Ham Radio S-Meter and Signal Strength Reference
The S-meter is the signal strength indicator on your receiver, calibrated in S-units from S1 through S9. Each S-unit represents a 6 dB change in signal level, meaning S9 is 48 dB stronger than S1. This page covers the complete S-unit reference table, the dBm equivalents, typical received signal levels, and a converter between S-units and dBm.
Interactive S-MeterClick any bar to see the signal description
The IARU standard defines S9 as -73 dBm into a 50 ohm input on HF bands. Each S-unit step is 6 dB. Signals above S9 are reported in dB over S9: S9+10, S9+20, S9+30, S9+40, S9+60.
| S | Description | HF (dBm) | VHF (dBm) | Signal bar |
|---|
| Reading | dB over S9 | HF (dBm) | Typical description |
|---|---|---|---|
| S9+10 | 10 dB | -63 dBm | Very strong -- full quieting on most receivers |
| S9+20 | 20 dB | -53 dBm | Extremely strong -- local station or strong DX |
| S9+30 | 30 dB | -43 dBm | Overpowering -- nearby high-power station |
| S9+40 | 40 dB | -33 dBm | Rare -- very close powerful transmitter |
| S9+60 | 60 dB | -13 dBm | Extremely rare -- almost always a local station |
Convert Between S-Units and dBm
The IARU (International Amateur Radio Union) published a recommendation for S-meter calibration that defines S9 as a signal producing 50 microvolts across a 50 ohm input impedance, which equals -73 dBm. Each S-unit step represents exactly 6 dB of signal level change. This standard is widely followed but not universally implemented — some manufacturers calibrate their meters differently, and many S-meters become progressively less accurate above S7 or S8 due to AGC circuit characteristics.
On VHF and UHF, the IARU definition changes: S9 is defined as -93 dBm (5 microvolts across 50 ohms) rather than the HF value of -73 dBm. This means an S9 signal on 2 metres is actually 20 dB weaker in absolute terms than an S9 signal on 20 metres. The 6 dB per S-unit relationship remains the same on both bands.
In practice, S-meter readings are most useful as relative measurements rather than absolute ones. If a station reports S5 this week and S7 next week after you raised your antenna, the 12 dB improvement is meaningful regardless of whether the absolute calibration is perfect. Comparing S-meter readings between different receivers requires caution because calibration varies significantly.
Typical Received Signal LevelsUnderstanding what signal levels correspond to real-world situations helps interpret S-meter readings correctly:
| Situation | Typical level | S-unit (HF) |
|---|---|---|
| Noise floor of a good HF receiver | -130 to -120 dBm | Below S1 |
| Weak FT8 decode (just above noise) | -115 to -110 dBm | S1-S2 |
| Weak CW signal, copyable with effort | -110 to -100 dBm | S2-S3 |
| Typical transatlantic HF path | -100 to -85 dBm | S3-S6 |
| Good DX contact on 20m | -90 to -75 dBm | S5-S8 |
| Strong domestic HF station | -75 to -65 dBm | S8-S9 |
| Local station on 40m at night | -65 to -50 dBm | S9 to S9+20 |
| Local VHF FM repeater | -80 to -60 dBm | Full quieting |
| Nearby 100W HF station | -50 to -30 dBm | S9+30 to S9+50 |
When a signal is stronger than S9, operators report it as “S9 plus” followed by the number of decibels over S9. Most modern transceivers extend the S-meter scale to show S9+10, S9+20, S9+40 and S9+60 dB over. On phone you would say “you are S9 plus 20” or “you are 20 over S9.” On CW the common shorthand is simply the number: “UR 59 + 20” or in RST reporting “599 + 20.”
A signal of S9+60 dB is extremely rare outside of local contacts and indicates the antenna is receiving approximately 1 milliwatt of signal — enormous by RF standards. Most receivers begin to have AGC and front-end compression problems at levels above S9+30 to S9+40, which can actually degrade readability even as the signal strength increases.
Other Reference Pages