Published on 10/13/2026
A radio datasheet lists transmit power as 23 dBm, an antenna as 5 dBi, and a cable loss of 1.5 dB. A new engineer asks if she can add them all. She can, but only if she knows which are absolute and which are relative, and that mixing the two the wrong way gives nonsense.
RF work runs on logarithmic units because signals span ten or more orders of magnitude. The decibel turns multiplication into addition, which makes link budgets and cable loss calculations easy. The trouble is that the letters after dB change the meaning.
This cheat sheet defines each unit, gives the formulas and a conversion table, lists the rules you use daily, and ends with a worked example. For how loss appears on cables see our article on insertion loss, return loss and VSWR.
On This Page
A decibel compares two powers. It has no absolute level.
| dB change | Power ratio | Meaning |
|---|---|---|
| +3 dB | 2x | Double the power |
| +10 dB | 10x | Ten times the power |
| +20 dB | 100x | One hundred times |
| -3 dB | 0.5x | Half the power |
| -10 dB | 0.1x | One tenth |
| -20 dB | 0.01x | One hundredth |
| dBm | Power | Typical meaning |
|---|---|---|
| -30 dBm | 1 microwatt | Weak received signal |
| -10 dBm | 100 microwatt | VNA port level |
| 0 dBm | 1 mW | Reference |
| 10 dBm | 10 mW | Bluetooth class |
| 20 dBm | 100 mW | Wi-Fi client |
| 30 dBm | 1 W | Small amplifier |
| 40 dBm | 10 W | Handheld radio |
| 43 dBm | 20 W | PIM test tone, cellular carrier |
| 50 dBm | 100 W | Base station amplifier |
Because dBm is absolute, you can add or subtract dB from it: 20 dBm minus 3 dB is 17 dBm (half the power). You cannot add two dBm values: 20 dBm plus 20 dBm is not 40 dBm. It is 23 dBm, because two equal powers sum to double power.
Antenna gain is relative to a reference antenna. dBi uses an ideal isotropic radiator, dBd uses a half-wave dipole. A dipole has 2.15 dB more gain than an isotropic source.
| Unit | Reference | Used for |
|---|---|---|
| dBc | Relative to the carrier | Spurious signals, phase noise, PIM |
| dBW | 1 watt (dBm minus 30) | High power, broadcast |
| dBuV | 1 microvolt (in 50 ohm: dBm + 107) | EMC and receiver sensitivity |
| dBmV | 1 millivolt | CATV and 75 ohm video systems |
| dBFS | Full scale of a converter | ADC and DAC levels |
| dBHz | Relative to 1 Hz | Noise density |
Cable loss is subtracted in dB, antenna gain is added in dBi, and the result is EIRP in dBm. For lower loss, a shorter cable or a larger cable would lift the EIRP. See our cable selection guide for loss by cable type.
Rules
Mistakes
What is the difference between dB and dBm?
dB is a ratio between two powers. dBm is an absolute power referenced to 1 milliwatt.
How do you convert dBm to watts?
Power in mW is 10 raised to dBm divided by 10. Then divide by 1000 for watts. 30 dBm is 1 W, 20 dBm is 100 mW.
What is 0 dBm in watts?
1 milliwatt, or 0.001 W.
What is the difference between dBi and dBd?
dBi is gain relative to an isotropic radiator, dBd relative to a half-wave dipole. dBi equals dBd plus 2.15.
Can you add dBm values?
Not directly. Convert to mW, add, then convert back. Two equal signals add 3 dB.
How many dB is double the power?
3 dB. A power ratio of 2 is 3.01 dB.
What is dBc?
dB relative to the carrier power. It is used for spurious signals, phase noise and intermodulation.
How do I convert dBW to dBm?
Add 30. 0 dBW is 30 dBm, which is 1 W.
Keep the references straight: dB is a ratio, dBm is power, dBi and dBd are antenna gain with a 2.15 dB offset, and dBc is relative to carrier. With 3 dB and 10 dB in your head, most RF arithmetic is addition.
SigmaRF assemblies are VNA-swept for insertion loss and VSWR in dB before shipment, 100 MHz to 40 GHz, so the numbers on your link budget match the cable that arrives. Our India (Vadodara) and USA (Cleveland area) facilities ship with lead times as short as 2 days.
Building a link budget?
Tell us your frequency and length and we will propose the cable.
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