dBm to Watts Calculator
Convert power in either direction. Enter a dBm value to get watts and milliwatts, or enter a power level to get dBm.
How this calculator works
The two input fields are linked: type a dBm value and the power field updates instantly. Type a power (in your choice of µW, mW, W, or kW) and the dBm field updates instead. The result panel always shows the same power expressed four ways, so you can read off whichever unit the datasheet, spec sheet, or signal meter in front of you uses.
Under the hood there is exactly one conversion: dBm is ten times the base-10 logarithm of the power in milliwatts. Everything else (the watts and microwatt readouts, the reference table below) is that single formula run forward or backward.
The formula
P(dBm) = 10 × log₁₀( P ÷ 1 mW ) P(mW) = 10^( dBm ÷ 10 )
The reference power is always 1 milliwatt. That's the "m" in dBm. Common values worth memorizing:
| dBm | Power | Typical of |
|---|---|---|
| -30 | 1 µW | |
| -20 | 10 µW | |
| -10 | 1 µW | |
| 0 | 1 mW | the reference point |
| +10 | 10 mW | |
| +20 | 1 mW | typical Wi-Fi router |
| +23 | 2 mW | typical cell phone max |
| +27 | 501 mW | high-power Wi-Fi / LoRa amp |
| +30 | 1 W | 1 watt |
| +33 | 2 W | |
| +37 | 5.01 W | handheld ham radio (5 W) |
| +40 | 10 W | |
| +47 | 50.1 W | 50 W mobile radio |
| +50 | 1 W | 100 W base station |
Worked example
Say a datasheet quotes a transmitter at +27 dBm and you want watts:
- Divide by 10: 27 ÷ 10 = 2.7
- Raise ten to that power: 10^2.7 = 501.2 mW
- Convert: ≈ 0.5 W
The mental-math route is faster: 27 dBm = 30 dBm − 3 dB. Since 30 dBm is 1 W and −3 dB halves it, 27 dBm ≈ 500 mW, matching the exact calculation to within a quarter of a percent.
Assumptions & tips
- Add dB to dBm, never dBm to dBm. A 10 dBm signal through a 6 dB attenuator is 4 dBm. That works because dB is a ratio. Adding two absolute powers (say, two transmitters) requires converting to watts first, adding, and converting back.
- Learn the three anchors. 0 dBm = 1 mW, +30 dBm = 1 W, and every ±3 dB is a factor of two. Nearly every conversion you'll meet in practice is reachable from those in two steps.
- Watch for dBW in the same document. Satellite and broadcast work often uses dBW (referenced to 1 W, not 1 mW): dBW = dBm − 30. Mixing them up is a thousand-fold error.
- Receive levels are negative. That's fine. −70 dBm into a receiver is a comfortable signal. The negative sign is not a problem to fix. It's just the log scale below one milliwatt.
- Regulatory limits are often EIRP. A rule that says "+36 dBm EIRP" limits transmitter power plus antenna gain minus cable loss, not the transmitter alone. Do the whole budget in dB before comparing to the limit.
Frequently asked questions
What is the difference between dB and dBm?
dB expresses a ratio (a gain or a loss) with no units attached: an amplifier has 20 dB of gain, a cable has 3 dB of loss. dBm is an absolute amount of power. The two combine by simple addition: 10 dBm into a 20 dB amplifier gives 30 dBm out. That additive arithmetic is the whole reason RF engineers work in decibels.
Why does adding 3 dB double the power?
Because 10 × log₁₀(2) ≈ 3.01. Every +3 dB multiplies power by two and every −3 dB halves it; +10 dB is exactly ten times the power. These two facts let you convert most values in your head: 27 dBm is 30 dBm (1 W) minus 3 dB, so 500 mW.
Can dBm be negative?
Yes: negative dBm means less than a milliwatt, and most received signals are. A Wi-Fi receiver showing −70 dBm is seeing 100 picowatts. A GPS signal at the antenna is around −127 dBm, about 0.2 femtowatts. Negative dBm is normal. Negative watts are impossible.
Sources
- Recommendation ITU-R V.574-5: Use of the decibel and the neper in telecommunications. International Telecommunication Union, 2015. itu.intThe telecommunications convention for absolute power level in decibels (§6.1), which is what makes "dBm" mean power referred to one milliwatt.
- NIST Special Publication 811, Guide for the Use of the International System of Units, §8.7 — Logarithmic quantities and units: level, neper, bel. National Institute of Standards and Technology, 2008. nist.govDefines level of a power quantity as 10 log₁₀(P/P₀) decibels and requires the reference power P₀ to be stated: the relation this calculator runs in both directions.
- Letter symbols to be used in electrical technology, Part 3: Logarithmic and related quantities and their units, IEC 60027-3. International Electrotechnical Commission, 2003. The underlying standard for decibel notation, which NIST SP 811 §8.7 names as the authority its treatment of the neper, bel and decibel is based on.
- 47 CFR §15.247 — Operation within the bands 902–928 MHz, 2400–2483.5 MHz, and 5725–5850 MHz. Federal Communications Commission, 2023 CFR edition. govinfo.govThe one-watt (30 dBm) conducted output limit and the rule that antenna gain above 6 dBi must be traded against transmitter power: the reason the tips warn that regulatory limits are stated as EIRP, not transmitter output.
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