An inch.

Maya Lin measures fourteen and a half inches across the hips.

Her seat width is fifteen and a half.

The chair my son bought was eighteen.

And here is why that is not a quibble, and here is the sentence from Debra Ngwenya that I have repeated to perhaps two hundred people.

“The width of the seat decides where her hands go.”

On a chair that is too wide, your shoulders have to abduct — go out sideways — to reach the hand-rims.