How to calculate fragmentation?

Imagine you have some memory containing a bunch of bytes:

++++ ++-- ---+ +++-
-++- ++++ ++++ ----
---- ++++ +

Let us say + means allocated and - means free.

I'm searching for the formula of how to calculate the percentage of fragmentation.

Background

I'm implementing a tiny dynamic memory management for an embedded device with static memory. My goal is to have something I can use for storing small amounts of data. Mostly incoming packets over a wireless connection, at about 128 Bytes each.