What is the difference between Nm3 and Sm3?

Unfortunately neither Nm3 (normal cubic meter) or Sm3 (standard cubic meter) are complete definitions in themselves. It is essential to know the standard reference conditions of temperature and pressure to define the gas volume since there are various debates about what normal and standard should be.

Most commonly used reference conditions are:
Normal cubic meter (Nm3) - Temperature: 0 °C, Pressure: 1.01325 barA
Standard cubic meter (Sm3) - Temperature: 15 °C, Pressure: 1.01325 barA

barA: absolute pressure

How do I calculate Nm3 and Sm3 and what is the conversion rate?

The volume of gases changes with temperature and pressure, therefore these parameters are also part of the conversion equation.

The conversion from Sm3 to Nm3:

V1/V2 = (P2xT1) / (P1xT2)
V1/V2 = (288.16x1.013) / (273.16x1.013) = 1.05491287

Temperature is entered in K; 273.16 is absolute zero
Interpretation: 1Nm3 is 5,49% larger than Sm3, 1Nm3>1Sm3

Some of our competitors use 15°C and 981mBar as reference conditions for standard cubic meter. The calculation is following:

V1/V2 = (288.16x1.013) / (273.16x0.981) = 1.08932389

Interpretation: 1Nm3 is 8,9% larger than Sm3. It also means the stated capacity of the generator stated in Nm3/h is 8.9% larger than same capacity defined in Sm3/h.
Assumption: It is essential to consider these facts when projecting the gas generating system or when actually making decision to purchase certain model because you might be actually buying less than you actually think you are.