Published on 10/13/2026
Convert dBm to watts with P(W) = 10^((dBm - 30)/10), or to milliwatts with P(mW) = 10^(dBm/10). The reverse calculation is dBm = 10 log10(P(W)) + 30 for positive power in watts.
A radio specified at 30 dBm supplies 1 W at its stated output reference plane. If its antenna feeder loses 3 dB, the antenna receives 27 dBm, about 0.501 W. That conversion is useful when checking power delivery, but it does not establish the cable's thermal or voltage rating.
Use the table and formula workflow below as a manual calculator. There is no executable widget in this body-only article; the CMS does not run scripts. Values are calculated conversions, not SigmaRF test measurements.
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dBm is an absolute power level referenced to 1 mW. It is not an impedance, a voltage or a power ratio by itself. A negative dBm reading still represents positive power: it is simply below 1 mW.
| Conversion | Formula | Input restriction |
|---|---|---|
| dBm to watts | P(W) = 10^((dBm - 30)/10) | Any finite dBm value |
| dBm to milliwatts | P(mW) = 10^(dBm/10) | Any finite dBm value |
| Watts to dBm | dBm = 10 log10(P(W)) + 30 | Power must be greater than 0 W |
| dBm to dBW | dBW = dBm - 30 | Different absolute reference |
Key idea: The dBm-to-watts conversion does not require a 50 ohm assumption. Converting that power to a voltage does require the load and the relevant measurement conditions.
The table rounds calculated values to useful engineering precision. At 3 dBm the power is 1.995 mW, not exactly 2 mW; at 33 dBm it is 1.995 W. The familiar 3 dB doubling rule is an approximation.
| Power in dBm | Power in mW | Power in W |
|---|---|---|
| -100 dBm | 1e-10 mW | 1e-13 W |
| -90 dBm | 1e-09 mW | 1e-12 W |
| -80 dBm | 1e-08 mW | 1e-11 W |
| -60 dBm | 1e-06 mW | 1e-09 W |
| -40 dBm | 0.0001 mW | 1e-07 W |
| -30 dBm | 0.001 mW | 1e-06 W |
| -20 dBm | 0.01 mW | 1e-05 W |
| -10 dBm | 0.1 mW | 0.0001 W |
| 0 dBm | 1 mW | 0.001 W |
| 3 dBm | 1.99526 mW | 0.00199526 W |
| 6 dBm | 3.98107 mW | 0.00398107 W |
| 10 dBm | 10 mW | 0.01 W |
| 13 dBm | 19.9526 mW | 0.0199526 W |
| 17 dBm | 50.1187 mW | 0.0501187 W |
| 20 dBm | 100 mW | 0.1 W |
| 23 dBm | 199.526 mW | 0.199526 W |
| 27 dBm | 501.187 mW | 0.501187 W |
| 30 dBm | 1000 mW | 1 W |
| 33 dBm | 1995.26 mW | 1.99526 W |
| 37 dBm | 5011.87 mW | 5.01187 W |
| 40 dBm | 10000 mW | 10 W |
| 43 dBm | 19952.6 mW | 19.9526 W |
| 47 dBm | 50118.7 mW | 50.1187 W |
| 50 dBm | 100000 mW | 100 W |
The weak-signal rows use scientific notation to avoid long strings of zeros. For example, 1e-12 W means 0.000000000001 W. Converting -90 dBm incorrectly to 1 nW introduces a 30 dB error: the correct value is 1 pW.
Step 01
Subtract 30 when converting to watts
For 43 dBm, the exponent is (43 - 30)/10 = 1.3. Do not divide the whole power expression by 10 after exponentiation.
Step 02
Raise 10 to that exponent
10^1.3 = 19.9526 W. Use the exponent or power function on your calculator, not the natural-exponential function e^x.
Step 03
Reverse-check the result
10 log10(19.9526) + 30 is approximately 43 dBm. Keep enough digits during the calculation, then round the final answer to the accuracy of the power measurement.
Specify the loss you can afford, not only the cable name
Send SigmaRF your frequency band, connector interfaces, installed length and insertion-loss target. Every assembly is VNA-swept for VSWR and insertion loss before shipment.
Specify a Custom AssemblyFor a matched feeder estimate, antenna input power in dBm equals radio output in dBm minus complete feeder insertion loss in dB. If the input is in watts, multiply by 10^(-loss/10). Both routes give the same result when the reference planes and assumptions match.
| Input power | Feeder loss | Output power |
|---|---|---|
| 30 dBm = 1 W | 1 dB | 29 dBm = 0.794 W |
| 30 dBm = 1 W | 3 dB | 27 dBm = 0.501 W |
| 30 dBm = 1 W | 6 dB | 24 dBm = 0.251 W |
Do not subtract 3 W when the cable loss is 3 dB. The loss is a ratio, so its dissipated power depends on input power. At substantial RF power, check the assembly rating at frequency, ambient temperature, duty cycle and mismatch conditions rather than relying only on this arithmetic.
Read SigmaRF's RF Link Budget Basics for antenna gains and path loss, and its dB, dBm, dBi and dBd Explained guide for unit references. This page is the numerical power conversion reference.
Illustrative transmitter and feeder: radio output 37 dBm with 2 dB complete feeder insertion loss. Assume the loss applies at the operating frequency and use a matched power estimate.
In this simplified matched estimate, about 63.1% of the input power reaches the antenna reference plane. A real system may also involve mismatch and reflected power; do not treat the difference as a verified cable-temperature prediction.
How many watts is 30 dBm?
30 dBm equals 1 W, or 1,000 mW. The watt conversion is 10^((30 - 30)/10) = 1.
How many milliwatts is 20 dBm?
20 dBm equals 100 mW, or 0.1 W. Each 10 dB increase multiplies power by 10.
Is -90 dBm the same as 1 nanowatt?
No. -90 dBm equals 1 picowatt, or 1e-12 W. 1 nanowatt is -60 dBm, so confusing them produces a 30 dB error.
Can I add power levels in dBm?
To add independent powers, convert each to watts or milliwatts, add those linear powers, then convert the total to dBm. Coherent signal combining also depends on phase.
How do I convert watts to dBm?
For positive power in watts, use 10 log10(P) + 30. For example, 5 W is about 36.99 dBm. Zero watts has no finite logarithmic dBm value.
Does a 3 dB cable loss always remove half a watt?
No. It leaves about half the input power, so the lost watts depend on the input. From 1 W, about 0.501 W remains; from 10 W, about 5.012 W remains.
Keep the reference explicit: dBm uses 1 mW, watts are linear power, and dB expresses a ratio. Subtract cable loss in dB from a dBm level, or multiply watts by the equivalent linear loss factor.
For a power-bearing cable specification, give SigmaRF the operating frequency, CW or peak power, duty cycle, ambient temperature and maximum insertion loss. Every assembly is VNA-swept for VSWR and insertion loss before shipment. SigmaRF supports requirements from 100 MHz to 40 GHz through Vadodara volume and Cleveland-area precision facilities, with lead times as short as 2 days for suitable builds.
Specify the loss you can afford, not only the cable name
Send SigmaRF your frequency band, connector interfaces, installed length and insertion-loss target. Every assembly is VNA-swept for VSWR and insertion loss before shipment.
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