Does the MongoDB stats() function return bits or bytes?

Bytes of course. Unless you pass in a scale as optional argument.

Running the collStats command - db.collection.stats() - returns all sizes in bytes, e.g.

> db.foo.stats()
{
    "size" : 715578011834,  // total size (bytes)
    "avgObjSize" : 2862,    // average size (bytes)
}

However, if you want the results in another unit then you can also pass in a scale argument.

For example, to get the results in KB:

> db.foo.stats(1024)
{
    "size" : 698806652,  // total size (KB)
    "avgObjSize" : 2,    // average size (KB)
}

Or for MB:

> db.foo.stats(1024 * 1024)
{
    "size" : 682428,    // total size (MB)
    "avgObjSize" : 0,   // average size (MB)
}

Related