Numbers can lie -- yet they're the best first step in determining whether a stock is a buy. In this series, we use some carefully chosen metrics to size up a stock's true value based on the following clues:

• The current price multiples.
• The consistency of past earnings and cash flow.
• The amount of growth we can expect.

Let's see what those numbers can tell us about how expensive or cheap Amazon.com (Nasdaq: AMZN) might be.

The current price multiples
First, we'll look at most investors' favorite metric: the price-to-earnings ratio. It divides the company's share price by its earnings per share (EPS). The lower the P/E, the better.

Then we'll take things up a notch with a more advanced metric: enterprise value to unlevered free cash flow. This tool divides the company's enterprise value (basically, its market cap plus its debt, minus its cash) by its unlevered free cash flow (its free cash flow, adding back the interest payments on its debt). As with the P/E, the lower this number is, the better.

Analysts argue about which is more important -- earnings or cash flow. Who cares? A good buy ideally has low multiples on both.

Amazon has a P/E ratio of 84.0 and an EV/FCF ratio of 43.0 over the trailing 12 months. If we stretch and compare current valuations with the five-year averages for earnings and free cash flow, we see that Amazon has a P/E ratio of 125.9 and a five-year EV/FCF ratio of 57.4.

A positive one-year ratio of less than 10 for both metrics is ideal. For a five-year metric, less than 20 is ideal.

Amazon is 0-for-4 on hitting the ideal targets, but let's see how it stacks up against some of its competitors and industry mates.

Company

1-Year P/E

1-Year EV/FCF

5-Year P/E

5-Year EV/FCF

Amazon.com 84.0 43.0 125.9 57.4
eBay (Nasdaq: EBAY) 23.8 17.2 29.1 18.5
Wal-Mart (NYSE: WMT) 11.7 19.3 14.1 22.2
Staples (Nasdaq: SPLS) 17.2 13.9 17.2 13.4

Source: Capital IQ, a division of Standard & Poor's; NM = not meaningful.

Numerically, we've seen how Amazon's valuation rates on both an absolute and relative basis. Next, let's examine …

The consistency of past earnings and cash flow
An ideal company will be consistently strong in its earnings and cash-flow generation.

In the past five years, Amazon's net income margin has ranged from 2.2% to 3.8%. In that same time frame, unlevered free cash flow margin has ranged from 5.0% to 8.7%.

How do those figures compare with those of the company's peers? See for yourself:

Source: Capital IQ, a division of Standard & Poor's; margin ranges are combined.

In addition, over the past five years, Amazon has tallied up five years of positive earnings and five years of positive free cash flow.

Next, let's figure out …

How much growth we can expect
Analysts tend to comically overstate their five-year growth estimates. If you accept them at face value, you will overpay for stocks. But even though you should definitely take the analysts' prognostications with a grain of salt, they can still provide a useful starting point when compared with similar numbers from a company's closest rivals.

Let's start by seeing what this company's done over the past five years. In that time period, Amazon has put up past EPS growth rates of 24.3%. Meanwhile, Wall Street's analysts expect future growth rates of 28.0%.

Here's how Amazon compares with its peers for trailing five-year growth:

Source: Capital IQ, a division of Standard & Poor's; EPS growth shown.

And here's how it measures up with regard to the growth analysts expect over the next five years:

Source: Capital IQ, a division of Standard & Poor's; estimates for EPS growth.

The bottom line
The pile of numbers we've plowed through has shown us the price multiples that shares of Amazon are trading at, the volatility of its operational performance, and what kind of growth profile it has -- both on an absolute and a relative basis.

The more consistent a company's performance has been and the more growth we can expect, the more we should be willing to pay. We've gone well beyond looking at an 84.0 P/E ratio, and we see a growth stock at a growth stock's price. Continuing innovation will be needed if these prices are to be proved "cheap" for Amazon. Check out what my fellow Fool Dan Caplinger thinks of Amazon's innovation ability.

If you find Amazon.com's numbers or story compelling, don't stop here. Continue your due-diligence process until you're confident one way or the other. As a start, add it to My Watchlist to find all of our Foolish analysis.

If you want some more stock ideas, check out my recent article: "The Greatest Companies of 2020."

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.