mW to dBm Converter

How to convert mW to dBm?

Despite the suggestively similar names, milliwatts (mW) and decibel-milliwatts (dBm) are *not* the same kind of quantity: mW is a linear power, dBm is a logarithmic ratio of a power to the fixed reference 1 mW. The exact conversion is P(dBm) = 10·log₁₀(P / 1 mW). Because the relationship is logarithmic, equal *additive* steps in dBm correspond to equal *multiplicative* steps in mW: every +3 dB doubles the power, every +10 dB multiplies it by ten, and 0 dBm is, by definition, exactly 1 mW (since log₁₀(1) = 0). This is the standard unit in RF and microwave engineering because real-world signal chains span enormous dynamic ranges — a Wi-Fi transmitter at +20 dBm = 100 mW and a received signal at −85 dBm ≈ 3.16 pW differ by 17 orders of magnitude in linear power, but only 105 dB on a logarithmic scale, which fits on one chart axis. Common reference points to memorize: 1 mW = 0 dBm, 2 mW ≈ 3 dBm, 10 mW = 10 dBm, 100 mW = 20 dBm, 1 W = 30 dBm, 1 kW = 60 dBm. Our tool computes dBm = 10·log10(mW) with full IEEE-754 precision; the function is undefined at 0 mW (log of zero is −∞) and undefined for negative power (no physical meaning). Use this for link-budget math, antenna EIRP, regulatory checks (FCC Part 15 limits 2.4 GHz Wi-Fi to +30 dBm EIRP), and receiver sensitivity comparisons.

P_(dBm) = P_(mW) × 1

Worked example

A consumer Wi-Fi router transmits at 100 mW. To express this in dBm: P(dBm) = 10·log₁₀(100) = 10·2 = 20 dBm. A high-gain antenna with 6 dBi gain raises the EIRP to 20 + 6 = 26 dBm = 10^(26/10) ≈ 398 mW, still under the FCC +30 dBm cap. Conversely, a Bluetooth LE Class 2 device at 2.5 mW emits 10·log₁₀(2.5) ≈ 3.98 dBm.

P_(dBm) = 1 mW × 1 = 1 dBm

How many dBm in a mW?

1 mW corresponds to exactly 0 dBm (because log₁₀(1) = 0). The relationship is logarithmic, not linear, so 2 mW is +3.01 dBm and 10 mW is +10 dBm — not 2 dBm or 10 dBm by direct equality.

What's the rule of thumb for converting in my head?

Memorize three anchors: +3 dB ≈ ×2, +10 dB = ×10, 0 dBm = 1 mW. Then 23 dBm = 10·10·2 mW = 200 mW; 27 dBm = 10·10·2·~1.26 mW ≈ 500 mW; 30 dBm = 10·10·10 mW = 1 W (1000 mW).

What is dBm and where is it used?

dBm (decibel-milliwatt) is a logarithmic unit of power used in radio frequency and microwave engineering, telecommunications, and wireless communications.

Why convert mW to dBm?

Converting mW to dBm is needed for RF engineering, antenna design, wireless communications, and working with equipment that uses dBm as the standard power unit.

Popular mW to dBm conversion table