As more of us use selective level meters to receive the longwave band,
I have put up this list that enables to convert from dBm to Voltage in
mV or uV and to Sunits, to get an idea about the signal strengths measured
with various receivers. For conversion of dBu to dBµV/m for my receiving
setup, have a look here.

dBu*
( R= 75 Ohm, 0 dbu= 0.775 V) 

Sunits
(S9=100 uV, R=50 Ohm) 

 7 dBm   17 dBu  99.88 mV  S9+60  

 23 dBu 



 17 dBm   27 dBu  31.58 mV  S9+50  

 33 dBu 



 27 dBm   37 dBu  9.99 mV  S9+40  

43 dBu 



 37 dBm  47 dBu  3.16 mV  S9+30  

53 dBu 



 47 dBm  57 dBu  1.00 mV  S9+20  

 63 dBu 



 57 dBm   67 dBu  315.85 uV  S9+10  

 73 dBu 



67 dBm   77 dBu  100 uV  S9  

 83 dBu 

S8 


 89 dBu 

S7 


 95 dBu 

S6 


 101 dBu 

S5 


 107 dBu 

S4 


 113 dBu 

S3 


 119 dBu 

S2 


 125 dBu 

S1 


 131 dBu 

S0 

 127 dBm   137 dBu  0.10 uV  S0 
Some further explanations:
Decibel (dB): Equal to onetenth
of a bel (After Alexander Graham Bell) The preferred
method and term for representing the ratio of different audio or rf
levels. It is a mathematical
shorthand that uses logarithms (a shortcut using the powers of 10 to
represent the actual number)
to reduce the size of the number. For example, instead of saying the
dynamic range is 32,000 to 1,
we say it is 90 dB [the answer in dB equals 20 log x/y, where x and
y are the different signal
levels]. Being a ratio, decibels have no units. Everything is relative.
Since it is relative, then it
must be relative to some 0 dB reference point. To distinguish between
reference points a suffix
letter is added as follows:
0 dBu A voltage reference point
equal to 0.775 Vrms. This reference originally was labelled dBv (lowercase)
but was too often confused with dBV (uppercase), so it was changed to
dBu (for unterminated).
0 dBm A power reference point equal
to 1 milliwatt. To convert into an equivalent voltage level, the impedance
must be specified. For example, 0 dBm into 600 ohms gives an equivalent
voltage level of 0.775 V, or 0 dBu (see above); however, 0 dBm into 50
ohms, for instance, yields an equivalent voltage of 0.224 V.