The S-meter is an instrument on most radio receivers that measures the strength of the signal being received. It uses a unit: the S-point. HF radios usually have a needle scale, whereas other bands often have LEDs displaying the S-Points.
S-Points is the common method of measuring signal strength between operators, however many programs like WSJTX use dB. So how do you convert between dB and S?
S-points go from S1 to S9, and each S-point is defined as a 6 dB change in signal strength. Therefore, each time the voltage is halved (–6 dB) the signal strength decreases by one point. S9 is already a very strong signal, but to describe larger signals, steps of 10 dB are used instead of 6 dB, noted “S9+20” meaning 20 dB above S9.
S-points for frequencies below 30 MHz
| Signal strength | Relative intensity |
| S1 | –48 dB |
| S2 | –42 dB |
| S3 | –36 dB |
| S4 | –30 dB |
| S5 | –24 dB |
| S6 | –18 dB |
| S7 | –12 dB |
| S8 | –6 dB |
| S9 | 0 dB |
| S9+10 | 10 dB |
| S9+20 | 20 dB |
| S9+30 | 30 dB |
| S9+40 | 40 dB |
| S9+50 | 50 dB |
| S9+60 | 60 dB |
S-points for frequencies above 30 MHz:
| Signal strength | Relative intensity |
| S1 | –48 dB |
| S2 | –42 dB |
| S3 | –36 dB |
| S4 | –30 dB |
| S5 | –24 dB |
| S6 | –18 dB |
| S7 | –12 dB |
| S8 | –6 dB |
| S9 | 0 dB |
| S9+10 | 10 dB |
| S9+20 | 20 dB |
| S9+30 | 30 dB |
| S9+40 | 40 dB |
| S9+50 | 50 dB |
| S9+60 | 60 dB |
Hopefully, this helps answer the question how to convert a dB signal strength to a S1 to S9 signal strength.
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