Measure the length of the tallest flower in your arrangement with a ruler or measuring tape.
Calculate the vase height by dividing the length of the flower by 2.5. For example, if the flower height is 25 inches, 25 divided by 2.5 is 10. So 10 inches is the correct height of the vase.
Another way to look at it is the tallest flowers should be the height of the vase plus 1.5 times the height of the vase in a finished flower arrangement. For example, if the vase is 10 inches high, the height of the tallest flower in the arrangement should be the height of the vase plus 1.5 times the height of the vase, or 25 inches. Or, simply multiply the height of the 10-inch vase 2.5 times to get the correct height of the flowers for the arrangement, which in this case is 25 inches.
Find a vase at the correct height. If you cannot find a vase at the preferred height, cut the flowers to the correct height by cutting the stems with a sharp pair of floral shears.