NOTE 1 – The decibel may also be used to express the ratio of two field quantities. To obtain the same numerical value as for the corresponding power ratio, the logarithm of the field quantity ratio is multiplied by the factor 20, assuming that the impedances are equal.

If P₁ and P₂ are two powers, their ratio expressed in decibels is:

${\displaystyle 10\text{lg}\frac{P_1}{P_2}}$

The relation between a field quantity ratio and the corresponding power ratio is dependent on the impedances.

If P₁ and P₂ represent the powers dissipated by currents I₁ and I₂ in resistances R₁ and R₂, then:

${\displaystyle 10\text{lg}\frac{P_1}{P_2}=10\text{lg}\frac{I_1^2}{I_2^2}\frac{R_1}{R_2}=20\text{lg}\frac{I_1}{I_2}+10\text{lg}\frac{R_1}{R_2}}$

NOTE 2 – Guidance for the use of the decibel and appropriate symbols to represent the ratios of various quantities is given in CCIR Recommendation 574, in CCITT Recommendation B.12, and in IEC publication 27-3.