Anglers who catch and release don’t want to keep a fish out of the water to weigh it. A quick length and girth measurement with these formulas gives a good estimate.
How to use the Fish Weight Calculator
- Species
- Custom factor k
- Length
- Girth (0 = estimate)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose the species, enter the length (nose to tail fork) and girth (around the widest part) – or leave girth at 0 to estimate it.
Tips
- Check local conditions and safety rules before going out.
- Start with conservative equipment choices and adjust with experience.
- Every field has its own unit menu.
- Everything is calculated in your browser.
Common mistakes to avoid
- Using estimates instead of a proper check in the water.
- Mixing feet and metres, or knots and km/h.
- Overestimating your skill in strong wind.
Inputs explained
| Field | What to enter |
|---|---|
| Species | Choose one: Bass / striper, Carp, catfish, walleye, Trout, herring, shad, Salmon, sturgeon … |
| Custom factor k | A number |
| Length | Units: in, cm |
| Girth (0 = estimate) | Units: in, cm |
Example values
How the main result changes when Custom factor k changes and everything else stays at the example values above:
| Custom factor k | Estimated weight (lb) |
|---|---|
| 400 | 3.364 |
| 600 | 3.364 |
| 800 | 3.364 |
| 1,000 | 3.364 |
| 1,200 | 3.364 |
| 1,600 | 3.364 |
Formula
Weight (lb) = length × girth² ÷ k (inches) · k = 800 for most fish, 900 for trout and shad, 1,200 for panfish; girth ≈ 0.58 × length if not measured
Worked example
Inputs — Species: Bass / striper; Length: 20 in; Girth (0 = estimate): 0 in.
Results — Girth used = 11.6 in; Estimated weight = 3.364 lb.
Key terms
- Girth
- Circumference at the widest part.
Frequently asked questions
How do I measure girth?
Wrap a soft tape around the fish just in front of the dorsal fin.
Is the estimate accurate?
Usually within 10–15%; spawning fish may be heavier.
Why do panfish use a different factor?
Their deep, flat bodies change the shape relation.
Can I use it for records?
No – records need a certified scale.