Physics

Instant, private, and free

Decibel (Power Ratio) Calculator.

Calculate decibels from an entered power and an explicit reference power.

On-device calculationNo signup
01

Set your values

Results update as you type.

Power ratio: 1,000 ×

Power ratio

1,000.000000×
Level: 30 dB

Level

30.000000dB

Results update automatically as you type.

The decibel route uses power ratios only. Absorption uses a measured power balance; it does not infer material ratings, room acoustics, or exposure limits.

Use Cases

Compare signal strengths

Use this calculator to compare the power of a signal against a known reference, such as in audio systems or telecommunications.

Example: Compare a 2 W signal to a 1 W reference: result is about 3.01 dB.

Verify amplifier gain

Check the gain of an amplifier by entering the output power as measured and the input power as reference.

Example: If output is 50 W and input is 5 W, gain is 10 dB.

Frequently Asked Questions

How do I calculate decibels from power?
Enter the measured power and the reference power in watts. The calculator computes 10 × log10(measured power / reference power) to give the power ratio in decibels (dB).
What does a positive or negative dB value mean?
A positive dB value means the measured power is greater than the reference power. A negative dB value means the measured power is less than the reference power. Zero dB means they are equal.
Can I use any units for power?
This calculator expects both powers in watts. Ensure you convert other units (like milliwatts) to watts before entering them to get accurate results.

Tips & Common Mistakes

Tips

  • Ensure both power values are in watts before entering them.
  • Use a reference power that is meaningful for your context, such as 1 W for absolute dBW.
  • Remember that decibels are logarithmic, so doubling power adds about 3 dB.
  • Double-check your entries for typos, as small errors can change the dB result significantly.

Common Mistakes to Avoid

  • Entering powers in different units (e.g., one in milliwatts and one in watts) without converting.
  • Using the formula for voltage ratios (20 log10) instead of power ratios (10 log10).
  • Confusing the reference power with the measured power, leading to an inverted sign.

Last updated: August 13, 2026