dBm to mW Converter for Fiber Optic Power
Fiber power meters read in dBm, but datasheets and lasers often quote milliwatts. This converter moves between dBm, mW and microwatts, and shows how much power arrives after a given loss.
Your numbers
Value shown on your power meter or transceiver datasheet.
Enter a positive value to convert to dBm.
Total loss between the source and the meter.
Your results
Mathematical conversion only. Confirm instrument readings against calibrated references.
Received power in mW versus link loss
Sweeps Link loss from 0 to 30, holding your other inputs fixed.
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How to use the converter
Type a power in dBm to see the equivalent in milliwatts and microwatts, or type a value in milliwatts to see the dBm figure. Then set a link loss in dB to see the received power both in dBm and mW, plus the percentage of the original power that is left. The chart shows how received milliwatts fall as loss increases. Use it to sanity check a power meter reading, or to translate a laser class limit into dBm.
- 0 dBm equals 1 mW
- Every 3 dB is about a factor of 2 in power
- Every 10 dB is a factor of 10
- Negative dBm means less than 1 mW
The formulas
Power in milliwatts equals 10 raised to the power of dBm divided by 10. Power in dBm equals 10 times the base 10 logarithm of power in milliwatts. Because dBm is a logarithmic scale referenced to 1 mW, losses in dB subtract directly: if you launch at -3 dBm and lose 6 dB, you receive -9 dBm. Converting losses to a percentage uses 10 raised to minus loss divided by 10. A 3 dB loss leaves about 50 percent of the power, and 10 dB leaves 10 percent.
Worked examples
A transceiver launching -3 dBm equals about 0.5 mW. After 6 dB of link loss, the receiver sees -9 dBm, about 0.126 mW or 126 microwatts, which is 25 percent of the launched power. A 1 mW source converts to exactly 0 dBm, and a 10 mW source is 10 dBm. A receiver sensitivity of -14.4 dBm equals only about 0.036 mW, or 36 microwatts, which shows how small the working power of an optical receiver really is.
Common mistakes
People often add dBm values together, but dBm figures cannot be added as power. Subtract a dB loss from a dBm level, but to combine two sources you must convert to mW, add, and convert back. Another mistake is mixing dB (a ratio) with dBm (an absolute level). Also, a meter set to the wrong wavelength reads incorrectly because detector responsivity varies with wavelength, so match the meter setting to the test source.
Frequently Asked Questions
What is 0 dBm in mW?
0 dBm is exactly 1 mW. dBm is a decibel scale referenced to 1 milliwatt, so positive dBm values are above 1 mW and negative values are below it.
How do I convert dBm to mW?
Divide the dBm value by 10 and raise 10 to that power. For example, -10 dBm is 10 to the power of -1, which equals 0.1 mW, and 3 dBm is about 2 mW.
Can I add two dBm values?
Not directly. dBm values are logarithmic. Convert each to milliwatts, add them, then convert the sum back to dBm. Two equal sources add 3 dB to the level of one.
What is the difference between dB and dBm?
dB is a ratio between two powers, such as a loss or gain. dBm is an absolute power level referenced to 1 mW. You subtract dB loss from a dBm level to get the new dBm level.
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