How much water does a running toilet use plus?

That very much depends on the toilet. Low-Flow toilets use in the neighborhood of 1 1/4 gallons. As of November 2009, you can get a 1.28 GPF toilet for around $100 in the southeast USA.

Older toilets can use several gallons.

If you don't know your toilet's manufacture:

1. Make sure no one else is using any water.
2. Read your meter.
3. Flush.
4. Read your meter.
5. Step 4 minus Step 2 reading is your answer.
6. Repeat Steps 2-5 to get your average.

Another test to learn how much water a tank-style toilet uses could be to turn off the water intake valve (under the tank) and flush the toilet to empty it. Then refill the tank using a one-gallon jug (perhaps an old milk jug). This will give you a rough estimate of that toilet's water use.

You can reduce water usage in older toilets to some extent by

1. Adjusting the fill level (When the water shuts off) OR
2. putting something weighty enough to not move and that won't decompose to the tank away from the water outlet.
3. Old-fashioned toilets (with a tank the size of a fish aquarium) may use 4-5 gallons of water per flush, but it really depends on the toilet.