1/3 the width of the drawers. Vary the pulls sizes with the drawer sizes.

Thanks
What about if using pulls on cabinets? Is there an equation for that also?

I don't know how other people figure out hardware, but I had the hardest time deciding hardware proportions. Eventually, I made cardboard cutouts the same color and size of the hardware I was considering and stuck them to the cabinets with painters tape, so that I could visualize size and placement before ordering. I ended up not following the 1/3 rule, but I think the 1/3 rule is a good place to start.

@sistertamala - for the pulls on cabinet doors, I would do something similar to what @theresa21 mentioned, though my method is to use aluminum foil shaped into the pulls of a couple sizes you are interested in. Hold them up against the cabinet doors, or stick them there with blue painters tape, and it will help you visualize which is the correct size.

Pull on drawers sized as suggested. IMO knobs are easier and more hand efficient on upper cabinets.

I used 1/3 rule for all cabinets and drawers (including the long drop open door in front of sink.) Cabinets are light grey and hardware is champagne bronze. I love the jewelry look. Pulls everywhere as I think they are easier for older hands.

I did not use a rule with my cabinet hardware. All of my pulls are the same, even for the paneled dishwasher. I threw in some knobs too.

I like pulls to be in the same proportion for all cabinets so often that means more than just 2 sizes I like the stainless pulls from LeeValley they come is lots of sizes .I use painters tape to figure what looks right and aslo to figure out what needs to be horizontal and what needs to be vertical