Conduct your buoyancy check according to the PADI Basic Weighting Guidelines found in the Peak Performance Buoyancy Course. Weigh yourself and your equipment before entering the water and record this weight.
A proper buoyancy check at the surface includes the following five distinct steps.
Put on all equipment.
Enter water too deep in which to stand, and deflate your BCD completely.
Hang vertical and motionless in the water while holding a normal breath.
Add or subtract a small amount of weight until you float at eye level while holding a normal breath.
As a test, exhale. You should sink slowly. Record this weight.
Ideally, you should be using a nearly depleted cylinder (300-500 psi). If one is not available, use a full tank and the calculator will compensate for the weight of the air. The calculator will assume a 10% reserve volume (90% usable air) when doing this.
Before using the calculator, you need a minimum of two things, your dry weight with equipment, and the amount of lead you need as determined in the above described weight check. The calculator can also calculate weight requirements between different size tanks. It contains some pre-defined cylinders as well as letting you enter your own by selecting the "User Input". Cylinder buoyancy characteristics can usually be found on the manufacturers website spec page.
The calculator defaults to fresh/salt water conversions. If you just need to know how much extra lead to put on when weighting with a different size tank, select "no conversion" for the type of water and enter the type of tank you weighted with and the tank you will be using.