It’s time for another one of my semi-regular FTSE 100 valuation and forecast updates, using the latest data for both the index’s price and its earnings.

Valuing the FTSE 100 using CAPE

My tool of choice in the valuation game is Robert Shiller’s CAPE (Cyclically Adjusted PE). CAPE is basically the same as the PE ratio that most investors are familiar with, except that is uses 10-year inflation adjusted earnings as the “E” part of the ratio.

It does that because the 10-year earnings average is a much more stable number from one year to the next than just a single year’s earnings. That’s important because we want something nice and stable to compare the FTSE 100’s price to.

In other words:

CAPE is like measuring the distance between two points on the ceiling while stood on a solid table, whereas the standard PE is like doing the same measurement stood at the top of a wobbly step-ladder.

You’ll get an accurate measurement with one and a dangerously inaccurate one with the other (dangerous if you fall off the ladder anyway).

Calculating “fair value” for the FTSE 100

Today, the FTSE 100’s 10-year inflation adjusted earnings stand at 530 index points. I’ve put the value into index points because that’s how the index is measured.

So with a current price of 6,700 index points, the FTSE 100’s CAPE is 12.6.

That’s actually pretty low. The long-term average, across multiple international markets, is somewhere in the mid-teens, around 15 or 16.

If the market were at “fair value” today, i.e. with a CAPE ratio of 16, it would be at 8,500. At that level it would have a dividend yield of 2.8%.

That sounds a little high (i.e. low yield) to me, but not significantly so as the market’s long-run average dividend yield is about 3% (we’re at 3.5% at the moment).

I could just stop there and say that the FTSE 100 is slightly below the typical long-run average of 16, and therefore it’s “slightly cheap”.

But that’s a bit dull, so here are a couple of visual tools to illustrate what the market’s valuation means for future returns.

CAPE is “mean reverting”, and that’s a good thing

First though I’ll take a little detour into the world of “mean reversion”, as that underpins everything else (skip this bit if you already know all this).

The FTSE 100’s CAPE is a mean reverting statistic, which simply means that the further it is from…

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