How much is home field worth? (NFL point spreads)

For years, I've heard that home field is worth three points. Occasionally you'll hear 3.5 points. But compared to what?

I wanted to know is home field worth 3 points relative to a neutral field, or relative to an away game?

To me, it makes sense to calculate this relative to a neutral field.

Covers.com just released points spreads for all games weeks 1-16, so I figured we could figure it out.

I wanted to start by looking at some teams not known for a HFA.

Rams
at home vs Cardinals: Rams by 4.5
on road vs Cardinals: pick 'em

at home vs 49ers: 49ers by 4
on road vs 49ers: 49ers by 7.5

Rams average HFA = 2 points (relative to neutral)

Bengals
at home vs Steelers: Bengals by 1
on road vs Steelers: Steelers by 2.5

at home vs Browns: Bengals by 6
on road vs Browns: Bengals by 3

Bengals average HFA = 1.625 points

Texans
at home vs Colts: Texans by 4.5
on road vs Colts: pick'em

at home vs Jags: Texans by 10
on road vs Jags: Texans 4.5

Texans average HFA: 2.5 points

Titans

at home vs Colts: Colts by 1
on road vs Colts: Colts by 4.5

at home vs Jags: Titans by 3.5
on road vs Jags: pick'em

Titans average HFA: 1.75 points


Okay, so far it seems like the fabled HFA is worth 3-3.5 really means relative to an away game. Let's take a look at teams that might have a perceived higher HFA.

Broncos
at home vs Chiefs: Broncos by 7.5
on road vs Chiefs: Broncos by 4

at home vs Chargers: Broncos by 6.5
on road vs Chargers: Broncos by 3

Broncos average HFA: 1.75 points

Packers
at home vs Lions: Packers by 4.5
on road vs Lions: Packers by 1.5

at home vs Vikings: Packers by 4.5
on road vs Vikings: Packers by 1

Packers average HFA = 1.625 points

Seahawks

at home vs Cardinals: Seahawks by 8.5
on road vs Cardinals: Seahawks by 6

at home vs 49ers: Seahawks by 2.5
on road vs 49ers: 49ers by 2.5


Seahawks average HFA = 1.825 points

Chiefs
Broncos = 3.5 point swing
Raiders = 4.5 point swing

Chiefs average HFA = 2

btw, Lions average HFA = 1.75

Conclusion

For these 9 teams, the average HFA in the NFL is 1.87. Double that and you'll get 3.74 which is near the 3.5 points that is thrown around.

But I think it's important to use the number relative to a neutral field, because you can't say that (versus the Raiders for example) that the Chiefs home field advantage is 4.5 points. Because then you're ignoring that the Raiders would have a similar home field advantage.

A quick google search turned up this quote: "So if the odds-makers believe that 2 teams are evenly matched, the home team will almost always be a 3 point favorite." 

I think that sums up conventional wisdom. But that would be a six-point swing, and in the 36 matchups I looked at, there weren't any that high.

Does this really help? I think so.

Let's take a look at week 3: Chiefs at Eagles. Eagles are favored by 3. The Eagles don't play at KC this year. But based on my numbers it seems that, contrary to conventional wisdom, Vegas thinks the Eagles are a bit better than the Chiefs. I would expect a line in KC to be around Chiefs by 1, and a line at a neutral field to be around Eagles by 1.

Does knowing that really help? Perhaps not. But it's something.