Fish weight calculator
A 20-inch bass with a 12-inch girth weighs about 2.4 pounds, or 1.09 kg: length times girth squared, divided by the shape constant for that species, the formula on angling weight-tape cards. Free in your browser.
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Worked example
- Species
- Bass in
- Length
- 20
- Girth
- 12
- Length in
- 20 in
- Girth in
- 12
- Divisor
- 1,200
- Weight lb
- 2.4
A landed fish's weight is the figure every catch-and-release angler wants after a photo, every club weigh-in needs when the scale stayed in the truck, and every fisheries homework set asks for from two tape readings. The practical field method is the girth formula published in angling magazines and on weight-tape cards: multiply total length in inches by the square of girth in inches, then divide by a species-specific shape constant to estimate pounds, and convert to kilograms with the international avoirdupois pound. This fish weight calculator accepts a closed species list, length and girth as finite positive numbers, and an optional unit of in or cm.
What the girth formula estimates and how species changes the divisor
The field method for estimating a landed fish's mass without a hanging scale is the girth identity used by angling magazines, club weigh-in cards, and fisheries teaching labs. Measure girth as the circumference at the thickest part of the body, usually just in front of the dorsal fin, with a soft tape snug against the skin or wet slime and not pulled into the flesh. Convert both measurements to inches if they were taken in centimetres by dividing each by 2.54, the exact international inch.
The divisor is a published shape constant, not a fitted curve: bass and panfish use 1200 because those bodies are more oval in cross-section; pike, muskie, trout, and striped bass use 900; salmon use 750 because a mature salmon is thicker for its length; walleye, catfish, carp, and the general bucket use 800. A twenty-inch bass with a twelve-inch girth therefore lands at 2.4 pounds, about 1.09 kilograms, which is the order of magnitude a club card would quote before you still confirm a tournament weight on a certified scale.
This calculator does not apply a Fulton condition factor, does not estimate from length alone, and does not replace a scale when a record, a bag limit in kilograms, or a scientific sample depends on a weighed mass.
Where a fish weight calculator earns its keep
Catch-and-release anglers use a fish weight calculator so the boat log can store pounds without killing the fish or guessing from a phone photo. Club weigh-ins and youth derbies batch length and girth from the card when the hanging scale is wet, missing, or not trusted, and they need a deterministic oracle that rejects a zero-inch paste instead of silently publishing a zero-pound fish. Failed validation does not consume balance.
Keep both measurements honest, pick the species that matches the body you taped, pin pounds and kilograms in tests, and you get a reproducible live-weight estimate for homework, LMS plugins, boat logs, and club spreadsheets. Related searches for a horse weight tape, a pond volume, an aquarium litre count, or a human BMI are different intents: stay on this page when the question is only the fish's estimated body weight implied by species, length, and girth.
This estimate is educational and does not replace a certified tournament scale, a fisheries laboratory balance, or a record application that requires a witnessed hanging weight.
What you can do with it
Club weigh-ins without a hanging scale
Youth derbies and club cards batch length and girth when the scale stayed in the truck, and get a deterministic estimate that rejects a zero-inch paste rather than inventing a zero-pound fish.
Centimetre tapes and mixed-unit kits
Send unit cm with both measurements in centimetres when the only tape in the tackle box is metric; the same species divisor and 2.54 conversion produce matching kilograms.
FAQ
How do I estimate a fish's weight from length and girth?
Send a species from the closed list plus finite positive numbers for length and girth, and unit in or cm (default in).
Which species divisors does the calculator use?
Bass and panfish use 1200; pike, muskie, trout, and striped bass use 900; salmon uses 750; walleye, catfish, carp, and general use 800. Those are the published girth-formula shape constants, not a fitted curve.
Where do I place the tape for length and girth?
Length is total length from the tip of the snout to the tip of the compressed tail. Girth is the circumference at the thickest part of the body, usually just in front of the dorsal fin, with the tape snug and not cutting in.
What if length or girth is zero, negative, or missing?
Length and girth must each be a finite positive number; missing fields, empty strings, and non-numeric values are rejected the same way and are not billed.
Can I measure the fish in centimetres?
Yes. Choose centimetres and enter both tape readings; each is divided by 2.54 to get inches before the formula runs. The kilograms match the inch measurement exactly, because there is one formula and no second metric approximation.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/sports/fish-weight \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"species":"bass","length":20,"girth":12}'const res = await fetch("https://api.kit.forhosting.com/sports/fish-weight", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"species": "bass",
"length": 20,
"girth": 12
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/sports/fish-weight",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"species": "bass",
"length": 20,
"girth": 12
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/sports/fish-weight", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"species":"bass","length":20,"girth":12}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"species":"bass","length":20,"girth":12}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/sports/fish-weight", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"species": "bass",
"length": 20,
"girth": 12
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "sports.fish_weight",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
max_length | 500 |
max_girth | 500 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
Missing, zero, or negative length or girth fail with invalid_input instead of inventing a mass.
What the girth formula estimates and how species changes the divisor
Measure length as total length: the tip of the snout to the tip of the compressed tail, with the fish lying straight and the mouth closed. Square the girth in inches, multiply by the length in inches, and divide by the species divisor: that quotient is weight_lb, the estimated live weight in pounds. Multiply weight_lb by 0.45359237, the exact international avoirdupois kilogram per pound, to obtain weight_kg. Zero and negative measurements are not a fish: they fail rather than invent a live weight.
Input contract, response shape, and error behaviour
Omit unit and the calculator treats both measurements as inches. Example: species bass, length 20, girth 12, unit in is a small largemouth. Example: the same fish measured in centimetres sends unit cm and the two centimetre readings; the kilograms match the inch path because there is one formula, not a second approximate metric divisor. POST a JSON body with three required fields and an optional unit. species is a closed list: bass, pike, muskie, trout, salmon, walleye, catfish, carp, panfish, striped_bass, or general (aliases fish and fish_species). Each numeric field may arrive as a JSON number or as a numeric string. Missing fields, empty strings, non-finite values, zero, negatives, values above the declared maxima, a species outside the list, and a unit other than in or cm fail with invalid_input naming the field. Those failures are not billed. On success the body echoes the cleaned species, length, girth, and unit, publishes length_in and girth_in as the working inches, publishes divisor as the integer used, and returns weight_lb and weight_kg rounded to four decimal places for stable goldens across runtimes. formula restates the girth identity so a notebook can print the substitution next to the answer. Pair the estimate with the species you actually taped; this endpoint does not identify a fish from a photo.
Where a fish weight calculator earns its keep
Fisheries homework and exam-prep generators pin a known twenty-by-twelve-inch bass, a centimetre trout pair, and the invalid_input path for a non-positive girth so a buggy grader cannot accept a negative mass.
Integration use cases
Catch-and-release boat logs
Tape total length and girth on a released bass or trout, pick the matching species, and store weight_lb and weight_kg in the log instead of guessing from a phone photo.
Technical questions
What does the fish weight calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.