This tool converts Noise Spectral Density with units nV/rt(Hz) or nV/√(Hz) to VRMS.
μVrms = nV/√(Hz) * √(BWkHz * 1.57)
The following graph shows a plot of input voltage noise vs frequency for an operational amplifier.
Between 200 Hz and 1 MHz, input voltage noise is approximately 6 nV/√(Hz). The value at any specific frequency can be converted to its equivalent dBm/Hz value.
Using the input voltage noise of 6 nV/√(Hz) for this op amp. The value of RMS noise over a 100 kHz bandwidth is 2.37 μVRMS.
This calculation uses a 1-pole filter with an equivalent noise bandwidth of 1.57. If we assumed a brick wall filter, this value would be 1. In that situation you could verify these results using the following calculators to go from VRMS to nV/rt(Hz)