Calculation to convert Nl/min to l/min

Hi,

What you say is absolutely right. If you say "10 Nl/min it means 10 l/min in normal conditions, so it means 10 l/min at 1 bar (absolute)."

And when I say "50 Nl/min at 7 bar", what I mean to say is this:
50 Nl/min is the flow/capacity. (50 l/min at normal conditions). But the system is 7 bar. So yes, the l/min at 7 bar will be about 8 times lower.*

About your situation: are you sure the compressors capacity is given in l/s at 7 bar (not Nl/sec). Very often, even compressor manufacturers use the wrong terminology.

If the word "FAD" (free air delivery) is used, that's close to Nl/min, but not exactly the same.*

For example: Capacity FAD: 99 l/s -- means: 99 l/s is sucked in and compressed to (for example 7 bar). So the output flow will be about 8 times smaller.*

To be sure, compare your compressor to these examples:

- 15 kW air compressor running at 7.5 bar, gives 155 m3/h FAD
- 22 kW air compressor running at 7.5 bar, gives 217 m3/h FAD

Hope this explain it more :) It can be really confusing sometimes, even for me writing this right now :)

I highly recommend to check out my free course "Compressed Air Basics" - all this is and more is explained!

Good luck. Let me know if you need any more info.

Cas

* Updated October 2026: corrected the pressure ratio (about 8 at 7 bar, not 7) and the comparison between FAD and Nl/min. See my comment below for the details.

Hi Cas, Thank you very much for your answer. It is everything very clear. You are right, my compressor capacity is given in free air delivery. It is a KAESER CSG-2 of 75 KW and the capacity is 11.76 m3/min. Comparing to your references about the m3/h FAD of 15 KW and 22KW compressors, it is clear that in my case the flow is given in m3/min Free Air Delivery. You say that m3/min FAD is practically the same as Nm3/min, so my compressor capacity finally is 11.76 Nm3/min. I have also been looking for data of this compressor on the internet and in the following link it says that the flow is free air delivery (2nd line): http://en.kaeser.co.th/Current_Affairs/press-A-CSG-2.asp I was confused because the manual says that the flow is given at 7 bar, but now I suppose that it means the working pressure of the compressor to give that flow is 7 bar. In a similar way as you said, it will be: - 75kW air compressor running at 7 bar, gives 11.76 m3/min FAD. Thanks because you have solved my problem about the calculation. I didn’t imagine that the mistake was in the capacity data, I thought that the mistake was in my way to do the conversion. Now my problem is that we don’t have enough air in the factory, but this is another issue. Regards, Iñigo

Good evening, the calculation is not right. You have to consider a ratio of absolute pressures. Nm3 is 1,01325 barA (101325 PaA) 7 bar(g) means in fact 8,01325 barA (7 x 1 + 1,01325) And so 1 m3 @ 7 bar is equal to 7,90 Nm3 1 x (1,01325 + 7 ) / 1,01325 We can also consider the Absolute temperature in calculation. P0*V0/T0 = P1*V1/T1 T = t +273,15 in °K t is in °C Hope it can help Regards

Hi Cas Thank you very very much for your answer. reza

Also, you need to consider that Nm3/h and FAD are not exactly the same. FAD flow will depend on atmospheric pressure (altitude) and ambient air temperature and humidity. I have found an excel conversion tool from Nm3/h to FAD here:

https://herramientasdeingenieros.com/conversion-de-nm3-h-a-fad-para-aire-comprimido/

An update on my answer above, because two things needed correcting.

1. At 7 bar, the air is about 8 times compressed, not 7. You have to use absolute pressure: 7 bar gauge is 8.013 bar absolute. So the ratio is 8.013 / 1.013 = 7.9. Yves was right.

2. FAD and Nl/min are not the same number. They use different reference conditions. Normal litres (DIN 1343) are at 0 °C, FAD (ISO 1217) is at 20 °C. Cold air is denser, so the same amount of air takes up less volume. The difference is about 8%:

Basis	Reference conditions	1 litre FAD equals
FAD (ISO 1217)	1 bar(a), 20 °C, dry	1 l
Normal litre, DIN 1343 (Nl, Nm³)	1.013 bar(a), 0 °C, dry	0.92 Nl
Normal litre, ISO 8778 ("ANR")	1 bar(a), 20 °C, 65% RH	1.02 Nl
SCFM	14.696 psi(a), 60 °F, dry	0.97 standard l
Standard air (ASME)	14.7 psi(a), 68 °F, 36% RH	0.99 standard l

So Iñigo's Kaeser with 11.76 m³/min FAD gives about 10.8 Nm³/min. If you need 350 Nl/min, you need about 380 l/min FAD.

Watch out: makers of pneumatic components often mean ISO 8778 (20 °C) when they write Nl/min. Then it's almost the same as FAD. If the datasheet doesn't say, ask.

And Matt is right too: FAD is the air at the compressor inlet, so in Nl/min a compressor delivers less at high altitude or on a hot day.

More in Free Air Delivery (FAD), and the flow converter does the conversion for you.

Cas