wooden framed windows

U-Value Calculator for Windows and Doors

Our U-Value calculator is a whole window calculator designed to calculate an accurate U-Value of windows and doors. This calculated value conforms with BS EN ISO 10077 Part 1.

We can advise on how you might achieve the required standard.  A U-value calculated by us should be sufficient to satisfy the requirements of Building Regulations.

Our calculator is primarily intended for use with wooden windows and doors. These can be made from hardwood or softwood, of any size with double or triple glazing. If you can provide us with the r-value of any other type of frame you are using we can provide a u-value for that too. Our u-value calculator is not restricted to products manufactured by one company.

  • Whole Window / Door Calculator

  • Conforms to BS EN ISO 1007 Part 1

  • Hardwood / Softwood / Accoya

  • Not restricted to one companies products

  • No Membership Required

  • Any Size of Window or Door

  • Discounts Available

  • Certificate of U-Value Issued

Why you require u-values

From 1st October 2010, window manufacturers needed to show that their windows complied with the energy efficiency requirements in the latest revision of Part L of the Building Regulations and the Approved Documents L1A (new dwellings) and L1B (existing dwellings).

Obtaining a precise U-value for a manufactured window can be extremely expensive especially for smaller manufacturers or architects with bespoke designs. U Value Calculator is able to calculate the u value for timber framed windows and doors at a very reasonable cost.

Architects and specifiers are now increasingly asking for the thermal transmittance values of windows, so that they can calculate the SAP ratings of the buildings where the windows are to be fitted. Clients require that value for completion certificates.

As a builder if you cannot provide U-values for your products, then your client may have to use the windows of a competitor who has that information to hand.

Building Regulations for windows and doors.

Below is a summary of the u-values that are specified in Building Regulations Documents L1A (New Dwellings) 2010 & L1B (Existing Dwellings) 2010. These are the values that your windows and doors may have to comply with. This was updated in June 2022.


  • U-Value = 1.6W/m²K
  • U-Value = 1.4W/m²K or Window Energy Rating = B and B

How our U-value calculator works

You supply us with the relevant information regarding the window or door for which you require a u-value. We will then calculate the U Value of your product using our specialist software and send you a certificate which specifies the result.

This certificate can be used as formal support when you supply U-value information to potential customers or Building Control.