Overview
The Price-to-Earnings Ratio (PER or P/E) is one of the most widely used equity valuation metrics, calculated as: Stock Price รท Earnings Per Share (EPS). It indicates how many years of current earnings investors are willing to pay for one share.
Types of P/E
- Trailing P/E (TTM): Based on the past 12 months of actual earnings. Most commonly reported by financial data providers
- Forward P/E: Based on analyst consensus estimates for next 12 months' earnings. Useful for growth companies
- Shiller P/E (CAPE): Cyclically Adjusted P/E uses 10 years of inflation-adjusted earnings, developed by Nobel laureate Robert Shiller. Reduces distortion from business cycles
Historical S&P 500 P/E Context
- Long-term average: ~15โ17x (since 1871, per Shiller data)
- Dot-com peak (March 2000): Shiller CAPE reached 44.2x โ the highest ever, preceding a 49% crash
- GFC bottom (March 2009): Forward P/E fell to ~10x
- COVID era (2021): S&P 500 forward P/E reached ~23x amid zero rates and stimulus
- As of 2024: S&P 500 trades at ~21x forward P/E, with technology sector at ~28x
Interpreting P/E
- High P/E (>25x): May indicate overvaluation OR high growth expectations (tech, biotech)
- Low P/E (<10x): May indicate undervaluation OR declining earnings/sector distress
- Negative P/E: Not meaningful โ company is losing money. Use Price-to-Sales (P/S) instead
Limitations
- Earnings can be manipulated through accounting choices (depreciation methods, one-time items)
- Not comparable across industries: utilities (14โ16x) vs. tech (25โ35x) have structurally different P/Es
- P/E collapses in recessions as earnings fall faster than prices โ the 'P/E trap'
- Does not account for debt levels (use EV/EBITDA for leveraged companies)
PEG Ratio: P/E รท Earnings Growth Rate. A PEG of 1.0 suggests fair valuation. Popularized by Peter Lynch in 'One Up on Wall Street' (1989).