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dBm to Watts Calculator

Convert dBm to watts and milliwatts.

Result

0.100000 W
100.00 mW

How it is calculated

dBm is a power level measured against 1 milliwatt on a logarithmic scale. The conversion to watts is P(W) = 10(dBm − 30) ÷ 10; the 30 comes from 1 W = 1,000 mW = 30 dBm. The tool also shows milliwatts, P(mW) = P(W) × 1,000. Any number is accepted, positive or negative: 0 dBm is 1 mW, and negative dBm values are fractions of a milliwatt (typical of weak radio signals). Every +10 dBm multiplies power by 10, and +3 dBm roughly doubles it. Watts are shown with six decimals so very small values do not collapse to zero; milliwatts with two.

Worked example

A transmitter outputs 20 dBm. Step 1: subtract 30: 20 − 30 = −10. Step 2: divide by 10: −1. Step 3: 10−1 = 0.1. The tool shows 0.100000 W, and in milliwatts 0.1 × 1,000 = 100.00 mW. Going up to 30 dBm would give 1 W.

Try this example →

Common mistakes

  • Confusing dBm with dB. dBm is an absolute level against 1 mW, while dB is a ratio between two powers.
  • Forgetting the −30 shift. Treating dBm like dBW gives a result 1,000 times too large.
  • Averaging dBm values. Convert to watts, average, then convert back; averaging in logarithms gives a different number.
  • Expecting +3 dBm to add 3 mW. It doubles the power, so the milliwatt change depends on the starting level.
  • Reading a negative dBm as negative power. −10 dBm is 0.1 mW, a small positive power.

About this calculator

dBm is a logarithmic power unit referenced to 1 milliwatt, defined as dBm = 10·log₁₀(P/1mW). Converting back to linear units uses P(W) = 10^((dBm−30)/10) (or P(mW) = 10^(dBm/10)); every +10 dBm multiplies the power by 10, and every +3 dBm roughly doubles it, which is why RF engineers think in dB for gains and losses but need watts for anything involving actual heat, battery life, or regulatory limits.

This conversion is a daily tool in radio-frequency and telecom work: Wi-Fi and cellular link-budget calculations, antenna and amplifier datasheets, and transmitter power limits are all specified in dBm, while power supplies, batteries, and thermal design are specified in watts. RF engineers, network technicians tuning Wi-Fi coverage, and satellite or ham-radio operators move between the two units constantly.

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