VHF/UHF S-meter

IARU Region 1 recommendation defines S9 for VHF/UHF to be a receiver input power of -93 dBm. 

This is the equivalent of 5 V in 50 Ω. Again, one S-unit corresponds to a difference of 6 dB,
equivalent to a voltage ratio of two, or a power ratio of four.
 

Conversion between power and VHF/UHF S-units

S-reading

Pin @50Ω

Vin @50Ω

[dB above1V]Vout[1V] @50Ω

S9 + 40 dB

-53 dBm

0.50 mV

54 dBV

S9 + 30 dB

-63 dBm

0.16 mV

44 dBV

S9 + 20 dB

-73 dBm

50 V

34 dBV

S9 + 10 dB

-83 dBm

16 V

24 dBV

S9

-93 dBm

5.0 V

14 dBV

S8

-99 dBm

2.5 V

8 dBV

S7

-105 dBm

1.26 V

2 dBV

S6

-111 dBm

630 nV

-4 dBV

S5

-117 dBm

320 nV

-10 dBV

S4

-123 dBm

160 nV

-16 dBV

S3

-129 dBm

80 nV

-22 dBV

S2

-135 dBm

40 nV

-28 dBV

S1

-141 dBm

20 nV

-34 dBV

 

HF S-meter

Many amateur radio and shortwave broadcast receivers feature a signal strength meter (S‑meter).4 
In 1981, the International Amateur Radio Union (IARU) Region 1 agreed on a technical
recommendation for S‑meter calibration of HF and VHF/UHF transceivers.5,6

IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input
power of -73 dBm.
 This is a level of 50 V at the receivers antenna input assuming the input impedance
of the receiver is 50 Ω.

The recommendation defines a difference of one S-unit corresponds to a difference of 6 dB, 
equivalent to a voltage ratio of two, or a power ratio of four. Signals stronger than S9 are given with
an additional dB rating, thus S9 + 20 dB, or, verbally, 20 decibel over S9, or simply 20 over 9
or even the simpler 20 over.

Well-designed S-meter on the DRS WJ-8711A HF transceiver. Source: N9EWO

Conversion between power and HF S-units

S-reading

Pout @50Ω

Vin @50Ω

[dB above1V]Vout[1V] @50Ω

S9 + 40 dB

-33 dBm

5.0 mV

74 dBV

S9 + 30 dB

-43 dBm

1.6 mV

64 dBV

S9 + 20 dB

-53 dBm

0.50 mV

54 dBV

S9 + 10 dB

-63 dBm

0.16 mV

44 dBV

S9

-73 dBm

50 V

34 dBV

S8

-79 dBm

25 V

28 dBV

S7

-85 dBm

12.6 V

22 dBV

S6

-91 dBm

6.3 V

16 dBV

S5

-97 dBm

3.2 V

10 dBV

S4

-103 dBm

1.6 V

4 dBV

S3

-109 dBm

800 nV

-2 dBV

S2

-115 dBm

400 nV

-8 dBV

S1

-121 dBm

200 nV

-14 dBV

source: https://hamwaves.com/decibel/en/