To calculate the gas density, you will need to convert the given units to standard units (SI units) and use the ideal gas law equation.

1. Convert nm3/h to m3/s:
   - 1 nm3/h = 1 × 10^-6 m3/s

2. Convert pressure from mbar to Pascal (Pa):
   - 1 mbar = 100 Pa

3. Convert temperature from Celsius (°C) to Kelvin (K):
   - Kelvin = Celsius + 273.15

Now, you can use the ideal gas law equation to calculate the gas density:

Density (ρ) = (Pressure (P) * Molecular Weight (M)) / (Gas Constant (R) * Temperature (T))

where:
- Pressure (P) is in Pascal (Pa)
- Molecular Weight (M) is in kg/mol
- Gas Constant (R) is 8.314 J/(mol·K)
- Temperature (T) is in Kelvin (K)

Make sure the units are consistent throughout the calculation.

Note: The molecular weight of the gas is required to calculate the density accurately. If you know the gas, you can look up its molecular weight online or use an appropriate average value.

For example, let's say you have:
- nm3/h = 1000 nm3/h
- Pressure = 500 mbar
- Temperature = 25 °C

1. Convert nm3/h to m3/s:
   - 1000 nm3/h = 1000 × 10^-6 m3/s = 0.001 m3/s

2. Convert pressure from mbar to Pascal (Pa):
   - 500 mbar = 500 × 100 Pa = 50000 Pa

3. Convert temperature from Celsius (°C) to Kelvin (K):
   - 25 °C + 273.15 = 298.15 K

Now, substitute the values into the ideal gas law equation:

Density (ρ) = (50000 Pa * M) / (8.314 J/(mol·K) * 298.15 K)

Simplify the equation and calculate the density.