How to Calculate Watts from dBm (Because Sometimes You Want to Know Just How Loud Your Signal Is) - OneSDR

⚡ So, you’ve come across this mysterious number: 30 dBm. Is that a little power? A lot? Enough to power a lightbulb? (Spoiler: not really.)

Let’s decode what dBm actually means — and how to turn it into Watts, a unit we’re all a bit more familiar with.

Table of Contents

🙋 Quick Refresher: What Is dBm?

dBm stands for decibels relative to 1 milliwatt. It’s a logarithmic way of measuring power, used a lot in radio, networking, and electronics.

Here’s the key idea:

It’s like turning up the volume on your stereo, but with math!

Here’s the magic:

Watts = 10 ^ ((dBm - 30) / 10)

Why minus 30? Because 30 dBm = 1 Watt. It’s the pivot point.

Let’s plug it in:

Watts = 10 ^ ((20 - 30) / 10)  
       = 10 ^ (-1)  
       = 0.1 Watts (or 100 milliwatts)

✅ So, 20 dBm = 0.1 W — not super powerful, but common in things like Bluetooth devices.

Easy, right?

Instead of using the table you can calculate the dBm for any Watt value.

Enter the value in dBm and the tool will calculate the Watt equivalent.

To understand signal power in a real-world sense
To check if a transmitter is within legal limits
To plan wireless systems (Wi-Fi, radios, antennas, etc.)
Because sometimes you just want to know if 36 dBm is a big deal (it is — that’s ~4 watts!)

Easy! Use this alternate version of the formula:

mW = 10 ^ (dBm / 10)

Then divide by 1000 if you want it in Watts.

Example:

10 dBm = 10 ^ (10 / 10) = 10 mW = 0.01 W

Now you’re fully equipped to convert dBm to Watts and make sense of the numbers flying around your signal analyzer or antenna manual.

Just remember:

Watts = 10 ^ ((dBm – 30) / 10)

So next time someone says, “Hey, this transmitter is 27 dBm,” you can confidently say, “Ah yes, that’s about 0.5 Watts!” 😎