dB Calculator
Convert between dBm, watts, power and voltage ratios, and S-meter readings — including the HF S9 reference. Everything runs locally in your browser. dBm、瓦特、比值与 S 表读数互换，所有计算都在本地完成。
dBm / WattsdBm 与瓦特互换
dB Gain / LossdB 增益 / 衰减
S-meter to dBmS 表读数转 dBm
Quick Reference常用速查
| dB | Power ratio 功率比 | Voltage ratio 电压比 |
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Decibels run the whole of radio engineering, and fluency with them turns guesswork into arithmetic you can do in your head. This calculator covers the four conversions a ham reaches for most: absolute power between dBm and watts, gain or loss between dB and power/voltage ratios, and S-meter readings converted to real signal levels.
The most useful landmarks are few. Doubling power is +3 dB; ten times the power is +10 dB; halving is −3 dB. On the absolute scale, 0 dBm is one milliwatt, so a 100 W HF transceiver peaks at +50 dBm and a 5 W handheld at about +37 dBm. Once you hold those anchors, station math becomes addition: a 100 W radio (+50 dBm) into an antenna with 6 dB gain through 2 dB of coax loss delivers the punch of roughly +54 dBm in the favoured direction — no multiplication required.
The gain/loss section converts the other way. Enter a gain in dB to see both the power ratio and the voltage ratio, or enter a known ratio to get its dB value. Keep the two formulas straight: power uses 10·log10 of the ratio, voltage uses 20·log10. Mixing them up is the most common decibel mistake — it is how a harmless “6 dB pad” (quarter power, half voltage) turns into a surprise.
The S-meter section uses the standard HF convention: S9 equals −73 dBm, with 6 dB per S-unit below S9 and any “over S9” amount added on top. It also shows the equivalent voltage across a 50 Ω input, which is how older receiver specifications are written — S9 is 50 µV. Bear in mind that many production radios drift several dB from this ideal; the converter tells you what a reading should mean, not what your particular meter guarantees.
Pair this tool with the SWR Calculator when you are budgeting a station: mismatch loss and feed-line loss from those results drop straight into dB sums here.
Frequently asked questions
Why does radio work use dB instead of watts?
Decibels are logarithmic: +3 dB doubles power, +10 dB multiplies it by ten, −3 dB halves it. They let you add up gains and losses through a whole station — antenna gain minus coax loss minus mismatch loss — instead of multiplying ratios at every step.
What is the difference between dB and dBm?
Decibels compare two values; dBm pins one end of the comparison to 1 milliwatt, so a dBm figure is an absolute power: 0 dBm = 1 mW, 30 dBm = 1 W, 50 dBm = 100 W. Use plain dB for gains and losses, dBm for actual power levels.
What power level is S9?
By the IARU convention, S9 on HF is a −73 dBm signal at the receiver input (50 µV into 50 Ω), and each S-unit below S9 is 6 dB weaker: S8 = −79 dBm, S7 = −85 dBm, and so on. Many real-world S-meters are only loosely calibrated, so treat reported values as approximate.
Why is it 10·log for power but 20·log for voltage?
Because voltage ratios ignore impedance. dB for power uses 10·log10, for voltage 20·log10 — the two agree only when the impedance is the same on both sides. In a 50 Ω system, doubling the voltage quadruples the power, which is why 6 dB means 2× voltage but 4× power.