# FFT amplitude vs IQ amplitude

I'm very new to digital signal processing, so please let me know if my question doesn't make sense.

I calculated the amplitudes (using the formula : $10 \log_{10}(i^2 + q^2)$) for some IQ data that I recorded using GNU radio. I compare the highest calculated amplitude to the highest amplitude shown when processing the IQ data with a GNURadio fft sink (using peak hold). The numbers are significantly different in some cases.

In one case, the highest amplitude signal I found was +2.98 db. When I process the same data with a GNURadio FFT sink, it shows the highest amplitude as about -0.5 db.

In a second case, the python calculations find an amplitude of -23 db, while GNURadio finds a highest amplitude of about -64 db.

Should I expect the highest FFT amplitude to be similar to the highest amplitude found by calculating it for each IQ sample? I was assuming that the IQ data amplitude and the FFT amplitude would be similar, and I'm not sure if I'm doing something wrong in the IQ amplitude calculation.