# Autocalculate field in a form

**URL:** <https://www.ruby-forum.com/t/autocalculate-field-in-a-form/185978>\
**Category:** Rails\
**Created:** [March 30, 2010, 2:00am UTC](https://www.ruby-forum.com/t/autocalculate-field-in-a-form/185978 "2010-03-30T02:00:29Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![newnw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/919ad9/32.png) [@newnw](https://www.ruby-forum.com/u/newnw)\
**Post date:** [March 30, 2010, 2:00am UTC](https://www.ruby-forum.com/t/autocalculate-field-in-a-form/185978/1 "2010-03-30T02:00:29Z")

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Hi All,

I have a form with three field X,Y and Z.

I want to filed Z automatically according to the X and Y values. x and y  
option fileds.

eg

if x filed value is “j” and Y field value “m” then Z field value must  
filed automatically with “k”.

I would like to do this with onfocus method.