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dBm ↔ watt converter

dBm is absolute power on a logarithmic scale referenced to one milliwatt: 0 dBm is exactly 1 mW, and every 10 dB is a factor of ten. So 30 dBm is 1 W, and −90 dBm — a typical Wi-Fi sensitivity limit — is one picowatt. This converter moves between dBm, dBW, watts, milliwatts and microwatts.

Watts, milliwatts and microwatts must be greater than zero — zero power is minus infinity dBm.

dBm
30.0 dBm
referenced to 1 mW
Watts
1 W
Milliwatts
1000 mW
dBW
0.0 dBW
referenced to 1 W
power ladder · dBm0 dBm = 1 mW · +10 dB = ×10+80+60+40+200−20−40−60−80−100−120+80 dBm · FM broadcast transmitter (100 kW)+60 dBm · microwave oven magnetron (1 kW)+33 dBm · GSM handset, max power (2 W)+20 dBm · Wi-Fi router (100 mW)0 dBm · Bluetooth LE beacon (1 mW)−60 dBm · strong Wi-Fi at the laptop−90 dBm · Wi-Fi sensitivity limit−127 dBm · GPS at the Earth’s surface30.0 dBm= 1 Wgreen = converted valuereference levels are typical magnitudes, not limits

The scale is fixed from −130 to +90 dBm, the range terrestrial radio lives in. Dashed lines mark typical power levels, from the GPS signal reaching the ground to a broadcast transmitter. The green line marks the converted value. A value outside the range pins to the top or bottom edge and is flagged ▲ or ▼.

dBm is absolute power; dB alone is a ratio. Mixing them is the classic RF beginner error. The reference levels on the ladder are typical figures, not regulatory limits.

How it works

  1. 01

    Fix the reference first

    The letter after dB names the reference. dBm is measured against 1 mW, dBW against 1 W, and since 1 W is 1000 mW the two scales sit exactly 30 dB apart: dBm = dBW + 30. A 20 W transmitter is 43 dBm or 13 dBW, the same power under two labels.

  2. 02

    Convert with the logarithm

    Going up, P (dBm) = 10·log10(P in mW). Coming back down, P (W) = 10(dBm − 30) / 10. The factor is 10 rather than 20 because these are power ratios. Voltage ratios use 20·log10, which is why the same 3 dB means half the power but 0.707 of the voltage.

  3. 03

    Learn the two shortcuts

    +10 dB = ×10 exactly, and +3 dB = ×2 to within 0.25%. Chain them and you rarely need a calculator: 27 dBm = 30 dBm − 3 dB, so 1 W halved, so 0.5 W. 37 dBm = 30 + 10 − 3 dB, so 10 W halved, so 5 W. Subtracting works the same way in reverse.

  4. 04

    Sanity-check against landmarks

    Anchor new numbers to levels you already know: a Wi-Fi access point radiates about 20 dBm (100 mW), a phone peaks near 33 dBm (2 W), a strong indoor Wi-Fi signal arrives at −60 dBm, and GPS reaches the ground at roughly −127 dBm. Almost everything in terrestrial radio lives between +60 dBm and −130 dBm. A result outside that range usually means the wrong input unit.

Formulas

Power level in dBm
P (dBm) = 10·log10( P1 mW )
  • P — power in the same unit as the reference
  • 1 mW — the reference that defines 0 dBm
Back to linear power
P (W) = 10 P (dBm) − 3010
  • P (dBm) — power level, dBm
  • 30 — the offset between the 1 mW and 1 W references, in dB
Relation to dBW
P (dBm) = P (dBW) + 30
  • dBW — power referenced to 1 W
  • 30 = 10·log10(1000), the mW-per-W ratio in dB

Worked example

Converting 27 dBm to watts, in your head
  1. 27 dBm = 30 dBm − 3 dB
  2. 30 dBm = 1 W ; −3 dB = ÷2
  3. 27 dBm ≈ 0.5 W (exact: 10(27−30)/10 = 0.501 W)
  4. → mental math with 3s and 10s gets you within 1%

FAQ

What is the difference between dB and dBm?
dB is a ratio between two powers and has no absolute meaning: an amplifier has 20 dB of gain, a cable 3 dB of loss. dBm is an absolute level referenced to 1 mW, so it names an actual power. You can add dB to dBm, as in 20 dBm + 10 dB = 30 dBm, but a bare dB figure never describes how much power is present.
Why do RF engineers use dBm instead of watts?
Because a radio link spans about twelve orders of magnitude, from a watt at the transmitter to a picowatt at the receiver, and every cable, antenna and path in between multiplies the signal. On a logarithmic scale those multiplications become additions, so a whole link budget is a column of numbers you can add up by hand.
What does a negative dBm value mean?
Simply less than 1 mW. −30 dBm = 1 µW, −60 dBm = 1 nW, −90 dBm = 1 pW. Received signals are almost always negative in dBm. A Wi-Fi client is usually somewhere between −40 dBm right next to the access point and −85 dBm at the edge of usable coverage.
How do I add two signals together in dBm?
Not by adding the dBm values: 0 dBm + 0 dBm = 3 dBm, not 0 dBm. Convert each level to milliwatts, add them, then convert the sum back with 10·log10. Only gains and losses, which are plain dB, can be added to a dBm level directly.

Now that the units make sense — see what −90 dBm looks like on a real map: simulate coverage down to your receiver’s sensitivity, over actual terrain.

Simulate signal levels →

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