How can I install a custom calculator for dog food requirements?

[dogmeatsbone]

Hi everyone, 

I'm looking to install a custom calculator that will assist in informing customers their dog's daily food requirements.  

it's  simple calculator with only 3 fields - weight, a percentage of that weight (this will need to be a drop down menu) and then the calculation.

can anyone recommend an appropriate app - ideally free given it's a simple calculation ?  

thank you