dBm to Watts Converter (and Why +3 dB Doubles Your Power)
To convert dBm to watts, first find milliwatts with P(mW) = 10^(dBm/10), then divide by 1000 for watts. Going the other way, dBm = 10 * log10(mW). The key shortcut: every +3 dB doubles power and every +10 dB multiplies it by 10. So 5 W is about 37 dBm, and a 3 dB gain antenna effectively doubles your radiated power.
Enter a value and unit to convert between watts, milliwatts, and dBm.
dBm (decibels relative to one milliwatt) is how RF engineers, radio spec sheets, and coax loss tables express power. It looks intimidating, but the whole system rests on two formulas and one habit of thinking in ratios instead of raw watts. Once you can move between dBm and watts in your head, antenna gain, feedline loss, and effective radiated power stop being mysteries.
This page gives you the exact math, a conversion table for common ham power levels, and the shortcuts that let you estimate without a calculator. It pairs naturally with our coax loss calculator, where every result is already expressed in dB.
dBm vs watts: what the unit actually means
A watt is an absolute amount of power. A dBm is a logarithmic measure of power referenced to 1 milliwatt (0.001 W). Zero dBm equals exactly 1 mW. Positive dBm values are more power than a milliwatt; negative dBm values are less.
Why bother with a log scale? Because in radio, power spans an enormous range, from a 100,000 W broadcast transmitter down to a received signal of 0.000000001 mW. Logarithms compress that range into readable numbers, and they turn multiplication into addition. When you add an antenna, lose some signal in coax, and gain some in a preamp, you simply add and subtract dB values instead of multiplying awkward fractions.
That additive property is the entire reason spec sheets and link budgets use dB. A +6 dB antenna feeding through a -2 dB coax run nets you +4 dB, no calculator required.
The two formulas you need
Everything comes down to these:
- dBm to milliwatts:
P(mW) = 10^(dBm / 10) - Milliwatts to dBm:
dBm = 10 * log10(mW)
To get watts, remember there are 1000 mW in a watt. So convert to milliwatts first, then divide by 1000. To go from watts to dBm, multiply watts by 1000 to get milliwatts, then take 10 * log10.
Worked example. Your HT runs 5 watts. That is 5000 mW. dBm = 10 * log10(5000) = 10 * 3.699 = 36.99 dBm, which everyone rounds to 37 dBm. Going backward: 37 dBm means 10^(37/10) = 10^3.7 = about 5012 mW, or 5 W. The math is reversible and self-checking.
Why +3 dB doubles power (and +10 dB is 10x)
This is the single most useful thing to memorize. Because dB is base-10 logarithmic, a few ratios recur constantly:
| Change in dB | Power multiplier | Meaning |
|---|---|---|
| +3 dB | x2 | Double the power |
| -3 dB | x0.5 | Half the power |
| +6 dB | x4 | Double, then double again |
| +10 dB | x10 | Ten times the power |
| +20 dB | x100 | Two 10x jumps |
What does 3 dB mean? It is the dB value where the power ratio is 2 (technically 3.0103 dB, always rounded to 3). Log10(2) = 0.301, and 10 * 0.301 = 3.01. That is why the number 3 shows up everywhere in RF: half-power bandwidth, filter roll-off, and antenna beamwidth are all quoted at the -3 dB points.
You can chain these. Need +13 dB? That is +10 (x10) plus +3 (x2), so 20x total. Need +9 dB? Three +3 dB steps, so 2 * 2 * 2 = 8x. This mental arithmetic is faster and more intuitive than punching the formula, and it is exactly how experienced hams reason about gain.
dBm to watts conversion table for ham power levels
These are the power levels you will actually encounter, from a Baofeng on low power to an HF base station at the legal limit.
| dBm | Power | Typical use |
|---|---|---|
| 0 dBm | 1 mW | Reference point |
| 20 dBm | 0.1 W (100 mW) | HT low power, some QRP beacons |
| 27 dBm | 0.5 W | FRS radios, HT lowest setting |
| 30 dBm | 1 W | HT low power |
| 37 dBm | 5 W | Most HT high power (UV-5R) |
| 39 dBm | 8 W | BF-F8HP high power |
| 44 dBm | 25 W | Small mobile / VHF-UHF |
| 47 dBm | 50 W | Typical mobile transceiver |
| 50 dBm | 100 W | HF base station (IC-7300) |
| 56.4 dBm | ~440 W | Near legal-limit amplifier region |
| 60 dBm | 1000 W (1 kW) | US ham legal max is 1500 W PEP |
Notice how little dBm changes for big watt jumps. 5 W to 50 W (10x the power) is only +10 dB. That is worth remembering when you are tempted to buy a bigger radio: going from 5 W to 8 W is barely +2 dB, well under the +3 dB you would need to double your power. Antenna and height almost always beat raw watts.
ERP, antenna gain, and effective radiated power
Antenna gain is quoted in dB (dBi relative to an isotropic radiator, or dBd relative to a dipole; dBi = dBd + 2.15). Because gain is already in dB, you just add it to your transmitter power in dBm to estimate effective radiated power (ERP).
Example. A 5 W HT (37 dBm) feeding a 3 dBd gain antenna radiates 37 + 3 = 40 dBm ERP, which is 10 W equivalent in the favored direction. The antenna created no new power; it focused the existing 5 W, trading coverage in other directions for gain toward the horizon. That is why a good gain antenna often outperforms a higher-wattage radio on a rubber duck.
Feedline loss subtracts. If that same setup ran through coax with 2 dB of loss, your net would be 37 - 2 + 3 = 38 dBm. This add-and-subtract link budget is the practical payoff of thinking in dB. See ham radio antenna basics for how gain patterns actually work, and remember that gain figures on cheap whips are frequently exaggerated.
Common mistakes and gotchas
- Confusing dB with dBm. dBm is an absolute power (referenced to 1 mW). Plain dB is a ratio between two powers. Antenna gain and coax loss are in dB; transmitter output is in dBm.
- Forgetting the milliwatt step. The formula gives milliwatts, not watts. Divide by 1000. 37 dBm is 5000 mW = 5 W, not 5000 W.
- Adding watts instead of dB. You cannot add watts through a chain of gains and losses in a simple way, but you can always add and subtract dB. That is the whole point.
- Treating 3 dB as exactly double. It is 3.01 dB, but 3 is close enough for field work.
- Assuming more watts equals proportionally more range. Doubling power (+3 dB) typically adds only a small percentage to line-of-sight range. Height and antenna gain matter far more, as our radio range calculator shows.
Quick mental-math method (no calculator)
Break any dBm value into 10s and 3s, starting from 0 dBm = 1 mW:
- Every +10 dB multiplies by 10.
- Every +3 dB multiplies by 2.
- Add the leftover (+1 dB is about x1.26, +2 dB about x1.58) only if you need precision.
Try 47 dBm. That is +10 +10 +10 +10 +3 +3 +1 from 0 dBm. Four tens = x10000, two threes = x4, so 40000 mW, times roughly 1.26 for the +1 = about 50000 mW = 50 W. Matches the table. With practice you can eyeball any spec-sheet dBm figure in a couple of seconds.