For a walking jacket to be deemed waterproof, the fabric and seams must keep out a minimum amount of water in laboratory conditions. Depending on the test method, the results are shown as PSI (pounds per square inch) or mm (millimetres) of pressure, the hydrostatic head.
All garments sold in the UK that state they are waterproof have had to meet the British Standard of 3 PSI, which is approx 1,500mm of pressure. Falling rain generates about 2 PSI (approx 1,000mm), however an 84kg person can generate about 8 PSI (4,000mm) when sitting and 16 PSI (approx 8,000mm) when kneeling, so your bargain waterproof may not be as waterproof as you think!
Berghaus and the North Face set their minimum at 20 PSI (10,000mm) and so should you, after all who wants to be wet?. For more extreme, windy and extended conditions adjust that to 40 PSI (20,000mm).
Bear in mind that keeping out the rain is more down to the design of the jacket, rather than how much water pressure the material can withstand. So even a 40 PSI jacket is no good if rain easily seeps in through the main zip! Always ask yourself: Are any zips exposed? Can the hood be adjusted to help prevent water going down your neck or absorbing into your hair? Can the cuffs be adjusted? Are the seams sealed in some way?
"Head" is not the same as pressure. Head is measured in units of feet or mm while pressure is measured in pounds per square inch, and is independent of pressure or liquid density.
The pressure at any point in a liquid can be thought of as being a vertical column of the liquid. The height of this column is called the static head and is expressed in terms of feet of liquid.
In a vertical column of water, the weight of the water at any given height determines the amount of pressure it exerts in any direction. Static head is defined as the height of this column expressed in terms of feet of liquid.
The principle of pressure head can be demonstrated with a drinking straw inserted into a glass of water. The water level in the straw will be the same as the water the level in the glass. If air is blown into the straw, pressure builds in the straw and pushes the water out. The pressure created in the straw is equal to the head pressure of the water in the glass. It is a simple matter to convert the created pressure to a desired unit of measure from wherever the water level may be in the glass.