How to use the Decibel (dB) Calculator
- 1. Pick the conversionPower ratio, voltage ratio, dB to ratio, watts to dBm and more.
- 2. Enter the valuesSI prefixes work, for example 2m for 2 milliwatts.
- 3. Read the answerWith the log steps written out.
Decibel formulas
For power, dB = 10 log₁₀(P2/P1). For voltage or current across the same resistance, dB = 20 log₁₀(V2/V1), because power goes with V². Going back, the power ratio is 10dB/10 and the voltage ratio is 10dB/20.
Absolute units use a fixed reference: dBm is relative to 1 mW and dBW to 1 W, so dBm = dBW + 30. 1 W = 30 dBm, 1 mW = 0 dBm and 1 µW = −30 dBm.
Worked example: an amplifier turns 10 mV into 1 V. The voltage gain is 100, so 20 log₁₀(100) = 40 dB. A link with +20 dB amplifier, −3 dB cable, −6 dB splitter and +15 dB amplifier has a total gain of 20 − 3 − 6 + 15 = 26 dB.
Useful values and exam questions
- 3 dB ≈ double the power (×1.995); 6 dB ≈ double the voltage.
- 10 dB = ×10 power; 20 dB = ×10 voltage.
- −3 dB is the half-power point used for bandwidth.
Typical questions: convert a 50 W transmitter to dBm (47 dBm), find the output power after a 12 dB loss, find the overall gain of cascaded stages, or find the voltage gain of a stage given in dB. Adding dB values is the same as multiplying ratios, which is why link budgets use decibels.
Frequently asked questions
When do I use 10 log and when 20 log?
Use 10 log for power ratios. Use 20 log for voltage or current ratios, assuming the same impedance on both sides.
What is the difference between dB and dBm?
dB is a ratio with no unit. dBm is an absolute power level compared with 1 milliwatt, so 0 dBm = 1 mW.
Can a dB value be negative?
Yes. A negative value means a loss: the output is smaller than the input. −10 dB means one tenth of the power.
Is this tool free?
Yes. You enter your name and WhatsApp number once to see your result, and all other Uncodemy tools then open without the form.