It can vary considerably and depends on:

• Type of surgery (large operations, e.g. removal of kidney or bowel, cancer surgery, aneurysm repair)
• Experience and skills of the surgeon
• Individual patient anatomy & build
• Underlying pathology (e.g. tumour size, other structures/blood vessels affected)
• Patients medical conditions (e.g. clotting)
• Medication that affects blood clotting (e.g. Warfarin, Clopidogrel, Rivaroxaban)
• Complications during an operation (tissue trauma, bleeding)
• Some anaesthetic techniques may help to reduce blood loss (e.g. Spinal, Epidural, Hypotensive anaesthesia)