What HHI really measures
HHI measures market concentration by summing squared market shares. The formula looks simple, but it is powerful because it captures both the number of players and the gap between their market positions in one single indicator.
Two markets can contain the same number of retailers and still have very different competitive structures. HHI helps surface that structural difference, as long as it is read in the right local context.
Why delta HHI matters as much as the final level
In a transaction, the final HHI level is not enough. What also matters is the structural effect of the deal itself. Delta HHI captures that effect and helps distinguish cases where the transaction adds little concentration from those where it materially changes the competitive balance.
That combined reading avoids common mistakes. An already concentrated market does not raise the same concerns when the deal has a marginal impact as when it significantly increases concentration.
- HHI after for the market snapshot
- delta HHI for the incremental effect of the deal
- combined reading for a more realistic risk view
Reading safe harbor thresholds correctly
Safe harbor thresholds are useful because they provide a first quick screen. But they never replace economic reasoning. They are designed to flag likely low-risk cases, not to mechanically conclude every merger assessment.
The sound approach is to read them alongside local market structure, geographic proximity, format substitutability and the quality of the chosen perimeter. Without that, a threshold can easily be over-read or local risk can be missed entirely.
Making HHI actionable
To be useful, HHI must be readable. A good interface or report should show before, after, delta, contributive shares and a concise interpretation of the resulting risk level.
That combination of mathematical precision and operational clarity is what saves time for M&A teams, external counsel and in-house legal teams. The best calculation is one that can be reviewed, discussed and reused quickly.