How Big Do Rainbow Trout Get? Average and Potential Size

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One of David’s average-sized rainbows during a fishing competition in Northeast Iowa.

Before you start dreaming of your lifetime rainbow trout, it’s worth clarifying how big these gamefish get. On average, a rainbow trout spans between 12 and 20 inches long. 

But, like many fishing questions, a better answer might be: “Depends on who you ask!” This is because anglers use “rainbow trout” to refer to various fish. 

Average Size of Rainbow Trout 

A rainbow trout’s growth depends on habitat and food availability. So, trout of different habitats have contrasting growth potential. 

For example, my region’s small brook trout streams will yield a different average-sized brookie than a Labrador river. The same applies to fluvial (fish that spawn in rivers), adfluvial (fish that spawn in lakes and streams), and steelhead trout. 

Fluvial Rainbow Trout

Fluvial (found in a river) rainbow trout usually have a modest size of 12-16 inches. Their swift-water environment burns more calories, slowing their growth.

Cory outfished David with dozens of stocked rainbow trout.
Cory outfished David with dozens of stocked rainbow trout

Also, a river’s cooler temperatures might slow their metabolism compared to a lake’s warmer water. The fish slow their metabolism to conserve energy as the river cools in cold months. 

A third possibility for their modest size is food availability. Invertebrates fill rivers to make a trout’s diet. Though opportunistic trout will bite at substantial-sized bait, quantity over quality defines their menu. 

Don’t be discouraged by a river trout’s moderate size. In my experience, these fish are experts at using the current to fight. The force they impose can surprise any angler! 

Adfluvial Rainbow Trout

As the world’s most prevalent trout, you can find ‘bows in lakes almost anywhere. For the most part, these adfluvial (stream/lake-dwelling) trout will have a similar size to those in rivers. 

However, fishing reports show their size potential is incredible. Some lake-dwellers weigh over 20 pounds and 40 inches. Unlike river-bound trout, adfluvial trout do not have to fight the water’s current.

These still-water fisheries are also stocked with sterile triploid trout. Without spawning activity, these fish can devote energy to feeding. Though the debate is ongoing, many anglers believe the typical triploid is larger than a non-sterile trout.

Fisherman catch colorful salmon at a pond.

Steelhead Trout

It would be unfair to leave out steelhead when considering a rainbow trout’s size. Though steelhead and rainbow trout are genetically the same, steelhead have exceptional proportions. 

Their average length is 24 inches, but measurements reaching 45 inches occur. Their migratory lifestyle and saltwater habitat create these impressive sizes. 

Even if you do not consider a Great Lake steelhead a true “steely,” its length and weight will impress you. Great Lake steelhead can grow to 36 inches and 20 pounds. Compared to an inland rainbow trout, that’ll be one heck of a fight.

Related Questions 

What Is A Good Size For Rainbow Trout?

No matter who you ask, 20 inches seems to be the standard mark of an inland trophy trout. As someone lucky enough to catch a few over twenties, it never gets old.

If you’re fishing a river and net a rainbow trout over 20 inches, it’s time to get out the camera. Do me a favor, though, and don’t hold the fish close to the camera to make it look bigger. If you do, a guide somewhere will be picking on you for the pose.

How Old Is A 12-Inch Rainbow Trout? 

Wild trout grow around 3 inches yearly, so assume that a 12-inch rainbow is about 3-4 years old. 

Farmed trout grow much faster than wild trout due to their high-protein diet. A healthy stocker grows 6 inches per year but occasionally grows an inch per month. 

How Long Can Rainbow Trout Live? 

Most rainbow trout only live for three to four years. But some rainbow trout live a long life of 11 years. 

Best to Eat? 

This answer depends on preference, but many anglers cite 10-12 inches for the perfect “keeper.” These trout are tender with a minimal fishy taste and fit into your frying pan. Convenient cooking that is tasty and effective? Count me in. 

Final Thoughts 

You never know what a lake or river will offer, but most rainbow trout are 12-20 inches on average. Keep in mind that rainbows over 20 inches and steelhead over 40 inches are out there, so don’t be discouraged from getting on the water. 

As they say, “You can’t catch ‘em from the couch!” With enough time on the water, the right conditions, and some luck, the fish of a lifetime might be waiting to take a gulp!

David Linsmeyer Avatar


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