Which strap size is suitable for Apple Watch Series 9?

lalamammad04 wrote:

in site has many strap sizes ex.42 44 40 mm

which size is appropriate for my watch series 9 45 mm

Other ways to approach this include using the Apple Store app.

You can select a band, and the app indicates if the band is compatible (“Compatible with [whatever]”) with your Watch, down in the details for the band:

Or you can use the Apple Store app to view all bands, and select the 45mm case size in the filter, as shown in the image below:

Apple bands are actually designed to be compatible across multiple Apple Watch generations that share similar case widths. So for your Apple Watch Series 9 (45mm), you're not limited to only bands labeled “45mm.” Bands that were originally made for the 42mm (Series 3 and earlier) and 44mm (Series 4–6, SE) models are also fully compatible with the 45mm size. Apple intentionally kept the lug width (the part of the watch the band slides into) consistent across these sizes to ensure backward compatibility.

So if you're browsing online and see a band marked as compatible with 42mm / 44mm / 45mm, it should fit your 45mm Series 9.

I suggest that you use Apple's sizing guide, or visit your local Apple Store, to make sure you get the exact band that you want.