The amount of air you need depends on several things, for instance how hard you are working, your heart rate, your fitness, temperature in the work area, and whether you speak.

When it comes to supplied air, two main factors come into play:

The VOLUME of air you consume. This is usually called minute volume, that is, how many litres of air you breathe during one minute's work. The minute volume is usually measured in litres per minute (l/min).

The SPEED of the air as you breathe it into your lungs. This is often called flow rate. In particular, the maximum speed of air through our mouth (called PIAF or Peak Inhalation Air Flow) is of interest, because this is the minimum flow rate the compressor or cylinder must be capable of supplying. If the air supply fails to meet your breathing requirements, you will 'out-breathe' the air supply, and contaminated air will leak into the system. The flow rate and PIAF are also measured in litres per minute (l/min), and it is important to differentiate between minute volume and flow rate.

Below are a few approximate minute volumes and peak inhalation flow rates in an average person performing different tasks WITHOUT speaking:

Remember, these are approximations only. Keep also in mind that speech will greatly influence your breathing, creating significant peaks and dips in the flow rate.

If your air supply is in the form of compressed air cylinders, one of the greatest obstacles is the limited duration of operation before you must switch to a new cylinder.

Below is a rough guide to the time you have before the warning whistle sounds, using different size bottles. All are calculated on an average 40 l/min air consumption per person, and the whistle set to sound at 55 bar:

When calculating the required capacity of a compressor, it is important to add up all air-driven tools. These can vary greatly. A small air-blower may consume very little air, while a sand blaster or jackhammer require a lot of air at high pressure.

An approximate guide may be a spray painting gun that consumes in the vicinity of 300–400 litres per minute. This is the capacity of an average medium-sized compressor.

There is no patent solution, ready reckoner or simple trick to calculate the required compressor delivery power or tank capacity. If the supplied air system serves many air tools and multiple users of breathing air, it may be best to consult an expert in the field. All systems are different. Two ground rules are:

• Compressor power and tank capacity MUST be greater than all tools and respirators put together
• All tools, pipes, hoses, tubes, valves and regulators MUST be rated at a higher pressure than that delivered by the compressor/tank
• At the very least for minimum requirements, the compressor should be capable of delivering 0.7 m³/min (25 cubic feet/min)