S-meter calculator: S-points, dBm, microvolts

A scale almost nobody keeps to

Under IARU one S-point is exactly six decibels, and S9 on HF is minus seventy-three dBm. In fact most transceivers are uncalibrated: the real step is 3–4 dB, and S9 differs between radios by a good ten decibels. So “five nine” is a ritual, not a measurement.

One S-point is six decibels; above S9 the scale carries on in «plus» figures

What to enter	S-meter pointsLevel in dBmVoltage in microvolts
Scale	HF, up to 30 MHz — S9 = −73 dBmVHF, above 30 MHz — S9 = −93 dBm
Points (1…9)
Plus above S9 (dB)
System impedance (Ω)
Result
Level
Voltage
Level in dBµV
Power
On the HF scale
On the VHF scale

📐 The formulas

The IARU Region 1 scale:

HF (below 30 MHz):  S9 = −73 dBm = 50.1 µV into 50 Ω
VHF (above 30 MHz): S9 = −93 dBm = 5.01 µV
One S-point = 6 dB;  S1 = S9 − 48 dB

Conversions:

U[µV] = 10⁶ · √( 50 · 10^((P[dBm] − 30)/10) )
dBµV = dBm + 107        (for 50 Ω)

Reference points:

Level	dBm	µV	dBµV
S1 (HF)	−121	0.20	−14
S9 (HF)	−73	50.1	34
S9+20 (HF)	−53	501	54
S9 (VHF)	−93	5.01	14
0 dBm	0	223,600	107

Where this calculator stops:

• Real transceivers are not calibrated to IARU — the calculator shows the reference scale, not what your radio will display
• The HF and VHF scales differ by exactly 20 dB; at 50 MHz the VHF scale formally applies
• The dBm → dBµV step depends on the system impedance: +107 dB holds only for 50 Ω
• The scale says nothing below S1 or above S9+60

🎯 In practice

• Turning a report into a level and back again.
• Calibrating your own S-meter with a generator of known output.
• Reading datasheets where sensitivity is given in microvolts and the noise floor in dBm.

Practical advice:

1. The gap between S7 and S9 ought to be 12 dB. If your radio shows 6, the scale is compressed by half, and that is normal.
2. To compare antennas use the built-in attenuator rather than S-points: it is better calibrated than the meter.
3. In digital modes a report is a signal-to-noise ratio in decibels — an entirely different quantity.
4. 0 dBm = 223.6 mV into 50 Ω is a handy number for checking any level calculator.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator

UR3PKI. «S-meter calculator: S-points, dBm, microvolts». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/s_meter (licence CC BY-NC-SA 4.0).

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