Trout and salmon
Rainbow trout
Oncorhynchus mykiss.
Length to weight chart, typical size by age, and how to photograph a rainbow trout so fishline measures it well.
- Typical top length
- 26 in
- about 6.5 lb
- A 20 inch fish
- 3.0 lb
- 2 lb 15 oz
- Age at 20 inches
- about 5 years
- rough, varies by water
- Growth levels off
- about age 8
- gains slow to a crawl after
Rainbow trout length to weight chart
Estimated weight for a rainbow trout in average shape at each length, from the formula W = L^3 / 2,700 (W in pounds, L in inches). It is the same relationship fishline uses when it measures a rainbow trout from a photo.
| Length | Weight | Rough age |
|---|---|---|
| 10 in | 0.37 lb | 1 to 2 years |
| 12 in | 0.64 lb | about 2 years |
| 14 in | 1.0 lb | 2 to 3 years |
| 16 in | 1.5 lb | 3 to 4 years |
| 18 in | 2.2 lb | about 4 years |
| 20 in | 3.0 lb | about 5 years |
| 22 in | 3.9 lb | 6 to 7 years |
| 24 in | 5.1 lb | 8+ years |
| 26 in | 6.5 lb | 8+ years |
| 28 in | 8.1 lb | 8+ years |
Total length, nose to the tip of the pinched tail. Weights are for a rainbow trout in average shape. A fat fish, like a female before the spawn, weighs more and a thin one less, which a girth measurement picks up. Rows past 26 inches are bigger than typical for the species.
Size by age
fishline's growth model puts a typical rainbow trout at around 26 inches when fully grown, with most of its growth done by about age 8. Younger fish add length fast; older fish mostly add weight.
Age from length is rough. Fish in warm water with plenty to eat grow faster than this, and fish in cold or crowded water grow slower. For a quick estimate from any length, use the fish age calculator.
| Age | Length | Weight |
|---|---|---|
| 1 year | 6.7 in | 0.11 lb |
| 2 years | 11.7 in | 0.60 lb |
| 3 years | 15.4 in | 1.4 lb |
| 4 years | 18.2 in | 2.2 lb |
| 5 years | 20.2 in | 3.1 lb |
| 6 years | 21.7 in | 3.8 lb |
| 7 years | 22.8 in | 4.4 lb |
| 8 years | 23.6 in | 4.9 lb |
From a von Bertalanffy growth curve. Fish in warm, food-rich water grow faster than this and fish in cold or crowded water grow slower, so treat every row as a rough middle.
Weigh your rainbow trout
Enter the length, and the girth if you taped it, for an estimate on your fish.
Photo tips for rainbow trout
fishline reads length from the photo using whatever is in frame for scale, so a clean, side-on shot gives the tightest number.
- 1Keep the fish wet and low. Cradle it just above the water with one hand under the belly and one near the tail, side-on to the camera.
- 2Silver sides throw glare. Put the sun behind you or off to one side, and skip the flash when the fish is right in front of the lens.
- 3A wet measuring board or a net with printed marks gives fishline the tightest read. Your hand in the shot works too.
More trout and salmon
Or skip the tape. Snap a photo and fishline measures it.
Species, length and weight from one photo. 5 free measurements every month.