Postage calculator for Royal Mail and Parcelforce, latest update is 3/9/23 to include price changes for 2nd October 2023.
Postage Pro has been regularly updated since 2012.

Includes a size guide, tips when it's cheaper to split your package into multiple smaller items and a lot more (see screenshots for details).
Postage Pro shows the postage rate you'd pay at the Post Office counter.
Weight is accepted in grams / kg / ounces and pounds.
Very useful for eBay, amazon and etsy traders.
Other useful features:
a detailed description of each service is available
automatic switching to Parcelforce for heavier items
automatic calculation of the most efficient way to use 1st/2nd class stamps (eg. for a parcel costing £4.80, use 3x1st class and 2x2nd class)
a size guide in both cm and inches
a handy shopping basket (to calculate totals, unfortunately you can't pay for postage with this app)
no adverts / banners
xmas posting dates

Check the screenshots to see this app in action.

Please email [email protected] if you have any questions or suggestions.