The trailing price-to-earnings ratio (P/E) divides a stock's price by the earnings per share (EPS) the company actually reported over the last 12 months; the forward P/E divides it by analysts' estimate of EPS for the next 12 months or the current fiscal year. Trailing P/E is based on facts but looks backward; forward P/E looks ahead but depends on forecasts. On October 9, 2026, the S&P 500 traded at 26.0 times trailing earnings and 19.1 times forward earnings, a gap that shows how much growth analysts expect [1]. The two numbers disagree most when earnings are about to change sharply, or when the last year included one-time gains.
What are trailing and forward P/E?
Both ratios use the same price and differ only in the earnings figure:
- Trailing P/E (also called TTM, for trailing twelve months) = share price divided by the sum of EPS over the last four reported quarters. Most data sites use EPS under generally accepted accounting principles (GAAP), which includes one-time items.
- Forward P/E = share price divided by the consensus EPS estimate for a future period. The consensus is the average of the published estimates of analysts who cover the stock. Some sources use the next 12 months, others the current or next fiscal year, so the period should always be checked.
Analysts usually estimate "adjusted" EPS, which excludes items such as investment gains or restructuring costs. A trailing P/E on GAAP earnings and a forward P/E on adjusted estimates are therefore not measured on exactly the same basis, which explains part of the gap for some companies.
Why is the forward P/E usually lower?
When analysts expect earnings to grow, the denominator of the forward P/E is larger, so the ratio is smaller. FactSet put the S&P 500's forward 12-month P/E at 19.1 on October 9, 2026, below its five-year average of 19.8 and equal to its ten-year average of 19.1 [1]. The trailing 12-month P/E was 26.0, above the five-year average of 24.4 and the ten-year average of 23.6 [1]. By that reading, the index looked expensive on past earnings and ordinary on expected ones, because expected earnings growth was high: FactSet's estimated third-quarter 2026 growth rate was 29.6% [1].
The forward multiple also moves when estimates change, not just prices. Between June 30 and October 9, 2026, the S&P 500 rose 3.5% while the forward 12-month EPS estimate rose 10.4%, so the forward P/E fell from 20.4 to 19.1 [1]. Sector gaps are wide: consumer discretionary traded at 23.2 times forward earnings and energy at 13.5 [1].
Five large companies, two ratios
| Company | Price, Oct 9, 2026 | Trailing EPS (12 months) | Trailing P/E | Consensus EPS, current fiscal year | Forward P/E |
|---|---|---|---|---|---|
| Microsoft (MSFT) | $535.07 | $17.95 | 29.8 | $19.77 (year to June 2027) | 27.1 |
| Nvidia (NVDA) | $229.28 | $7.91 | 29.0 | $9.31 (year to January 2027) | 24.6 |
| Alphabet (GOOGL) | $347.86 | $19.93 | 17.5 | $12.11 (2026) | 28.7 |
| Micron Technology (MU) | $1,029.00 | $73.84 | 13.9 | $176.15 (year to September 2027) | 5.8 |
| Exxon Mobil (XOM) | $168.94 | $7.77 | 21.7 | $12.46 (2026) | 13.6 |
Source: DataPorium stock pages, prices and consensus estimates as of October 9, 2026; ratios are DataPorium calculations [2][3][4][5][6].
Microsoft and Nvidia show the normal pattern: expected growth makes the forward P/E a few points lower than the trailing one. The other three show what happens when the two ratios tell different stories.
When does the trailing P/E mislead?
One-time gains inflate trailing earnings
Alphabet's trailing P/E of 17.5 is lower than its forward P/E of 28.7, the opposite of the normal pattern [4]. The reason is in its income statement: second-quarter 2026 results included gains on equity securities that lifted EPS to $9.11 for that quarter alone [7]. Those gains count in trailing GAAP earnings, but analysts do not forecast them, so the trailing P/E makes the stock look cheaper than its recurring business implies.
Microsoft's trailing EPS of $17.95 also included gains, smaller ones. Its fiscal 2026 10-K reconciles reported diluted EPS of $17.95 to adjusted EPS of $17.28 after removing net gains from its investments in OpenAI [8]. On adjusted EPS, its trailing P/E would be 31.0 rather than 29.8 [2][8].
