag limits for recreational fishing in New Zealand. Rules may vary by region — always confirm at mpi.govt.nz before retaining fish.

Species Min. Length Daily Bag Limit Notes
Barracouta
Thyrsites atun
 — 30 / day No minimum size, 30/day. Primarily a South Island species.
Black Marlin
Makaira indica
 — 1 / day Tag and release strongly encouraged
Blue Cod
Parapercis colias
 30 cm 10 / day 30 cm min; check area-specific SI rules
Blue Marlin
Makaira nigricans
 — 1 / day Tag and release strongly encouraged
Blue Moki
Latridopsis ciliaris
 40 cm 15 / day 40cm all areas. 15/day South Island; 20/day North Island.
Bluenose
Hyperoglyphe antarctica
 — 5 / day
Butterfish
Odax pullus
 35 cm 15 / day 35cm all areas. 15/day South Island; 20/day North Island.
Eel
Anguilla spp.
 — 6 / day No minimum size, 6/day all areas (freshwater and saltwater).
Elephant Fish
Callorhinchus milii
 — 5 / day 5/day South Island; 20/day North Island.
Flatfish
Rhombosolea spp.
 25 cm 20 / day General flatfish (excl. sand flounder). 25cm all areas.
Grey Mullet
Mugil cephalus
 — 20 / day
Gurnard
Chelidonichthys kumu
 25 cm 20 / day
Hapuka
Polyprion oxygeneios
 — 5 / day Combined Groper/Hapuku/Bass daily bag limit of 5 (no more than 3 kingfish in that 5).
John Dory
Zeus faber
 25 cm 10 / day
Kahawai
Arripis trutta
 — 15 / day No minimum size. 15/day South Island areas; 20/day North Island.
Kingfish
Seriola lalandi
 75 cm 3 / day 75cm NI / 65cm SI — check your area
Red Cod
Pseudophycis bachus
 25 cm 10 / day 25cm all areas. 10/day South Island; 20/day North Island.
Red Moki
Cheilodactylus spectabilis
 40 cm 15 / day 40cm all areas. 15/day South Island; 20/day North Island.
Rig
Mustelus lenticulatus
 — 5 / day No minimum size. 5/day South Island; 20/day North Island.
Sand Flounder
Rhombosolea plebeia
 23 cm 20 / day
School Shark
Galeorhinus galeus
 — 5 / day No minimum size. 5/day South Island; 20/day North Island.
Snapper
Pagrus auratus
 30 cm 7 / day 30cm min (SNA1 east coast); 27cm west coast. 7/day SNA1; 3/day Marlborough Sounds. Always check MPI for your area.
Striped Marlin
Kajikia audax
 — 1 / day Tag and release strongly encouraged
Tarakihi
Nemadactylus macropterus
 25 cm 20 / day
Trevally
Pseudocaranx georgianus
 25 cm 20 / day
Trumpeter
Latris lineata
 35 cm 15 / day 35cm, 15/day. Primarily a South Island species.
Warehou
Seriolella brama
 — 15 / day No minimum size, 15/day. Primarily a South Island species.
Yellowfin Tuna
Thunnus albacares
 — 15 / day Pelagic species — confirm current WCPFC/MPI rules

Legal size data sourced from NZ MPI Fisheries regulations 2024. This table is a guide only — verify current rules at mpi.govt.nz.

Frequently Asked Questions

Measure the fork length — from the tip of the snout to the fork in the tail — in centimetres. Select Snapper in the calculator and enter the length. The result uses the formula W = 0.0107 × L3.13, a NIWA-derived constant for the New Zealand snapper population. A 40 cm snapper weighs approximately 0.8 kg; a 60 cm snapper approximately 2.7 kg.

The calculator covers 14 New Zealand species: Snapper, Kingfish, Kahawai, Trevally, Hapuka, Bluenose, Blue Cod, John Dory, Tarakihi, Yellowfin Tuna (length-weight formula) and Striped Marlin, Black Marlin, Blue Marlin, Mako Shark (girth-based billfish formula).

Estimates are accurate to within 10–15% for a healthy fish in average condition. Fish in spawning condition, unusually fat or lean may fall outside this range. The estimate reflects an average fish of that length for the New Zealand population. For competition purposes, always use certified scales — this calculator is a guide only.

The minimum legal size for snapper is 30 cm (fork length) in most New Zealand areas, with a daily bag limit of 10 fish. Some areas have additional restrictions — always confirm current rules at mpi.govt.nz.

Kingfish minimum legal size is 75 cm in the North Island and 65 cm in the South Island, with a daily bag limit of 3 fish. Always verify at mpi.govt.nz for your exact area.

Snapper on a measure mat

Measure fork length — from the tip of the snout to the fork in the tail, with the fish lying flat and the tail in its natural closed position.

For billfish and sharks, also measure girth at the widest point of the body (usually behind the pectoral fins). All measurements in centimetres.

For inshore species: W = a × Lb, where L is fork length in cm and W is weight in kg. The constants a and b come from FishBase and NIWA research for New Zealand populations. For billfish and sharks: W = G² × L ÷ divisor, where G is girth in cm — consistent with the IGFA / MBGLAC standard used by NZ big game fishing clubs. Full methodology →

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