My tenant’s 1 year tenancy is completing on the 14th of January.

Tenant paid his last rent on the 15th of December which will take them upto 14th of January.

He will be moving out only on the 20th of January. That means he will be staying an additional 6 days more.

How do I calculate the rent for the remaining days?

He is paying £750 every 15th to 14th.

So, should I calculate the rent for the remaining 6 days like

Divide total rent of 750 by 30 days and multiply by 6 days or

Divide total rent of 750 by 31 days and multiply by 6 days