8.9-9.55mm ropes are considered to be thin ropes, and they are good for situations where weight is critical, like steep sport climbs. These are usually favored for their lightweight, making them great for long multi-pitch climbs. However, thin ropes also have downsides. They tend to be less durable, less easy to grab, and have a lower UIAA fall rating. They also slide through belay devices much faster, meaning that you will need to have an experienced and concentrated partner when using a thin rope.
The most popular diameter of a single rope is 9.8mm. These ropes tend to be very versatile, decently durable, and easier to use. However, the thicker the rope, the easier it is to grab. For example, a 10.2-10.7mm rope will offer a quicker grip. Not to mention, a thicker rope is also easier to lock into your belay systems. Thick ropes are also known for being more durable than thin ropes.
This style is best suited for gym climbing or top-roping, which puts quite a strain on the ropes. You should always pay close attention to the wear and tear of your rope, no matter the thickness. On average, lead-climbing ropes range from about 9.8mm to 10.2mm in diameter. And you can’t really go wrong with a rope in this range, but finding the particular rope diameter that hits the comfort spot for you is a personal choice that takes some safe trial and error.
Now, most of this information applies to single ropes. The average diameter of half and twin ropes is different. A half rope typically has a diameter of 8 or 9mm, and a twin rope is most frequently seen with a diameter of 7 or 8mm thick. As you can see, these styles of ropes are thinner and will most likely wear out faster. However, they allow you to tackle more technical and wandering terrains.