"The Extension Charm is advanced, but subject to strict control, because of its potential misuse."
—Description of the charm[src]

The Undetectable Extension Charm, or simply Extension Charm (Capacious extremis[1]) is a charm that extends the internal dimensions of the target object without affecting the external dimensions. This charm is advanced magic and heavily controlled by the Ministry.[1]


"Undetectable Extension Charm. Tricky, but I think I've done it okay; anyway, I managed to fit everything we need in here."
Hermione Granger explaining her use of the charm[src]

Undetectable Extension Charms are advanced magic and subject to strict Ministry of Magic control. This is because of its potential to create breaches of the International Statute of Wizarding Secrecy. There is a law in place that Extension Charms are not to be cast for private use, but only for the production of objects. These objects have been individually approved for manufacture by the relevant Ministry Department.[1] Hogwarts school trunks (like the majority of wizarding luggage) are issued with these charms.[1] Family tents are also infused with this charm, in order to make them more inhabitable. 

Extension charm

Newt Scamander placed an Undetectable Extension Charm on his case

Newt Scamander used the Extension Charm on a case during the 1920s. He used the enlarged space to house the many magical creatures he collected during his trip around the world while gathering information for his upcoming book Fantastic Beasts and Where to Find Them.

The Extension Charm can be used on cars. Arthur Weasley used this spell, among others, on his Flying Ford Anglia. It was able to fit all of the Weasleys living at the Burrow at that time, Harry Potter, and all of their luggage comfortably. Molly did not recognise it for what it was, stating "Muggles do know more than we give them credit for, don't they?" Mundungus Fletcher cast this spell on an automobile he stole on Christmas of 1995 to transport the Weasleys and company to St Mungo's Hospital for Magical Maladies and Injuries. It managed to fit all of the above minus Arthur and Percy, as well as William Weasley, Hermione Granger, Alastor Moody, Remus Lupin, and Mundungus himself.[2]

Hermione Granger used this charm on her small beaded handbag in 1997 so that she was able to pack all the supplies she believed she would need while hunting down Lord Voldemort's Horcruxes with Harry Potter and Ron Weasley. This included books, changes of clothes, money, Harry's Cloak of Invisibility, and healing supplies.[3] Eventually, Hermione also placed the portrait of Phineas Nigellus Black that had previously been in 12 Grimmauld Place in her bag as well[4] to prevent Severus Snape from spying on them.


This sophisticated charm has two main effects: to expand an object's internal space while retaining its external dimensions. The charm also magically disguises itself so that no one but the caster (and those the caster confides about the charm in) can realise that this spell has been cast over the object, even if they are experiencing it.

Theoretically, a hundred wizards could take up residence in a toilet cubicle if they were sufficiently adept at these spells.[1]

Known practitioners


Capacious is an English word meaning "having a lot of space"[5], and can alternatively be thought to be a portmanteau of the words capacity and spacious. Extremis comes from a conjugation of the Latin extrēmus, meaning "extreme".

Behind the scenes

  • The effects of this spell are similar to the effects of the TARDIS in Doctor Who.
  • The effects of the spell are also very similar to Mary Poppins's carpet bag.
  • This charm appears to have a number of interesting side-effects when cast over an object. For example, when Hermione drops the bag she has cast this charm over in Harry Potter and the Deathly Hallows it makes a suspiciously loud thump (making it seem heavy), yet she carries the bag with ease (despite the number of objects inside).
  • In the American versions of Harry Potter and the Order of the Phoenix, the spell used on cars to increase their capacity is identified as the Enlarging Spell, though the spell goes unidentified in the original British text. Pottermore, however, clarifies that the spell used on the Weasley's Ford Anglia was the Extension Charm, thus making this embellishment non-canon.


Notes and references

  1. 1.0 1.1 1.2 1.3 1.4 1.5 Writing by J.K. Rowling: "Extension Charms" at Pottermore
  2. Harry Potter and the Order of the Phoenix, Chapter 23 (Christmas on the Closed Ward)
  3. Harry Potter and the Deathly Hallows, Chapter 9 (A Place to Hide)
  4. Harry Potter and the Deathly Hallows, Chapter 12 (Magic is Might)
  5. Merriam Webster Dictionary - capacious