Estimate the weight of your catch using scientifically backed, species-specific formulas. Enter length and girth for the most accurate fish weight estimation.
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Girth is optional. If not provided, we'll estimate it based on species-typical proportions. Providing girth improves accuracy significantly.
Estimating the weight of a fish without a scale is a common challenge for anglers. Whether you practice catch-and-release or simply forgot your scale at home, a fish weight calculator can give you a reliable estimate using just two measurements: length and girth.
The standard formula used by fisheries biologists and wildlife agencies across North America is:
Weight (lbs) = (Length × Girth²) ÷ Factor
Where Length is measured from nose to tail tip, Girth is the circumference at the widest point, and Factor is a species-specific divisor.
The species-specific factor (also called the "shape factor" or divisor) accounts for differences in body shape between fish species. A largemouth bass, for example, has a deeper, rounder body than a northern pike, which is long and slender. This means the same length and girth measurements will produce different weights depending on the species.
For species like walleye, northern pike, and muskellunge, a length-only formula is also commonly used:Weight = Length³ ÷ 3500. This is less accurate than the length-and-girth formula but useful when girth cannot be measured.
Place the fish on a flat surface or hold it horizontally. Measure from the tip of the nose (or lower jaw for billfish) to the tip of the tail fin. This is called Total Length (TL) and is the standard for most freshwater and inshore saltwater species. For tuna, mahi-mahi, and mackerel, use Fork Length (FL) — measured to the fork of the tail.
Wrap a flexible tape measure around the fish at its widest point — typically just in front of the dorsal fin. This circumference measurement is the girth. It's the single most important measurement for accuracy because it accounts for how "fat" or "skinny" the fish is. A well-fed bass will weigh significantly more than a lean one of the same length.
Pro Tip: If you don't have a tape measure, use a piece of fishing line or string to wrap around the fish, then measure the string against a ruler or known reference (like a rod marking). Many modern fishing rods have measurement markings built into the blank for exactly this purpose.
Each species has a unique shape factor that accounts for body proportions. Below are the formulas used in our fish weight calculator for the most popular freshwater and saltwater sportfish.
| Species | Formula | Factor | Avg. Weight |
|---|---|---|---|
| Largemouth Bass | (Length × Girth²) / 800 | 800 | 1–5 lbs |
| Smallmouth Bass | (Length × Girth²) / 800 | 800 | 1–4 lbs |
| Striped Bass | (Length × Girth²) / 800 | 800 | 5–30 lbs |
| Rainbow Trout | (Length × Girth²) / 900 | 900 | 1–5 lbs |
| Brook Trout | (Length × Girth²) / 900 | 900 | 0.5–3 lbs |
| Brown Trout | (Length × Girth²) / 900 | 900 | 1–8 lbs |
| Steelhead | (Length × Girth²) / 900 | 900 | 5–12 lbs |
| Walleye | (Length³) / 3500 | 2700 | 1–6 lbs |
| Northern Pike | (Length³) / 3500 | 2700 | 2–10 lbs |
| Muskellunge | (Length³) / 3500 | 2700 | 8–25 lbs |
| Channel Catfish | (Length × Girth²) / 1100 | 1100 | 1–10 lbs |
| Blue Catfish | (Length × Girth²) / 1100 | 1100 | 5–40 lbs |
| Flathead Catfish | (Length × Girth²) / 1100 | 1100 | 3–20 lbs |
| Bluegill | (Length × Girth²) / 1200 | 1200 | 0.25–1 lb |
| Black Crappie | (Length × Girth²) / 1200 | 1200 | 0.5–2 lbs |
| White Crappie | (Length × Girth²) / 1200 | 1200 | 0.5–2 lbs |
| Chinook Salmon | (Length × Girth²) / 800 | 800 | 10–30 lbs |
| Coho Salmon | (Length × Girth²) / 800 | 800 | 6–12 lbs |
| Common Carp | (Length × Girth²) / 800 | 800 | 3–15 lbs |
| Yellow Perch | (Length × Girth²) / 1200 | 1200 | 0.25–1 lb |
| Species | Formula | Factor | Avg. Weight |
|---|---|---|---|
| Redfish (Red Drum) | (Length × Girth²) / 900 | 900 | 3–15 lbs |
| Speckled Trout (Seatrout) | (Length × Girth²) / 900 | 900 | 1–5 lbs |
| Snook | (Length × Girth²) / 900 | 900 | 3–15 lbs |
| Tarpon | (Length × Girth²) / 800 | 800 | 40–120 lbs |
| Bonefish | (Length × Girth²) / 900 | 900 | 3–8 lbs |
| Blacktip Shark | (Length × Girth²) / 800 | 800 | 15–50 lbs |
| Bull Shark | (Length × Girth²) / 800 | 800 | 100–300 lbs |
| Sand Shark | (Length × Girth²) / 800 | 800 | 50–150 lbs |
| Mahi-Mahi (Dolphinfish) | (Length × Girth²) / 900 | 900 | 5–25 lbs |
