dB to Watts Converter
A level of 30 dBm is exactly 1 watt, and 30 dBW is 1,000 watts.
Quick answer: Divide the dB by 10, raise 10 to that power, then multiply by the reference power. For example, 30 dBm is 1 W, 0 dBm is 0.001 W and 10 dBW is 10 W.
The dB to watts formula
Power equals the reference power times 10 to the power of dB divided by 10.
The reference is 1 milliwatt for dBm and 1 watt for dBW.
Take 30 dBm. 1 mW × 10^(30 ÷ 10) gives 1 W.
Take 0 dBm. 1 mW × 10^(0 ÷ 10) gives 0.001 W.
A dB level needs a reference
A dB value only has meaning with a reference, so you must always state it (Engineering ToolBox: Decibel).
dBm uses 1 milliwatt and dBW uses 1 watt, so 1 dBW is the same as 30 dBm (Wikipedia: Decibel).
How to convert dB to watts step by step
- Read the level in dB and note whether it is dBm or dBW.
- Divide the dB by 10.
- Raise 10 to that power.
- Multiply by 1 mW for dBm or 1 W for dBW, then read watts or milliwatts.
Example 1: 30 dBm
- Start with 30 dBm.
- 1 mW × 10^(30 ÷ 10) = 1.
- The power is 1 W.
30 dBm = 1 W
Example 2: 0 dBm
- Start with 0 dBm.
- 1 mW × 10^(0 ÷ 10) = 0.001.
- The power is 0.001 W.
0 dBm = 0.001 W
Example 3: 10 dBW
- Start with 10 dBW.
- 1 W × 10^(10 ÷ 10) = 10.
- The power is 10 W.
10 dBW = 10 W
The three examples below use common sizes.
Each one shows the sum in full.
dB to watts conversion table
| Level | Power (W) | Power (mW) |
|---|---|---|
| 0 dBm | 0.001 W | 1 mW |
| 3 dBm | 0.001995 W | 1.995 mW |
| 10 dBm | 0.01 W | 10 mW |
| 13 dBm | 0.02 W | 19.953 mW |
| 20 dBm | 0.1 W | 100 mW |
| 27 dBm | 0.5012 W | 501.19 mW |
| 30 dBm | 1 W | 1,000 mW |
| 43 dBm | 19.953 W | 19,952.62 mW |
| 0 dBW | 1 W | 1,000 mW |
| 10 dBW | 10 W | 10,000 mW |
| 20 dBW | 100 W | 100,000 mW |
| 30 dBW | 1,000 W | 1,000,000 mW |
The highlighted row is the default of the calculator. The last column gives mW.
A quick mental shortcut for dB to watts
Every 10 dB is a factor of 10 in power, and every 3 dB is about a factor of 2.
So 23 dBm is about 200 mW, because 20 dBm is 100 mW and 3 dB doubles it.
dBm and dBW differ by 30
The same power gives a dBW number that is 30 less than its dBm number (Wikipedia: Decibel). So 30 dBm is 0 dBW, and 40 dBm is 10 dBW.
Always check which reference a data sheet uses.
Zero dB is not zero power
At 0 dBm the power is 1 milliwatt (Wikipedia: DBm). At 0 dBW it is 1 watt.
Zero dB just means the power equals the reference.
Negative levels
A negative level means less than the reference. A level of minus 10 dBm is 0.1 mW, which is 100 microwatts.
The answer is never negative in watts.
Safety with radio power
A transmitter of a few watts or more can heat tissue close to its antenna, and it can damage equipment connected to it by mistake.
Follow the maker limits for safe distances and loads.
Quick conversion answers
| Conversion | Result |
|---|---|
| 30 dBm | 1 W |
| 0 dBm | 0.001 W |
| 10 dBW | 10 W |
| 43 dBm | 19.953 W |
Where this conversion is used
Radio and Wi-Fi designers turn a dBm figure into watts for a power budget.
Students practise logarithmic units.
Common mistakes to avoid
- Mixing dBm and dBW. They differ by 30 dB, which is a factor of 1,000.
- Using 20 instead of 10. Power uses 10 log, not 20 log.
- Forgetting the reference. A bare dB value is a ratio, not a power.
- Adding dBm values directly. Add the powers in watts, not the dB numbers.
Related converters
More dBm and dBW levels
-10 dBm is 0.0001 W. -3 dBm is 0.0005012 W. 6 dBm is 0.003981 W.
23 dBm is 0.1995 W. 33 dBm is 1.995 W. 36 dBm is 3.981 W.
40 dBm is 10 W. 3 dBW is 1.995 W. -10 dBW is 0.1 W.
Doubling the first number
-20 dBm is 0.00001 W. -6 dBm is 0.0002512 W. 12 dBm is 0.01585 W.
46 dBm is 39.81 W. 66 dBm is 3,981 W. 72 dBm is 15,849 W.
80 dBm is 100,000 W. 6 dBW is 3.981 W. -20 dBW is 0.01 W.
Common dBm levels
0 dBm is 0.001 W. 10 dBm is 0.01 W. 20 dBm is 0.1 W.
30 dBm is 1 W. 40 dBm is 10 W.
More worked examples
13 dBm is about 20 mW, and 20 dBm is exactly 100 mW. 27 dBm is about 0.5 W, and 36 dBm is about 3.98 W.
43 dBm is about 19.95 W. 50 dBm is exactly 100 W.
Going from watts to dB
To go back, take 10 times the log of the power over the reference. So 100 mW is 20 dBm and 2 W, which is 2,000 mW, is about 33.01 dBm.
500 mW is about 26.99 dBm, and 100 W is 20 dBW.
A quick check
Each plus 10 dB multiplies the power by 10. So 30 dBm is 1 W, 40 dBm is 10 W and 50 dBm is 100 W.
If your answer breaks that pattern, look for a slip in the reference.
Frequently asked questions
How do I convert dB to watts?
Divide the dB by 10, raise 10 to that power, and multiply by the reference power.
How many watts is 30 dBm?
Exactly 1 watt, or 1,000 milliwatts.
How many watts is 30 dBW?
1,000 watts.
What is 0 dBm?
1 milliwatt.
How much power does 3 dB add?
About double. It is a factor of 1.995.
Can I add dBm values?
No. Convert to watts, add, and convert back.
Sources
- Engineering ToolBox: Decibel (engineeringtoolbox.com)
- Wikipedia: DBm (en.wikipedia.org)
- Wikipedia: Decibel (en.wikipedia.org)
