I have had the phone for 6 months and the battery capacity has gone down fast

Hello AliH121, 

Thanks for reaching out on the Apple Support Communities. Having your iPhone performing at its best, including the battery performance is important. We're happy to look into this with you. 

In the iPhone Battery and Performance article you'll find this information under the, "Your battery's maximum capacity," section: 

"Maximum battery capacity measures the device battery capacity relative to when it was new. A battery will have lower capacity as the battery chemically ages which may result in fewer hours of usage between charges. Depending upon the length of time between when the iPhone was made and when it is activated, your battery capacity may show as slightly less than 100%.

A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge. Learn more about charge cycles.

As your battery health degrades, so can its ability to deliver peak performance. The Battery Health screen includes a section for Peak Performance Capability where the following messages may appear."

Based on this information the length of time between when the iPhone 8 was manufactured (it was announced in September, 2017), and when you activated it earlier this year will explain why you're seeing 95% maximum capacity. 

With the Peak Performance Capability showing that the battery is currently supporting normal peak performance, your battery is showing that it is currently in good health. 

If you're experiencing issues with the battery life, follow the information provided in this article: About the battery usage on your iPhone, iPad, and iPod touch. 

If you're experiencing any changes in the overall performance of your iPhone, this article will be helpful: About iOS performance on your iPhone, iPad, or iPod touch.

All the best.