Milliwatts to Watts

Putting a Transmitter's Milliwatts on the Watt Scale

Radio datasheets live in milliwatts and decibel-milliwatts. Link budgets, amplifier specs and regulatory tables mix all three — a module rated "+14 dBm", a limit written "25 mW ERP", a transceiver sold as "5 W". The moment you have to compare them, or feed a figure into a path-loss sheet that expects watts, the milliwatt number has to move three decimal places.

Conversion factor: 1 mW = 0.001 W, so divide the milliwatt figure by 1 000. A LoRa node transmitting at the European 868 MHz ceiling of 25 mW is putting out 0.025 W — and +14 dBm on the logarithmic scale, since 10 · log₁₀(25) ≈ 13.98.

Three Scales Describing the Same Transmitter

dBm is the milliwatt written logarithmically

The reference for dBm is one milliwatt exactly, which is why 0 dBm and 1 mW are the same thing. Gains and losses along a chain then add and subtract instead of multiplying.

Conducted power stops at the connector

The milliwatt figure on a module datasheet is measured at the antenna port. Feeder loss subtracts from it before anything radiates, so a 100 mW module is already under 0.1 W by the time the signal reaches the mast head.

Antenna gain multiplies it back up

A 25 mW node on a 6 dBi antenna radiates as if it were roughly 100 mW in the main lobe — the same 0.025 W of real power, concentrated into a narrower pattern.

Regulators write ceilings in mixed units

One band is capped in milliwatts of radiated power, the next in watts of conducted power, a third in dBm. Getting them onto one scale is usually the first step of a compliance check.

Getting a Module Spec into a Link Budget

The sequence below is what happens when a radio module's numbers meet a spreadsheet that was built in watts.

Enter the transmit power from the datasheet

Type the milliwatt figure — 1, 10, 25, 100, 200 — into the left field and the watt equivalent appears immediately. Dot or comma both work as the decimal separator and spaces in the number are ignored.

Cross-check against the dBm on the same page

Most datasheets print both. If the table says +20 dBm the milliwatt column should read 100, and the watt field here should show 0.1. A mismatch usually means one column is EIRP and the other is conducted power.

Copy the watt value into the budget sheet

The copy button puts the plain number on the clipboard with no unit and no thousands spacing — exactly what a cell expects. Ctrl + C from inside a field does the same.

Reverse it when the spec arrives in watts

Amplifier and transceiver datasheets often quote whole watts. Press the swap button (↔) for W → mW and a 5 W handheld reads as 5 000 mW, ready to sit in the same column as a 25 mW sensor node.

Check what the limit is measured on: a milliwatt ceiling quoted as ERP or EIRP includes antenna gain, while a conducted limit does not. Comparing a conducted transmit power against a radiated limit is the classic way to fail a test-house measurement.

Transmit Power by Radio Class, in mW, W and dBm

Everyday radio equipment spans four orders of magnitude in transmit power. Set out on all three scales, the pattern is easy to keep in your head.

Radio class	Transmit power (mW)	Transmit power (W)	Level (dBm)
Bluetooth LE beacon, low-power class	1 mW	0.001 W	0 dBm
Bluetooth LE long-range module	10 mW	0.01 W	+10 dBm
LoRa node, 868 MHz European ceiling	25 mW	0.025 W	+14 dBm
Wi-Fi access point, 2.4 GHz EU limit	100 mW	0.1 W	+20 dBm
Cellular handset, 3GPP power class 3	200 mW	0.2 W	+23 dBm
Indoor small cell / pico base station	250 mW	0.25 W	+24 dBm
Wi-Fi, US conducted ceiling	1 000 mW	1 W	+30 dBm
Handheld VHF transceiver, high power	5 000 mW	5 W	+37 dBm

Read down the dBm column and the compression is obvious: the whole span from a beacon to a handheld — a factor of five thousand in watts — is only 37 dB. That is exactly why link budgets are kept in decibels, and why the watt column is the one that surprises people when they first see how little power a wireless sensor actually radiates.

Handy Details on a Bench Full of Radios

Both fields track while you sweep power steps

Modules step their output in discrete levels. Type each one in turn and the opposite field keeps up, so a whole power table converts without any clearing or re-entering.

Flip the direction for an amplifier datasheet

Power amplifiers and transceivers are specified in watts. The swap button turns the page around so the whole chain — module, amplifier, handheld — can be listed on one scale.

Microwatts on the dropdown for receive levels

A receiver sensitivity of −30 dBm is one microwatt; the searchable unit lists on both sides reach µW, so the weak end of the budget converts on the same page as the strong end.

Very small results drop into scientific notation

Results carry up to eight decimals, and anything below a millionth switches to exponential form automatically, which keeps a −70 dBm received level legible instead of a row of zeros.

Radio Power and dBm Questions

Why does a 25 mW transmitter show up as +14 dBm?

Because dBm is ten times the base-ten logarithm of the power in milliwatts: 10 · log₁₀(25) = 13.98, rounded to 14 on every datasheet. The same relation gives you 0 dBm for 1 mW, +20 dBm for 100 mW and +30 dBm for 1 000 mW — one watt. Going the other way, milliwatts = 10^(dBm/10).

Does doubling the milliwatts really only add 3 dB?

Yes — 10 · log₁₀(2) = 3.01, so every doubling is one 3 dB step: 1 mW to 2 mW, 100 mW to 200 mW, 2.5 W to 5 W. In free space that buys about 41 % more range, since path loss grows with the square of distance. Ten times the power (+10 dB) roughly triples it. This is why chasing range with raw output gets expensive fast, and why a better antenna or a lower data rate usually wins.

Is the milliwatt figure in the datasheet what actually leaves the antenna?

No. It is conducted power, measured into a 50 Ω load at the module's output port. What leaves the antenna is EIRP: conducted power in dBm, plus antenna gain in dBi, minus feeder and connector losses in dB. A 100 mW (0.1 W, +20 dBm) access point with a 5 dBi antenna and 1 dB of cable loss radiates +24 dBm EIRP — about 250 mW equivalent in the main beam, though it is still only drawing 0.1 W of RF from the amplifier.

What is the difference between ERP and EIRP when a limit is quoted in mW?

They use different reference antennas. ERP is referred to a half-wave dipole, EIRP to an isotropic radiator, and a dipole already has 2.15 dB of gain over isotropic — so EIRP = ERP + 2.15 dB. The European 868 MHz limit of 25 mW (0.025 W) is written as ERP, which corresponds to about 41 mW EIRP. Mixing the two is a 2.15 dB error, small on paper but enough to put a borderline design over a legal ceiling.

Why is Wi-Fi capped near 100 mW while a handheld ham radio runs 5 W?

Different licensing regimes, not different physics. Licence-exempt bands are shared by everybody, so ceilings are set low — 100 mW (0.1 W) EIRP in the European 2.4 GHz band, 25 mW ERP at 868 MHz — to keep neighbouring users from swamping each other. Amateur allocations are licensed and coordinated, so an operator may run 5 W (5 000 mW) from a handheld and far more from a base station. Regional rules differ, so always check the ceiling for the exact band and country.