Each spell that is remembered (as opposed to carried around in a magic scroll or hag stone) takes up a certain amount of this space, proportional to the thaums added to the room when casting. This amount might also be related to the difficulty of the spell in question.
Amount of head space
A player's head space is determined as their magic.spells.special bonus plus 30.
Amount used by spells
The amount of head space taken up by a spell is five times the number of thaums the spell adds to the room when cast. This amount is the same even if the spell is stopped before the end of casting or if casting fails.
Be advised that the dynamic enchantment of a room returns to normal (0 thaum) over time and can spoil measures if this happens between thaumometer readings. Being in a room with no dynamic enchantment can reduce this concern, although it can still happen while casting the spell.
It is thus recommended to verify by measuring several times and/or fill your head with spells (until less than 5 units left) and see if it all fits with your head space of magic.spells.special + 30. Then, you can further confirm by forgetting the (smallest) 5 units spell and try to fill it with others of the same size, then two of them with the 10 size spells, etc. This is easier for witches that can learn all the spells from Granny Weatherwax than for wizards who have to go to the books or remember from scrolls.
Exceeding head space
If a wizard has insufficient remaining head space for a spell, then:
- They will be unable to remember that spell from the appropriate tome.
- They will be unable to remember that spell from a magic scroll.
- Granny Weatherwax and Aunty Ogg-San will refuse to teach it.
- She will be unable to educe that spell or trick from a hag stone.
- She will be unable to dub the current squint being researched.
Note that the checks on exceeding head space only apply at the point of attempting to learn a new spell or trick or dubbing a squint; spells, tricks or dubbed squints will not then subsequently leave the magic user's head if their head space is later reduced (by way of stat changes affecting the magic.spells.special bonus).