How do I measure for a replacement gas hob?

I am wanting to know how to measure for a replacement gas hob (new )
Do I measure the width and depth of the original one ? Thankyou.

The best way to be sure you're getting it right is to look under the bottom for a manufactirer and model number and order that part. If for some reason that's impossible, contact the supplier and ask what measurements they need. An incorrectly installed gas hob can be a very dangerous thing.