Alphabet reports Q2 results after the bell today. The number that decides the stock isn't revenue or earnings -- it's capex.

Alphabet (NASDAQ: GOOG)(NASDAQ: GOOGL) reports second-quarter results after the market closes today, with the earnings call scheduled for 4:30 p.m. ET. The revenue and earnings may end up being the focus on many of the headlines. But I'd argue the number that actually has more implications for the stock sits further down the report. It's capital expenditures -- the money Alphabet is pouring into data centers and the computing infrastructure behind its artificial intelligence (AI) push.

After all, nobody doubts that the business is growing. The question is whether the company's AI spending is an investment compounding into more growth or a cost rising faster than the returns it generates.

The spending curve keeps steepening

In April, alongside first-quarter results, Alphabet raised its 2026 capital expenditure guidance to a range of $180 billion to $190 billion, up from $175 billion to $185 billion. Chief financial officer Anat Ashkenazi also said the company expects its 2027 capital expenditures to "significantly increase" from there.

And Alphabet spent $35.7 billion on capital expenditures during Q1 specifically. So, even to reach even the low end of its full-year range, spending would need to average about $48 billion per quarter for the rest of the year -- a step-up of more than 30% from the first quarter's pace.

To be fair, the tech company's growth has been impressive. Alphabet's first-quarter revenue rose 22% year over year to $109.9 billion, the company's 11th consecutive quarter of double-digit growth. Google Cloud revenue climbed 63% year over year to $20 billion -- an acceleration that made the segment the company's most powerful growth catalyst. And Alphabet notably said its cloud backlog swelled to more than $460 billion.

Further, Alphabet remains compute-constrained.

"We are compute constrained in the near term," CEO Sundar Pichai said in the company's first-quarter earnings call. "Our cloud revenue would have been higher if we were able to meet the demand."

What would be reassuring -- and what wouldn't

As for the earnings line, it has gotten noisy recently. Alphabet's first-quarter net income rose 81% year over year, but a $36.9 billion pre-tax gain on equity securities added $28.7 billion to the bottom line, a swing factor that has nothing to do with the operating business. That's exactly why profit is a poor scoreboard for this report, and the capital expenditure line is a better one.

So what would a reassuring report look like?

Capital expenditure guidance held at $180 billion to $190 billion, cloud growth still running near 60%, and clear evidence that the more than $460 billion cloud backlog is converting into revenue. That combination would say the spending is buying growth at a steady exchange rate.

The worrying version is the opposite. Another guidance raise stacked on top of April's, paired with decelerating cloud growth, would suggest the price of keeping up in AI is rising faster than the payoff. Investors could probably forgive either one on its own. Both together, however, could hit the stock hard.

Valuation frames the stakes. At about $347 per share, Alphabet trades at about 27 times earnings -- closer to 32 times without the first quarter's equity gains, but hardly extreme either way for a company growing revenue 22%. Shares also sit about 15% below their 52-week high of $408.61, so some caution is already priced in.

But a multiple like that still assumes Alphabet's strong growth persists as its investments pay off.

Alphabet has earned patience from investors on this front before. Google Cloud spent years absorbing investment before it became the profit driver it is now, and the company's balance sheet gives it more room for error than almost any business on Earth. The bull case, therefore, is simply that history repeats: spend heavily, wait, collect a bigger business on the other side.

Ultimately, the report lands this afternoon, and the reaction will come fast. When it does, I'll go straight past revenue and earnings to the capital expenditure line -- and I think investors should, too. If Alphabet holds the range while cloud keeps compounding, the stock's premium valuation looks earned. But if the spending number jumps again without growth to match, investors may have some cause for concern.

Daniel Sparks and his clients do not have positions in any of the stocks mentioned. The Motley Fool has positions in and recommends Alphabet. The Motley Fool has a disclosure policy.