A multiple compresses assumptions about earnings, growth and risk. It does not replace them.
Calculate the same ratio
Price-to-earnings is share price divided by earnings per share. A hypothetical £100 share with £5 of annual earnings per share trades at 20 times those earnings. Record the price date and the earnings period.
Trailing earnings use a past period; forward earnings use an estimate. Keep those labels separate, and distinguish reported figures from adjusted figures. A zero or negative earnings denominator cannot be interpreted as an ordinary positive P/E.
Interrogate apparently cheap earnings
A one-off gain or a cyclical peak can make last year’s earnings unusually high and the P/E unusually low. A high multiple can reflect growth expectations, but those expectations may fail.
Compare businesses with attention to margins, debt, cash generation, accounting and the durability of demand. A peer group is a starting point for questions, not an automatic fair-value calculator.
Keep enterprise and equity measures separate
The share price measures equity. Enterprise-value measures usually also account for debt and cash, with further adjustments that vary by definition. Do not mix an enterprise numerator with an equity-only earnings measure without understanding the calculation.
Write a range of assumptions and the evidence that would challenge them. Neither a low ratio nor a precise spreadsheet output is an instruction to buy.
Change one assumption
Using the hypothetical £100 share, compare £5 reported EPS with £2 EPS after removing a clearly identified one-off gain: 20× versus 50×. This arithmetic is not a forecast. Record why the adjustment is justified and what evidence would make it wrong.
Terms in this guide
Work through the evidence: Microsoft: revenue growth and cash after investment →
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Educational notes, not personalised financial advice. Hypothetical examples and chart shapes are not performance claims or trading signals.