A question that comes up often is
Can you convert dB to dBi?
The answer is no.
Let’s understand why 
dB is short for deciBels. It represents the logarithm of the equivalent linear quantity. Power in dB is calculated using the formula:
PdB = 10*Log10(Plinear)
Use this calculator to compute the dB value from its linear equivalent.
An antenna’s dBi value is a measure of its gain compared to an isotropic antenna. Isotropic antennas radiate signal equally in all directions. An antenna with a higher dBi value has more gain than an antenna with a lower dBi value.
Antenna gain can be calculated using this tool. It is calculated using the formula:
Gain (dBi) = 20*Log10(9.73/(λ*(10)AF/20))
Where
- λ = Wavelength = c/f, where c is the speed of light and f is the frequency
- AF = Antenna Factor