Determining the exact amount of wort that is available in any cylindrical kettle at any given time in a boil is easy to do with the aid of a stainless steel ruler or measuring tape and a little math. All one needs to do is to know the inside diameter of one's kettle and the height of one's wort.

kettle_inside_radius = kettle_inside_diameter / 2

wort_volume_in_cubic_inches = wort_height x 3.14 x kettle_inside_radius x kettle_inside_radius

There are 231 cubic inches in a gallon; therefore, wort_volume_in_gallons = wort_volume_in_cubic_inches / 231.

Example:

kettle_inside_diameter = 14 inches

kettle_inside_radius = kettle_inside_radius / 2 = 7

wort_height = 8 inches

wort_volume_in_cubic_inches = 8 x 3.14 x 7 x 7 = 1231 cubic inches

wort_volume_in_gallons = 1231 / 231 ~= 5.33 gallons

Now, a quick and dirty way to know the approximate wort volume in gallons at any given point is to calculate the volume of one quarter inch of wort in gallons. Using the kettle diameter above, one_quarter_inch_of_wort_in_gallons = 0.25 x 3.14 x 7 x 7 / 231 = 0.1665 gallons per quarter inch of height, 1/3rd of a gallon per half inch of height, or one gallon for every 1.5 inches of height.

Tracking the height of one's wort every 15 minutes allows one make adjustments to the boil.

With that said, there are other factors that will limit the amount of wort that can be collected from one's kettle. Whole cone hops retain a significant amount of wort, as much as 12 fluid ounces of wort per ounce of hops. Additionally, break material that is left behind in the kettle will affect primary volume. The impact of break material and hops on primary volume should be tracked. Over time, one will be able to adjust for these losses on the fly without needing to perform any calculations. For example, I learned that I needed to boil down to one quart more than I expected to yield to my primary fermentor when brewing 5-gallon brews with up to two ounces of whole cone hops (i.e., boil down to 5.75 gallons to achieve a primary volume of 5.5 gallons).