Why Does G-Force Knock You out?

The acceleration that causes blackouts in fighter pilots is called the maximum g-force. Fighter pilots experience this force when accelerating or decelerating quickly. At high g's the pilots blood pressure changes and the flow of oxygen to the brain rapidly decreases.

How many G's does it take to pass out?

Most of us would pass out with head-to-toe G forces of just 4 or 5 because our hearts can't summon the necessary pressure. Blood pools in our lower extremities, and our brains fail to get enough oxygen.

How many G's can a human survive?

Normal humans can withstand no more than 9 g's, and even that for only a few seconds. When undergoing an acceleration of 9 g's, your body feels nine times heavier than usual, blood rushes to the feet, and the heart can't pump hard enough to bring this heavier blood to the brain.