Fish Weight by Length Chart (and the Formulas Behind It)
A scale isn't always in the boat, and a fish you're about to release shouldn't be hanging on one for long anyway. A length — plus a girth measurement when you can take one — gets you a weight that's close enough to log.
The formulas below are the ones published by the Wisconsin DNR in its guide to estimating fish weight. Length and girth go in as inches, and the answer comes out in pounds. They work because weight scales roughly with the cube of length — the divisor is what adapts that to a body shape, which is why a pike and a sunfish of the same length are nowhere near the same weight.
The formulas
| Fish | Formula (inches → pounds) | Needs a girth measurement? |
|---|---|---|
| Bass | length × length × girth ÷ 1,200 | Yes |
| Trout | length × girth × girth ÷ 800 | Yes |
| Pike | length × length × length ÷ 3,500 | No |
| Walleye | length × length × length ÷ 2,700 | No |
| Sunfish / bluegill | length × length × length ÷ 1,200 | No |
Pike weight by length
Northern pike (length ÷ 3,500)
| Length | Estimated weight |
|---|---|
| 20 in (51 cm) | 2.29 lb (1.04 kg) |
| 24 in (61 cm) | 3.95 lb (1.79 kg) |
| 28 in (71 cm) | 6.27 lb (2.84 kg) |
| 30 in (76 cm) | 7.71 lb (3.50 kg) |
| 32 in (81 cm) | 9.36 lb (4.25 kg) |
| 34 in (86 cm) | 11.2 lb (5.09 kg) |
| 36 in (91 cm) | 13.3 lb (6.05 kg) |
| 38 in (97 cm) | 15.7 lb (7.11 kg) |
| 40 in (102 cm) | 18.3 lb (8.29 kg) |
| 42 in (107 cm) | 21.2 lb (9.60 kg) |
| 44 in (112 cm) | 24.3 lb (11.0 kg) |
| 48 in (122 cm) | 31.6 lb (14.3 kg) |
Walleye weight by length
Walleye (length ÷ 2,700)
| Length | Estimated weight |
|---|---|
| 12 in (30 cm) | 0.64 lb (0.29 kg) |
| 14 in (36 cm) | 1.02 lb (0.46 kg) |
| 16 in (41 cm) | 1.52 lb (0.69 kg) |
| 18 in (46 cm) | 2.16 lb (0.98 kg) |
| 20 in (51 cm) | 2.96 lb (1.34 kg) |
| 22 in (56 cm) | 3.94 lb (1.79 kg) |
| 24 in (61 cm) | 5.12 lb (2.32 kg) |
| 26 in (66 cm) | 6.51 lb (2.95 kg) |
| 28 in (71 cm) | 8.13 lb (3.69 kg) |
| 30 in (76 cm) | 10.0 lb (4.54 kg) |
| 32 in (81 cm) | 12.1 lb (5.50 kg) |
Sunfish and bluegill weight by length
Sunfish (length ÷ 1,200)
| Length | Estimated weight |
|---|---|
| 5 in (13 cm) | 0.10 lb (0.05 kg) |
| 6 in (15 cm) | 0.18 lb (0.08 kg) |
| 7 in (18 cm) | 0.29 lb (0.13 kg) |
| 8 in (20 cm) | 0.43 lb (0.19 kg) |
| 9 in (23 cm) | 0.61 lb (0.28 kg) |
| 10 in (25 cm) | 0.83 lb (0.38 kg) |
| 11 in (28 cm) | 1.11 lb (0.50 kg) |
| 12 in (30 cm) | 1.44 lb (0.65 kg) |
Bass and trout: take the girth
Bass and trout vary too much in condition for length alone. Run a tape around the fish at its thickest point, just behind the pectoral fins, and keep it snug without squeezing.
A 20 in largemouth with a 14 in girth: 20 × 20 × 14 ÷ 1,200 = 4.7 lb (2.1 kg).
A 24 in trout with a 13 in girth: 24 × 13 × 13 ÷ 800 = 5.1 lb (2.3 kg).
The same girth formula used for trout — length × girth² ÷ 800 — is the usual fallback for any species you don't have a divisor for.
These are estimates, and they behave like estimates. A fish full of spawn, one that's just fed hard, or a lean fish late in the season can sit well outside the number. Log it as an estimate rather than a weight, and note whether you had a girth measurement — otherwise your personal-best list quietly mixes two different things.
Converters
The formulas only work in inches and pounds, so convert before you calculate if you measure in metric — then convert the answer back.
Length: cm ↔ in
1 cm = 0.393701 in. Type a value to convert instantly — no sign-up.
Weight: kg ↔ lb
1 kg = 2.20462 lb. Type a value to convert instantly — no sign-up.
Getting a usable estimate in the boat
- Measure length the same way every time — total length, snout to pinched tail. See how to measure a fish if you're not sure which method to use.
- Take the girth at the thickest point if the fish will hold still for it.
- Pick the divisor for the body shape, not the exact species — pike-shaped, bass-shaped, or panfish-shaped.
- Do the sum later; write the measurements down now, while the fish is going back in the water.
- Store the numbers with a photo, so a doubtful estimate can be sanity-checked years later.
Frequently asked questions
How accurate is estimating fish weight from length?
Close enough for a logbook, not close enough for a record claim. Length-only formulas assume an average body condition, so a fish in unusual shape can be well off. Adding a girth measurement narrows it considerably, which is why the bass and trout formulas ask for one.
Is there one formula for all fish?
The general fallback is length × girth × girth ÷ 800, with length and girth in inches. It's the trout formula, and it's the one to use for a species you don't have a specific divisor for. Long, slender fish such as pike come out closer with their own divisor.
How do I calculate fish weight in kilograms?
Convert first: the formulas are built for inches and pounds. Measure in cm, convert to inches with the converter above, run the formula, then convert the pounds back to kg. The charts on this page already list both.
How do I keep these estimates in my log?
Record the length, the girth if you took one, and the estimate — and be consistent about it, because your personal bests are only comparable if every entry was measured the same way. Anglerly stores length and weight in metric or imperial and switches the whole log between them at once, so you can enter what you measured and read it back however you like.