The amount of gallons of water that brushing your teeth uses is often 4 gallons of water.
However the actual amount of water that you use when brushing your teeth will also depend on your faucet flow rate and if you leave the water running or not when brushing your teeth.
If you brush your teeth for 2 minutes and the water is left on for those 2 minutes you will use around 4 gallons of water.
If you shut the water off during those 2 minutes of brushing your teeth then you could only use around 2 to 3 gallons of water.
You do save some water when shutting the water off while brushing your teeth but for me I find it more inconvient to shut the water off and then turn it back on.
The savings is also minimal for me as I rarely even use more than 2,000 gallons of water a month which is the minimum for my water bill anyway.
Some people need to save water if they are in drought prone areas or have high water bills but for most leaving the water on while brushing your teeth is not much of a waste.
And if you brush your teeth twice a day and leave the water running for 2 minutes you would use around 8 gallons of water a day or 200 gallons of water a day which seems like a lot but it's not even close to the 1,000 gallons of water that you're charged for.
My water rates are $5.00 per 1,000 gallons so if I used 200 gallons of water a month it would take me almost 3 months to use $5.00 worth of water.
That's not too much money to just leave my water running and we have an abundance of water here so no fear of running out any time soon.