Around 7–8% of an adult's body weight is blood. The body can easily replace a small amount of lost blood, which makes blood donation possible. If a person loses around 15% or more of their blood, there may be a risk of shock.

https://www.medicalnewstoday.com/articles/321122#:~:text=Around%207%E2%80%938%25%20of%20an%20adult's%20body%20weight%20is%20blood,be%20a%20risk%20of%20shock.