If the landlord (or landlord's agents) serve a Section 21 notice on the tenant that forces them to leave the property, the tenant must be repaid any rent paid for the rental period that they were not at the property. The formula used is R x D/P where:
Scenario A
Facts: Mr A has an assured shorthold tenancy (AST) for 12 months, with rent being paid monthly (£1,000) in advance on the 1st of the month for that month (ie the rental period is for the whole month). The tenancy includes a break clause of 6 months. Mr A's landlord served a section 21 notice, with the expiry date falling in the middle of the month (on the 15th of the month). Mr A complies with the section 21 notice and vacates the property on the 15th. Mr A is entitled to a refund of the portion of the rent that was paid for the whole month because he paid a month's rent, but was evicted partway through the rental period.
Answer: Using the above formula, with January (which has 31 days) as an example month, the calculation would be £1000 x (16/31) = £516.13. This means that the landlord would need to refund Mr A a total of £516.13 of the rent paid for the month.
Scenario B
Facts: Mrs B has an AST for six months, with no break clause. Mrs B pays £1,500 rent per month. The section 21 notice cannot be served until after four months into the tenancy from the original start date. If the tenancy started on 1 January, notice cannot be served any earlier than 1 May. If notice is served on 2 May, and the tenant refuses to leave, possession proceedings cannot commence until 2 months later (ie 2 July). If the landlord goes to court and gets a possession order for 21 July, then the tenant will have to leave on that date by order of the court. If the tenant pays rent in advance for the whole rental period for July, they would need to be refunded a portion of the prepaid rent.
Answer: Using the above formula with rent being £1,500, the month is July and the date the tenant left being 21 July, the calculation would be £1,500 x (10/31) = £483.87. As Mrs B wasn't in occupation of the property for 10 days of the month, she will need to be refunded a portion of the rent she paid.