A commonly encountered phrase while renting a flat or room is "PCM," which means "per calendar month" and refers to the amount of money the property's landlord wants as a rent payment each month.
In most British town and cities, weekly rentals make up a section of the residential properties available for rent, particularly at the lower end of the market. A flat or bedsit let on a weekly basis is denoted as "PW" (per week) as opposed to "PCM" or "per calendar month."
PCM is a more useful rate most of the time, because there are 52 weeks a year, but only 12 months. If each month were actually four weeks, then the PCM and PW would balance out at 48 weeks each. However, only February is exactly four weeks long, so PCM gives a more realistic idea of what you will actually pay each month.
PW is better than PCM when leases are short-term, and especially if the tenant may have to vacate the property at any time. When a tenant might have to leave at any time, a PW arrangement can be made so the lease can be cancelled at short notice. This makes the arrangement ideal for people who travel a lot for work, military personnel or tourists. However, if the lease really does need to be flexible, rather than simply short-term, it is up to the tenant to make sure it includes those terms.
PCM rent isn't a familiar concept in many other countries, but that does not mean it is completely unheard of. A general rule of thumb is that if you see PCM on "To Let" boards and adverts, it is because the local rental market is dramatically overheated and landlords can offer PCM rentals to get a competitive advantage.
PCM is also often used to describe other monetary transactions. For example, a person who is paid a monthly, as opposed to bi-weekly or weekly salary, may say "I am paid £1,200 PCM."