Measurements:

Fractions to Decimals to Millimeters

Measurements between lugs, where the band or strap is attached, on your watch.
MilimetersInches
261
2415/16
227/8
2113/16
2025/32
193/4
1811/16
1721/32
165/8
1519/32
149/16
131/2
1215/32
117/16
103/8
911/32
85/16

Here are several ways to measure a watch and buckle.
This watch would require a band that is 20mm at the lugs and 16mm at the buckle.