I am thinking from the UX perspective about implementing a control where there are four parameters (see example below). The user can select the values for any three of them to get the value of the fourth (parameters are related).


The 4 parameters are E (energy), V (voltage), I (current), t(time). When I select any 3 out of the 4 - the last one is obtained from the equation - E = V x I x t (energy = voltage x current x time).

  • The simplest solutions is 4 inputs with good written logic?
    – Piotr Żak
    Commented Jan 14, 2020 at 12:14
  • The problem is that 4th input becomes an output when I have the rest 3 inputs.
    – vkrouglov
    Commented Jan 14, 2020 at 12:42
  • If you have 1,2,3 (4 automatic), when you have 2,3,4 (1 automatic), when you have 3,4,1(2 automatic) - You have 24 combinations - so i think the inputs should be identical - if there is one unknown in the equation - it doesn't matter which input.
    – Piotr Żak
    Commented Jan 14, 2020 at 12:43
  • 2
    I have found a very similar question discussed here: ux.stackexchange.com/questions/50511/… - I do not like the solutions there. Has to be a more intuitive way?
    – vkrouglov
    Commented Jan 14, 2020 at 12:47
  • 2
    A similar question: ux.stackexchange.com/q/60022/36883. It's about a calculator and seems almost the same.
    – jazZRo
    Commented Jan 14, 2020 at 14:19

1 Answer 1


vkrouglov You can try this approach:

By offloading the work upfront, ask for the missing details (i.e output) enter image description here

  • Yes, it's seem optimized & easy - also You can "pull" values from dropdown to buttons / tabs.
    – Piotr Żak
    Commented Jan 14, 2020 at 12:51

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