dB/hz to dB Conversion
Recall from Physics class that density is, mass per unit volume. As an example, the approximate density of water is 1 g/cm3, so 100 cubic centimetres of water weighs 100 grams.

Power Spectral Density (PSD) is simply average noise power per unit of bandwidth. Power is expressed in units of watts. In telephony measurements, power is commonly expressed in logarithmic terms with respect to a milliwatt, i.e., dBm. The unit (volume) of bandwidth is
1 Hz. So PSD may be expressed as dBm/Hz, pronounced, “dBm per Hz.”

PSD is to Total Power as Density is to Weight. All transmission systems have finite power and bandwidth. Let’s consider an ideal system with a flat PSD signature.

At each frequency between f1 and f2 Hz, Let PSD to be a flat -10 dBm/Hz, or 0.1 mW/Hz.

Then 

PSD (dB) = PSD(dbm/Hz) * Bandwidth(Hz)

where bandwidth can be defined as f2 –f1.

If you want to calculate the total power there are more info available in the reference. 

reference: PSD techniques by Peter Walsh, NCE