One half-inch

[The Wizard of BC]

Alternately you can consult with a kiltmaker or competent tailor.

It is far easier to make a kilt smaller than to make one larger by moving the straps and buckles.

But, and this is very important. The straps and buckles are NOT sewn to the outer Tartan fabric!
They are actually sewn to a strip of fabric inside the kilt and hidden from view by the liner you can see on the inside.

The stitching just goes through the outer Tartan to get to the strip.
This strip of fabric is called the stabilizer.

When you take the kilt to the shop explain to the tailor that the process is the remove the buckles on the right hip and the strap on the left hip.

Put the kilt on and chalk mark the strap and buckles new location.

Then stitch the one or two right buckle tabs and left strap at the new location insuring that the stitching goes fully through the outer fabric and is anchored to the hidden stabilizer strip.

Viola! you may now wear you kilt until such time that it takes for the beer or desert diet program takes effect.