Adding to Holy Water

The common custom (though not officially recognized) is that so long as the water added to the holy water does not amount to half the amount of the holy water then all water in the container is considered blessed water.

The reverse is also the common understanding. Adding a thimble of holy water to a gallon of regular water does not make the container full of holy water. The holy water would need to be more than 50% of the combined water.

As previously stated, this is common custom and not official teaching. Since it does not take very long for a priest or deacon to bless water, adding regular water to holy water should only be done when absolutely necessary.