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The S-meter scale (IARU R.1)
The only widely used definition of S-units is IARU Region 1 Technical Recommendation R.1 (adopted 1981). It fixes S9 at a set power into the receiver's 50 Ω input and makes each S-unit 6 dB. The table is calculated from that definition, not copied from any meter.
| Reading | HF (below 30 MHz) | VHF/UHF (above 144 MHz) |
|---|---|---|
| S9+40 | -33 dBm · 5006 µV | -53 dBm · 501 µV |
| S9+20 | -53 dBm · 501 µV | -73 dBm · 50 µV |
| S9+10 | -63 dBm · 158 µV | -83 dBm · 16 µV |
| S9 | -73 dBm · 50 µV | -93 dBm · 5.01 µV |
| S8 | -79 dBm · 25 µV | -99 dBm · 2.51 µV |
| S7 | -85 dBm · 13 µV | -105 dBm · 1.26 µV |
| S6 | -91 dBm · 6.3 µV | -111 dBm · 0.63 µV |
| S5 | -97 dBm · 3.16 µV | -117 dBm · 0.316 µV |
| S4 | -103 dBm · 1.58 µV | -123 dBm · 0.158 µV |
| S3 | -109 dBm · 0.793 µV | -129 dBm · 0.0793 µV |
| S2 | -115 dBm · 0.398 µV | -135 dBm · 0.0398 µV |
| S1 | -121 dBm · 0.199 µV | -141 dBm · 0.0199 µV |
Worked example
A station on 20 m gives you S7. On the HF scale that is −73 + (7 − 9) × 6 = −85 dBm, about 12.6 µV across 50 Ω, or 3.2 picowatts. To move you to S9 they would need 12 dB more, which is about 16 times the power (100 W to roughly 1.6 kW), so turning up from 100 W to 400 W only buys one S-unit.
Why your radio's meter may disagree
- Calibration: R.1 is a recommendation. Most transceiver S-meters are not calibrated to it, often read high, and many give fewer than 6 dB per S-unit near the bottom of the scale.
- Preamp and attenuator: a preamp makes the meter read higher and an attenuator lower, without changing the signal on the air.
- 6 m and other 30–144 MHz bands: R.1 names an HF scale (below 30 MHz) and a VHF scale (above 144 MHz) only. We default to the VHF scale there and let you switch.
Signal reports and S-meters are not in the FCC rules; Part 97 does not define them. For on-air rules, see Can I transmit here? and the station ID builder.
Sources
- IARU Region 1, Technical Recommendation R.1 (S-meter calibration: S9 = −73 dBm HF, −93 dBm VHF, 6 dB per S-unit), in the IARU Region 1 VHF and HF managers' handbooks
- S meter (background, including the R.1 values)
- dBm definition (power relative to 1 mW)
Formulas checked against R.1 and our worked-example tests on 2026-10-23. Planning aid, not a calibration standard.