Ernie Willson wrote:

The calculation is correct and likely the number is accurate enough
for most purposes. If not then I suggest the following.

Turn off the water inlet valve to the toilet. Then remove the top and
mark the water height. Flush the toilet, and then refill the tank (to
your mark) with water from a known size container, like a gallon jug.

Remember to turn the water back on when you are finished.

EJ in NJ

That tells you how much water is used by the tank, but the toilet also fills
and uses the water in the bowl, too, which is filled by a mechanism other
than straight filling the tank.

To measure the water used by the bowl, remove the little hose that fills the
riser column in the tank, put it into an adequately sized container, and
flush the toilet.

When the tank finishes filling, measure the amount of water which
accumulated in the adequately sized container, and add that amount to the
volume of water used by the tank.

That will give you the total amount of water used by your toilet, including
both tank and bowl, per flush.

Jon