Price-to-earnings: which earnings are in the ratio?

A price-to-earnings ratio relates a quoted share price to one earnings-per-share figure. Before interpreting the multiple, identify the earnings period and whether the figure reports a result or estimates one.

The SEC's P/E definition divides price per share by earnings per share, using the past 12 months for its earnings explanation.

Fix the share and the period

An invented company has one million identical common shares at the price-observation time and throughout the preceding 12 months. There are no preferred shares, options, convertible securities or share-count changes in this model. Reported net earnings available to those common shares for those 12 months, after all company expenses and taxes, are $2 million.

On this supplied basis, earnings per share (EPS) are $2 million divided by one million: $2. At that observation time, a share is quoted at $30. Dividing $30 by $2 gives a trailing P/E of 15.

The price is fifteen times that period's EPS. That earnings record does not promise cash distributions or a fifteen-year recovery of the purchase price. The profit-and-cash reference follows that separate distinction.

Change the estimate, not the price

Now supply a hypothetical estimate of $3 million in earnings available to the same one million shares over the next 12 months, with the same share conditions. Estimated EPS is $3. Use the same $30 observation price. Dividing by that estimate gives a forward P/E of 10.

Nasdaq's glossary distinguishes past-12-month earnings from prospective next-12-month estimates. A report using a different forecast period would need its own label.

Both 15 and 10 can describe this same share price. The smaller number has not come from a price reduction. It depends on the higher earnings estimate becoming the denominator, without establishing that the company will earn it.

When the denominator gets small

For separate sensitivity checks, hold the illustrative price at $30. EPS of $0.02 gives a P/E of 1,500; EPS of $0.01 gives 3,000. The price has not doubled between those calculations. The positive denominator has halved.

At zero EPS, division is undefined. At negative $2 EPS, the arithmetic gives minus 15, but the denominator now describes a loss. Sorting that number below a positive P/E would not establish a bargain.

Recover the comparison

Try expanding a bare label such as P/E 10 into a sentence: a $30 price at this observation time, divided by $3 of estimated EPS for the next 12 months. Replacing estimated with reported would change the claim, not merely its wording.

Before comparing two multiples, check the price dates, earnings periods and earnings bases. FINRA's stock-evaluation guide puts valuation ratios alongside the business, its debt, industry and risks. A multiple alone does not supply those facts or determine which future earnings will occur.

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