Why is it when your taking a shower and someone flushes the toilet you lose all water pressure?

This is because most homes have a single main water supply line that feeds both the shower and the toilet. When the toilet is flushed, a large amount of water is drawn from the main supply line, which can cause the water pressure to drop in other fixtures in the house, including the shower.

Additionally, many shower valves are designed to automatically reduce the flow of water when the toilet is flushed, in order to prevent the shower from flooding. This can also contribute to the loss of water pressure in the shower when the toilet is flushed.

To avoid this problem, you can try installing a pressure-balancing shower valve, which is designed to maintain a consistent water pressure even when other fixtures in the house are being used. You can also try staggering your water usage, so that you don't flush the toilet while you're taking a shower.