How does the PEG ratio fold growth into the P/E?

The single biggest reason two companies deserve different P/E ratios is different growth. The PEG ratio is a quick attempt to bake that growth right into the multiple, so you compare fast and slow growers on a fairer footing.

The formula and the idea

PEG = P/E ÷ annual earnings growth rate (in %)

Take the P/E and divide it by how fast earnings are growing. A company with a P/E of 30 growing earnings at 30% a year has a PEG of 30 ÷ 30 = 1.0. A company with a lower P/E of 15 but growing at only 5% has a PEG of 15 ÷ 5 = 3.0.

Notice what just happened. On raw P/E, the second company looked half as expensive (15 versus 30). On PEG, once growth is accounted for, it looks three times as expensive. That inversion is the entire point of PEG: a high P/E paired with high growth can be more reasonable than a low P/E paired with no growth.

The rough rule of thumb

The traditional read is that a PEG near 1.0 means the P/E is roughly "in line" with the growth rate, above 1 means you're paying up relative to growth, below 1 means the price is modest relative to growth. Treat these as loose observations, not laws — the number 1.0 has no magic in it; it's a convention.

Handle with real care

PEG is a rule of thumb, and a fragile one. Its problems:

• It depends entirely on the growth figure, which is a forecast — change the growth estimate and PEG swings wildly. Trailing growth, forward growth, one-year, five-year — they give different PEGs for the same company.
• It breaks at the extremes. Near-zero growth makes PEG explode toward infinity; negative growth makes it meaningless.
• It treats all growth as equally valuable, ignoring how risky, sustainable, or profitable that growth is. Fast growth that burns cash is not worth the same as fast growth that funds itself.

So PEG is a useful first-glance screen for whether a rich P/E might be justified by growth — and a poor final answer. Use it to raise questions, never to close them.

In the data

PEG comes published, right next to the P/E it was built from:

It is the clearest case of a published ratio that hides its own definition: nothing beside it says which growth rate, over which horizon, sits in the denominator. Divide the P/E by the PEG and the growth figure you back out matches none of the analyst estimates published alongside it. Where earnings are negative the PEG is simply left blank rather than reported as a meaningless number.

Try it now

1. Read the P/E and the PEG in the table above. Divide the first by the second. The number you get is the growth rate the published PEG used.
2. Go looking for that growth rate among the published estimates and fail to find it:

Your backed-out figure is a growth rate, so compare it against the growth rates the table implies, not against the EPS levels themselves: trailing EPS to this year's estimate, trailing EPS to next year's, one estimate year to the next, and the published quarterly earnings growth. Nothing matches, and nothing on either table says what should have. Write down how far off the nearest candidate is — that distance is what "published ratio" is worth when the definition is missing. 3. So build your own instead, twice. Take the P/E ÷ the implied growth from trailing EPS to next year's estimate, then the P/E ÷ the quarterly earnings growth. Two defensible PEGs for one company on one day — that instability is why PEG is a hint, not a verdict. 4. Do the same for a low-P/E, low-growth company:

Compare the two PEG figures and the two P/E figures, and say whether the ranking flips between them. Then a loss-maker:

Note what its PEG row shows instead of a number, and what a screen sorting on it would do with the company.