Turning points make the past a poor guide
A trailing P/E assumes the last year is representative. For a company whose earnings are rising fast, it overstates the price of future earnings; for one whose earnings are about to fall, it understates it.
When does the forward P/E mislead?
Forward P/E is only as good as the estimate behind it, and estimates are most wrong at turning points.
Cyclical companies at the top of the cycle
Micron sells memory chips, a business whose prices swing with supply and demand. Its trailing EPS was $73.84, and analysts expected $176.15 for the fiscal year to September 2027, which puts the stock at 5.8 times forward earnings [5]. A very low forward P/E for a cyclical company can mean the market expects peak earnings that will not last: investors pay a low multiple because they doubt the high earnings will be repeated. For cyclicals, analysts often compare price with average earnings over a full cycle instead.
Commodity producers and the price assumption
Exxon Mobil's forward P/E of 13.6 against a trailing 21.7 reflects estimates that assume oil stays high: Brent crude closed at $104.72 on October 9, 2026 [6][10]. If oil prices fall, the estimates fall with them and the forward P/E rises. Energy had the lowest forward P/E of any S&P 500 sector, at 13.5 [1].
Estimates drift
Analysts' estimates usually fall as a quarter goes on, so a forward P/E taken early can be too low. FactSet reports that the index-level EPS estimate has fallen by 2.2% on average during a quarter over the past five years, although it rose in the third quarter of 2026 [1].
Trailing P/E tells you what you pay for earnings already earned; forward P/E tells you what you pay for earnings analysts expect. The gap between them is the forecast.
How to use both ratios
- Read the gap. Forward much lower than trailing means analysts expect strong growth; check whether that growth is plausible.
- Check the earnings basis. Look for one-time gains or losses in trailing GAAP EPS, as with Alphabet's and Microsoft's investment gains [7][8].
- Check the period. Know whether the forward figure covers the next 12 months, the current fiscal year or the next one; fiscal years differ (Microsoft's ends in June, Nvidia's in January) [2][3].
- Be wary of very low forward P/Es in cyclical industries such as memory chips, energy and materials.
- Compare like with like: a company against its own history and its sector, not against the whole market. Growth can be added with the PEG ratio, explained in DataPorium's P/E vs PEG guide [9].
Frequently asked questions
Is forward P/E or trailing P/E better?
Neither alone. Trailing P/E uses reported earnings but looks back; forward P/E looks ahead but relies on estimates. For the S&P 500 on October 9, 2026, they were 26.0 and 19.1 [1].
Why is forward P/E higher than trailing P/E?
Because analysts expect earnings to fall, or because trailing earnings included one-time gains. Alphabet's trailing P/E was 17.5 and its forward P/E 28.7 in October 2026, after large investment gains in 2026 [4][7].
What is a good forward P/E ratio?
It depends on growth and sector. The S&P 500's ten-year average forward P/E is 19.1; in October 2026 energy traded at 13.5 and consumer discretionary at 23.2 [1].
What is the S&P 500 forward P/E in 2026?
19.1 as of October 9, 2026, according to FactSet, below the five-year average of 19.8 and down from 20.4 on June 30 [1].
Sources & References
- [1] FactSet, Earnings Insight (October 9, 2026)
- [2] DataPorium Microsoft (MSFT) stock page (price, trailing EPS and consensus estimates)
- [3] DataPorium Nvidia (NVDA) stock page (price, trailing EPS and consensus estimates)
- [4] DataPorium Alphabet (GOOGL) stock page (price, trailing EPS and consensus estimates)
- [5] DataPorium Micron Technology (MU) stock page (price, trailing EPS and consensus estimates)
- [6] DataPorium Exxon Mobil (XOM) stock page (price, trailing EPS and consensus estimates)
- [7] Alphabet Inc., Form 10-Q for the quarter ended June 30, 2026
- [8] Microsoft Corporation, Form 10-K for the fiscal year ended June 30, 2026
- [9] DataPorium Articles, P/E Ratio vs PEG Ratio
- [10] DataPorium Commodities (Brent crude oil price, October 9, 2026)
