How to use the calculator for dBs (decibels) and electronics units
This decibel (dB) calculator has two columns. The one on the left is used to calculate voltages [V] or currents [A]. The one on the right is for power [W]. To use it, fill in any field and click the button. All other fields will be calculated automatically. The highlighted field is the last introduced input.
If you want to learn more about voltage, current and power concepts, visit our article, where we explain in simple terms those concepts from physical and electrical point of view.
What are Decibels (dBs) and the units of voltage or current
Decibels are a logarithmic function useful to express very large and small quantities, such as voltage, current, or power. You probably already know that the unit we use for voltage is the Volt and for current the Ampere. Depending on the context, it can be useful to give them in 'millis' or 'micros' to avoid typing lots of zeros. For example, instead of writing 0.001 Volt, we can write 1mV. In electronics, we often use dBV (decibels referenced to 1 Volt) or dBA (decibels referenced to 1 Ampere) when we want to express huge or very small quantities of volts and amperes. Also, when we work with alternating current (AC) we must specify whether we are referring to the peak voltage (Vpeak) or the effective voltage (Vrms). Vpeak is the maximum voltage value along the wave. Vrms gives us that value which, applied to a resistor, would dissipate the same power as if we had direct current (DC) instead of AC. If the current has a sinusoidal waveform, we can relate them with the following equation:
$$V_{peak} = V_{rms}\cdot\sqrt{2}$$
It is also useful to express the voltage or current in decibels (or dBs for short). Decibels (dBs) are useful for making very large or very small figures manageable using the logarithm function. Beware, the dBs are not a unit of measurement like the Volt or the Liter. dBs are a way of scaling quantities. Therefore, when we use dBs we have to indicate which unit it accompanies. In this case, we speak of dBVolts (dBV) or dBAmperes (dBA). You can convert between dBs and 'normal' (also called linear) values using the following expressions:
$$dBV = 20\cdot\log_{10}\left(\dfrac{V_{[V]}}{1V}\right)$$
$$V_{[V]} = 10^\frac{dBV}{20}$$
The important part of the formula is the denominator of what is inside the logarithm. This is called the reference. In reality, a dB is a ratio of a given number to a reference. This reference can be anything, but engineering communities tend to use the same ones to ease communication. In the previous equation we have seen an example for Volts. The same would apply to Amperes, just replacing [V] with [A]. A different case is for audio levels, where 0.775V RMS is taken as the reference level. When we do this, we call it dBu. This is because when 0 dBu is applied to a 600 Ohm load, it results in 1 mW of power, which was a common reference in professional audio equipment. So, in summary: dBV → reference is 1 Volt; dBu → reference is 0.775 Volts (used in audio); dBA → reference is 1 Ampere, dbW → reference is 1 Watt; dBm → reference is 1 milliWatt.
Decibels (dBs) and power units
To measure power we use Watts [W]. If the power varies over time, we can also speak of peak Watts or average Watts. What you can't do is talk about Watts RMS, because it doesn't make physical sense (although they can be calculated mathematically). In general, when talking about power, it refers to the average, so if you ever see it without specifying, it is probably for this reason. It is also useful in some cases to use dBs for Watts, which gives us dBW. For some applications a dbW is too much, so dBmW (dB milliWatt) is used, which is usually written without the final W because we are a bit lazy, that is just dBm. The dBm and the dB are conceptually the same, but one refers to the Watt and the other to the milliWatt. The following equations will tell you how to calculate them (note that they are similar to the voltage ones, but not the same):
$$dBW = 10\cdot\log_{10}\left(\dfrac{P_{[W]}}{1W}\right)$$
$$P_{[W]} = 10^\frac{dBW}{10}$$
$$dBm = 10\cdot\log_{10}\left(\dfrac{P_{[W]}}{1mW}\right)$$
$$P_{[mW]} = 10^\frac{dBm}{10}$$
You may have noticed that the expressions for [W] have a 10, and that those for voltage and current have a 20. This is because the expressions for voltage and current are derived from the power expression and we can relate them using the following logic and properties of the logarithm:
$$10\cdot log_{10}\left(\dfrac{P_{[W]}}{1W}\right)=$$
$$ = 10\cdot log_{10}\left(\dfrac{\frac{V_{[V]}^2}{R}}{\frac{1V^2}{R}}\right)=$$
$$ = 10\cdot log_{10}\left(\dfrac{V_{[V]}^2}{1V^2}\right)=$$
$$ = 10\cdot log_{10}\left(\dfrac{V_{[V]}}{1V}\right)^2=$$
$$ = 20\cdot log_{10}\left(\dfrac{V_{[V]}}{1V}\right)$$
Don't worry if this last part is not clear to you, just remembering that power goes with 10 and voltage/current with 20, you can use the dBs with ease.
Practical Uses of Decibels
Decibels are used everywhere in electronics, audio, microwaves and signal processing. Engineers use dB to express gain, loss, attenuation, and signal-to-noise ratios (SNR). In audio systems, dB measures sound pressure level (SPL), while in communication systems it quantifies link budget or antenna gain. The logarithmic nature of the decibel makes it ideal for comparing signals that vary across wide dynamic ranges.
Frequently Asked Questions
-
What is the difference between dBW and dBm?
dBW is referenced to 1 Watt, while dBm is referenced to 1 milliwatt. 0 dBW equals 30 dBm. One or the other is used depending on the power levels involved.
-
Can decibels be negative?
Yes. A negative value means the signal is smaller than the reference level (attenuation). Positive means it is larger (gain).
-
Why use decibels?
They allow expressing very large or small ratios in manageable numbers and make calculating gains and losses easier by simply adding or subtracting.
-
How can I calculate fast dB values?
There are some tricks. For example, every time you double a voltage, you add approximately 6 dB (20*log10(2) ≈ 6.02 dB). Similarly, halving a voltage subtracts about 6 dB. For power, doubling adds about 3 dB (10*log10(2) ≈ 3.01 dB), and halving subtracts about 3 dB. These approximations can help you quickly estimate changes in dB without a calculator. Also, adding 10 dB corresponds to multiplying the power by 10, and adding 20 dB corresponds to multiplying the voltage or current by 10. Conversely, subtracting 10 dB means dividing the power by 10, and subtracting 20 dB means dividing the voltage or current by 10.
