7.7.3. Criteria for key validity

The trust level is used to calculate the validity of a key. Being the trust level only locally significant, as a consequence the calculated validity of a key is only relative to your local installation of OpenPGP. That is, a key you consider valid might be considered invalid by someone else.

A key is considered valid if it has been signed by a fully trusted key, or by at least 3 marginally trusted keys. In addition to that, the path leading from that key back from your own key must be 5 steps or shorter.
Note that, as a consequence, keys signed personally by your own key are always considered valid.

Ultimate trust bypasses any restrictions on how many fully trusted signatures are needed to make a key valid: any key signed by an ultimately trusted key is always valid. You can set ultimate trust to a particular key you want to allow to make trust decisions for you.

You can see the calculated validity and trust value in the Key Management or Key Properties window, in the fields Key validity and Owner trust.