# Why are Signal Strength Numbers Negative?

Signal strength numbers, particularly in the reception of wireless signals, are often negative because they are typically expressed in decibel-milliwatts (dBm). dBm is a logarithmic unit of power relative to 1 milliwatt.

In this article we present key reasons why these numbers are negative

Table of Contents

Table of Contents

## Reference Level and Measurement

### dBm Reference

• The unit dBm is based on a reference level of 1 milliwatt (mW). A signal strength of 0 dBm corresponds to 1 mW of power.
• When the power level is less than 1 mW, the logarithmic calculation results in a negative dBm value.

### Logarithmic Scale

The formula for calculating dBm is:

PdBm = 10*log10(PmW)

Where PmW is the power in milliwatt.

When PmW is less than 1, the dBm value is negative.

Conversely when PmW is greater than 1, the dBm value is positive.

Use the calculator below to convert from mW to dBm.

## What are typical Transmit Signal Levels?

A Wi-Fi access point will transmit signal levels up to a max of +20 dBm. A cellular base station will transmit levels up to +40 dBm.

## What are typical Receive Signal Levels?

Receive signal levels depend primarily on distance away from the transmitter.

These signals decay exponentially before they arrive at a laptop or phone and as a result the signal drops to below 0 dBm. The received signal is therefore a negative dBm value.

Try the RSSI calculator to see that the signal level at 10 feet away from a Wi-Fi access point is -39 dBm. That’s a difference of almost 60 dB! Even at a distance of only 2 feet away, the signal drops to -11 dBm.

Let’s look at some practical scenarios next.

## Practical Examples

### Wi-Fi Signal Strength

• Wi-Fi signal strengths are often measured in the range of -30 dBm to -90 dBm.
• A signal strength of -30 dBm is very strong, while -90 dBm is very weak and likely close to the noise floor of the receiver.

### Cellular Signal Strength

• Cellular signals typically range from -50 dBm (excellent signal) to -110 dBm (poor signal).
• These values indicate the received power level from the cell tower at the device.

## Understanding the Negative Values

### Power Levels

• Higher (less negative) numbers indicate stronger signals. For instance, -50 dBm is stronger than -70 dBm.
• Lower (more negative) numbers indicate weaker signals, often leading to poorer performance or loss of connectivity.

### Receiver Sensitivity

• The sensitivity of a receiver is also expressed in dBm and indicates the minimum signal strength that the receiver can detect.
• For example, a receiver sensitivity of -100 dBm means the device can detect signals as weak as -100 dBm.

## Why This is Useful

### Signal Quality

Measuring signal strength in dBm provides a precise and standardized way to quantify signal quality and performance.

### Troubleshooting

Technicians and engineers use these measurements to troubleshoot and optimize wireless networks, ensuring adequate coverage and performance.

## Summary

Understanding that negative dBm values are a result of power levels being less than 1 mW helps clarify why signal strength numbers are negative. This convention allows for a consistent and meaningful way to measure and compare signal strengths across different devices and systems.