I’ve been bullish on digital payments enabler Square Inc (NYSE:SQ) for a long time. As Square stock has crossed the $20, $30, and $40 thresholds, I have maintained my bullishness. After all,Square historically has had big growth prospects and a valuation that had yet to run up to those big growth prospects.

That combination has allowed Square to more than quadruple over the past two years.

This run higher in Square stock can continue. But it will continue with much less velocity. This stock won’t quadruple over the next 2 years. The valuation is finally starting to catch up to the growth prospects, and that means Square is beginning to look fully valued.

Here’s a deeper look.

Fundamentals Supporting Square Stock Remain Strong

At its core, Square is a pure-play on the secular growth in digital payments.

The company has emerged a go-to enabler of digital payments for merchants of all sizes, mostly due to its convenience. It used to be that accepting digital payments at some locations was a hit or miss due to the complexity of accepting such digital payments.

But today, as opposed to requiring several moving parts to support digital payments, all retailers need to complete essentially any transaction is a phone and Square technology.

Because of this enhanced convenience and the massive shift away from cash, Square has benefited from explosive growth.

This big growth isn’t going anywhere anytime soon. Payments processing is a $26-billion-and-growing market. With the addition of ancillary markets like small-business loans and food delivery (Square’s subscription and services businesses), Square believes its total addressable market is around $60 billion.

Revenues this year are expected to be just $1.34 billion. From this perspective, SQ is tapping into just over 2% of its addressable market. Consequently, today’s 20% to 30% and up net revenue growth rates are here to stay.

Moreover, the company’s biggest growth segment is also its highest-margin segment. SQ’s subscriptions and services business runs at 70% gross margins, versus 36% for the transaction business. But the subscription and services business nearly doubled last quarter, while transaction revenue rose by 30%.

Thus, the more subscription and services growth outpaces transaction growth, the higher margins should go. Plus, revenue scale (30% and up revenue growth) should allow for healthy opex leveraging.

All together, Square is a big top-line growth stock with huge margin drivers. That combination promises exciting earnings growth potential, and is why Square stock has come so far in such a hurry.

Valuation Is Starting to Look Full

But in the upper $40’s, Square stock looks close to fully valued.

Even if you assume revenue growth continues to run at a 25-30% pace over the next 5 years and adjusted net profit margins zoom to 20% (from 5% last year), that still only gets you to $7.46 billion in net revenue and $1.49 billion in net profits in 5 years. On presumably 500 million diluted shares by then, that would equate to nearly $3 in earnings per share in 5 years.

Peers Paypal Holdings Inc (NASDAQ:PYPL) and Visa Inc (NYSE:V) have both historically traded around 25-times forward earnings. An industry-average 25-times multiple on $3 in earnings per share implies a 4-year forward price target of about $75. Discounting that back by 10% per year, you get to a present value of just over $50.

Bottom Line on Square Stock

SQ is a big growth stock with a valuation that is finally catching up to its growth prospects. As such, gains from here won’t be linear. The stock will head higher, but not by the same amount it has over the past several years, and trade should be choppy.

But that doesn’t deter me from owning it here and now. I like SQ stock below $50 because the risk-reward profile is favorable. Above $50? I’m not so sure Square is ready for those levels just yet.

As of this writing, Luke Lango was long SQ and PYPL.