You have a list of air tools, such as breakers, drills, sandblasters or spray guns, and two compressor offers in different units. This guide shows how to choose an air compressor CFM figure from your own tools, and how to test each offer against it before you hire or buy.
What do CFM and pressure mean for a compressor?
A compressor makes two things, and a tool needs both.
Capacity is the volume of air the compressor delivers per unit of time. It is written in CFM (cubic feet per minute), litres per minute or cubic metres per minute. Pressure is the force of that air, written in bar or psi.
A tool slows or stalls if the compressor has the right pressure but too little volume.
Plates and quotes use different units. Convert them to one.
| Convert | Multiply by | Example |
|---|---|---|
| Bar to psi | about 14.5 | 7 bar is about 100 psi. |
| Cubic metres per minute to CFM | about 35.3 | 2 m³/min is about 71 CFM. |
| Litres per minute to CFM | divide by about 28.3 | 1,000 L/min is about 35 CFM. |
How do I find how much air my tools use?
Every pneumatic tool has a data plate or a manual page with two figures: air consumption and working pressure. Write both down for each tool. Do not reuse a figure from another tool of the same type; sizes and makers differ.
Many common air tools are rated at around 6 to 7 bar (about 90 to 100 psi). That is only a general picture. The tool's own figure is what counts.
If a plate is missing, ask the tool's supplier or maker for both figures in writing. Not sure yet which tool the job needs? Start with which site power tool does which job, or breaker, ripper or rock drill for hard ground.
How do I add up the demand for several tools?
Add the consumption of the tools that will run at the same time. A tool in the rack does not count. Then ask how each is used:
- Continuous tools, such as sandblasters and paint sprayers, use air the whole time the trigger is held. Count them at their full figure.
- Burst tools, such as a breaker used in short spells, stop and start. Count them at less than the full figure only if the crew really works that way.
- If you cannot say how often a tool pauses, count it as continuous.
Pressure is not added up. Take the highest working pressure among the tools that run together; the compressor must deliver the total volume at that pressure. A tool rated for a lower pressure then needs a regulator, so it never receives more than its own rating.
Tip. Plan for the busiest moment of the day, not the average one. That is when every crew picks up its tools together.
Worked example (invented numbers)
Keep one line per tool in a demand sheet. It also gives the supplier what they need to quote. These figures are made up to show the method; read your own data plates. Say the two hand tools work about half the time, and you add a 20% margin for this example.
| Tool | Air use | Pressure | Pattern | Counted |
|---|---|---|---|---|
| Sandblaster | 4 m³/min | 7 bar | Continuous | 4.0 m³/min |
| Breaker | 2 m³/min | 7 bar | About half the time | 1.0 m³/min |
| Chipping hammer | 1.2 m³/min | 7 bar | About half the time | 0.6 m³/min |
- Total: 5.6 m³/min. With the margin: 5.6 × 1.2 = about 6.7 m³/min.
- In CFM: 6.7 × 35.3 = about 237 CFM, delivered at 7 bar or more.
How much extra capacity should I allow?
Add a margin on top of the total. Real systems lose air in places a data plate does not show:
- leaks at couplings, hoses and tool seals;
- pressure drop along long or narrow hoses;
- worn tools, which can use more air than when new;
- hot, dusty working conditions, which can make a compressor deliver less than its rating.
There is no single correct margin. The 20% above only shows the arithmetic. Ask the supplier what margin they advise, and tell them if the tools are old or the hose runs are long.
Portable diesel or electric compressor: which fits the site?
| Type | Suits | Watch out for |
|---|---|---|
| Portable diesel | Open sites with no mains supply, moving work, civil and road jobs | Fuel supply, towing, noise, exhaust in enclosed spaces. |
| Electric | Workshops, factories and sites with a suitable supply | Supply capacity, cable length, the motor's starting demand. |
A large electric compressor needs a supply or generator that copes with the motor's starting demand, which is higher than its running load. The method is in how to size a generator: kVA, kW and power factor.
Heat and dust matter on Saudi sites. Ask how the machine copes with high ambient temperature, and place it so cooling air can flow. Requirements differ by project, site owner and insurer, so confirm before mobilising.
What do hoses, moisture and filters change?
Hoses. A long or narrow hose loses pressure, so the tool works below its rating. Use the hose diameter the tool maker specifies, keep runs short and check couplings for leaks.
Watch out. A compressor's maximum pressure can be higher than a hose, coupling or tool is rated for. Never exceed the lowest rated pressure in the line, and use hoses and fittings in good condition that are rated for the working pressure.
Moisture. Compressed air carries water. Sandblasting, painting and precision tools need dry, clean air: water makes blasting media clump and spoils paint finishes.
Ask the supplier:
- Is there a dryer, an after-cooler or a moisture separator, and who maintains it?
- Which filters are fitted, and when were they last changed?
- Is the air clean and dry enough for painting or blasting? Ordinary compressor air is not breathing air. Never feed a respirator or blasting helmet from it unless the supplier confirms in writing that the equipment is built and tested for breathing air.
How do I compare two compressors in quotes?
Compare the same points on both quotes. A headline size or motor power is not enough.
- Delivered air (often written FAD) at the working pressure. This is the air actually available to your tools. Use one unit for both.
- Working pressure. It should meet or exceed the highest tool pressure you listed.
- Maximum pressure. A small set can have a high maximum pressure and still deliver too little air at your working pressure.
- Type and fuel or supply. Diesel or electric, and what the site must provide.
- Accessories. Hoses, couplings, dryer, filters and an air receiver, and whether they are included.
- Condition and inspection. Ask for the current service record and any inspection or test certificate the site owner requires for the machine's air receiver and safety valve.
- Operator and servicing. Who runs, fuels and services the machine during the hire, and what happens if it breaks down.
If a quote gives only one unit, ask for delivered air in the unit on your plates.
How to do this on Maedah
Browse air compressors in your city, or post one request with the machine, the city and site on the map, the dates, the delivered air and working pressure you calculated, and the tools it must run. Say whether you need a dryer, filters, hoses or a diesel set. What to write in an equipment rental request has the full list.
Sealed quotes line up side by side, so you can compare delivered air, inclusions and response time. Agree price and payment with the supplier directly; Maedah does not process payments.
