WinAtt  is a small utility to aid in attenuator design. It calculates the exact values  of resistor required for a given attenuation and system impedance, and also  shows the performance of attenuators built using real-world components with preferred  values. Both  pi and T designs are covered, and the power dissipated in each resistor is  shown. Recalculation is automatic if the attenuation or power is changed.
Simply enter a value for your desired attenuation (and power input, if you  wish) in the box at the top left. If you want to design for a system impedance  other than 50 ohms, then change the value in the system impedance box just  below the title. 

WinAtt is available for Windows as a zip file, containing the installer and all files required. Click to download WinAtt. (Latest version, May 2021, minor bug fix)
LEGAL 
    NOTICE 
    The WinAtt software is copyright of the 
    author: 2004-2021, Roger Blackwell who 
    retains all rights, but has made WinAttn available for non-commercial use 
    under the conditions stated below. 
    The software is provided "as is" and without warranties 
      or liability, whether express or implied. Because the various hardware and 
      software environments in which this software may be used are beyond our control, 
      no warranties of functionality or fitness for a particular purpose are offered 
      or implied.
    The WinAtt software is not permitted to be used 
    for any commercial purpose, offered for sale in any way, or supplied in association 
    with any commercial product.