Practice catch and release with all fish.
Take a photo of the fish.
Record a measurement of the fish's length and girth.
Keep the fish in the water as much as possible during measurement.
Be sure to take a few extra minutes to revive and release all fish after measurement.

Use this calculator for figuring the weight of your fish.

The algorithm used is W=(L * (G^2))/R, where:

W = Weight in decimal pounds
L = Length in inches
G = Girth in inches (default is .58 * L)
R = 900 for steelhead, 800 for Salmon

Fish Weight Calculation This form requires a browser that supports Javascript. The calculation is fairly accurate, but cannot include the many genetic variations in fish body shape.
Length of Fish	Girth of Fish (not required)	Shape of Fish	Weight of Fish
		Steelhead Salmon

This calculator adapted from work by: Joel Dunn @ UNC