How to Calculate dB (Decibels) — Explained with Formula + Calculator

If you’ve ever seen numbers like 60 dB or +3 dB on audio gear, radios, or electronics, you’re looking at a decibel (dB) value. But what does it actually mean — and how do you calculate it?

Let’s break it down in plain language — no engineering degree needed!

🙋 What is a dB?

dB (decibel) is a logarithmic unit used to compare two power or voltage levels.

It’s not an absolute number — it’s a ratio. You use it to express how much stronger or weaker one signal is compared to another.

🧮 The dB Formulas

Here are the two main formulas used to calculate dB, depending on whether you’re comparing power or voltage:

🔌 Power Ratio Formula

dB = 10 × log10(P2 / P1)

• P2 = measured power
• P1 = reference power

⚡ Voltage Ratio Formula (same impedance*)

dB = 20 × log10(V2 / V1)

• V2 = measured voltage
• V1 = reference voltage

⬆️ Example 1: Power Increase

You go from 1 mW to 10 mW:

dB = 10 × log10(10 / 1) = 10 × 1 = 10 dB

That means the power has increased by 10 dB.

⬇️ Example 2: Voltage Drop

Voltage drops from 5V to 2.5V:

dB = 20 × log10(2.5 / 5) = 20 × log10(0.5) ≈ -6.02 dB

This means a decrease of 6 dB.

💡 Bonus Tip: What Do dB Values Mean?

• +3 dB ≈ double the power
• -3 dB ≈ half the power
• +10 dB = 10× more power
• -10 dB = 10× less power

Sure! Here's a brief and easy-to-understand explanation you can add to the article:

📌 * What Does Impedance Mean in dB Calculations?

When using the voltage formula for dB (20 × log₁₀(V₂ / V₁)), it assumes that the impedance stays the same across both measurements.

Impedance is like resistance, but for AC signals — it affects how voltage and current behave in circuits. If the impedance changes between V₁ and V₂, the dB result may not accurately reflect a power difference.

So in short:

When using the voltage dB formula, make sure the signals are measured across the same or equal impedance — usually 50Ω or 75Ω in electronics.