| Red Snapper | (Length × Girth²) / 900 | 900 | 3–10 lbs |
| Grouper | (Length × Girth²) / 800 | 800 | 5–40 lbs |
| Flounder | (Length × Girth²) / 1000 | 1000 | 1–8 lbs |
| Yellowfin Tuna | (Length × Girth²) / 800 | 800 | 20–150 lbs |
| King Mackerel | (Length × Girth²) / 900 | 900 | 5–20 lbs |
| Spanish Mackerel | (Length × Girth²) / 900 | 900 | 1–4 lbs |
| Cobia | (Length × Girth²) / 900 | 900 | 10–40 lbs |
| Pompano | (Length × Girth²) / 900 | 900 | 1–4 lbs |
| Sheepshead | (Length × Girth²) / 900 | 900 | 2–6 lbs |
| Black Drum | (Length × Girth²) / 800 | 800 | 5–20 lbs |
| Wahoo | (Length × Girth²) / 900 | 900 | 15–60 lbs |
Two fish of the same species and identical length can have dramatically different weights. A 20-inch largemouth bass from a nutrient-rich lake might weigh 5 pounds, while the same length bass from a less productive body of water might weigh only 3 pounds. The difference? Girth.
Girth measures the circumference of the fish at its fattest point and is the strongest predictor of weight beyond length. In the weight formula, girth is squared — meaning a small increase in girth produces a large increase in estimated weight. This is why fisheries biologists consider girth the most important measurement for accurate weight estimation.
When girth is not available, our calculator uses species-typical girth-to-length ratios derived from fisheries research data. For example, bass typically have a girth that is about 60% of their length, while pike have a girth closer to 46% of their length. These ratios provide a reasonable estimate but cannot account for individual variation in body condition.
Formula-based fish weight calculators have been the standard for decades, and they work well — especially when you have accurate length and girth measurements. But they have limitations: you need a tape measure, you need to handle the fish long enough to take measurements, and the formulas can't account for every individual variation in body shape.
FishScale.AI takes a different approach. Using advanced artificial intelligence and computer vision trained on thousands of fish images with verified weights, the app estimates weight directly from a photograph. No tape measure required. No formulas to remember. Just snap a photo and get an instant estimate.
| Feature | Formula Calculator | FishScale.AI App |
|---|---|---|
| Tape measure needed | Yes | No |
| Speed | 1–2 minutes | Instant |
| Species identification | Manual | Automatic |
| Handles fish | Extended | Minimal |
| Catch-and-release friendly | Moderate | Excellent |
| Catch logging | No | Built-in Trophy Room |
| Share results | No | One-tap sharing |
| Requires internet | No (manual calculation) | Yes (needs cell or Wi-Fi) |
When using accurate length and girth measurements with species-specific formulas, you can expect estimates within 5–10% of actual weight. The accuracy improves significantly when girth is measured rather than estimated. Factors like spawning condition, time of year, and regional forage availability can cause individual fish to deviate from formula predictions.
Yes, but it's less accurate. Length-only formulas use a cubic relationship (Length³ ÷ Factor) and assume average body proportions for the species. For species like walleye, pike, and muskellunge, length-only formulas are commonly used and reasonably accurate. For deep-bodied species like bass and panfish, adding girth dramatically improves accuracy.
Total Length (TL) is measured from the tip of the nose to the very end of the tail fin. Fork Length (FL) is measured from the nose to the fork (notch) in the tail. Most freshwater species and inshore saltwater species use total length. Offshore species like tuna, mahi-mahi, and mackerel typically use fork length.
The species-specific factor (divisor) in the formula accounts for differences in body shape and density. A largemouth bass has a deep, round body (factor 800), while a northern pike is long and slender (factor 2700 for length-only). These factors have been calibrated using fisheries research data with verified weights.
Yes — FishScale.AI uses artificial intelligence to estimate fish weight directly from a photograph. Instead of manually measuring length and girth, you simply take a photo of your catch. The AI analyzes the fish's proportions, identifies the species, and provides a weight estimate. It's faster, more convenient, and great for catch-and-release fishing where minimizing handling time matters.