If you have all or part of a UK address, but need to find the corresponding postcode, jump online. You can find this information for free via a search tool at either the Royal Mail or Post Office websites. You can also phone Royal Mail for help finding local or international postcodes. Expect to pay for these calls.
Go online to find any UK postcode. You can use the Royal Mail's postcode finder tool to search for up to 15 postcodes per day, for no charge. The Post Office website offers the same service. Enter as much of the address as you know into the search boxes, for example house number, street and county. Then click "find postcode".
Undertake a "detailed search". If you can't find the postcode using the regular search tool, use the detailed search option on either the Royal Mail or Post Office sites. This allows you to enter flat numbers and business names to refine your search.
Find a UK postcode by phone. If you can't find it online, call Royal Mail on 09063 021 222 between the hours of 8 am to 5 pm weekdays for assistance. You will be charged 50p/minute from a land line. Between 5pm and 7 pm weekdays, 9 am and 5.30 pm Saturdays and 9 am and 2 pm Sundays, call 08457 111 222 for the same service. Calls during these times are charged at local rates. Note that Royal Mail will provide a maximum of three postcodes per phone call.
Submit an enquiry to the Post Office. If the online tool isn't providing you with a result and you don't need the postcode immediately, submit an online enquiry form to the Post Office. Describe the problem and they will get back to you as soon as possible, hopefully with a solution. This is free of charge.
Fin an international postcode. Call the Royal Mail on 08702 403 838 to find international postcodes. Remember to have the address handy to give to the operator. Calls are charged at local rates. If you want to avoid the cost and possible call queue times, try typing the international address into Google Maps first. The postcode may come up along with the map search results.
You can also search for a street address using the Royal Mail online tool if you know the postcode and house number.