Equivalent RSA modulus for NIST P-192 and P-521 elliptic curves
Security strength of RSA in relation with the modulus size outlines the formula that is used for the strength of an RSA key (as a function of the size of the modulus).

For ECC it's a simpler "the prime field size / 2".

So ECC-192 => 96 bits of security.  $\sqrt[0.43]{96 / 4}$ = $\sqrt[0.43]{24}$ = $1621$, with $1624$ being the next highest number divisible by 8.

ECC-521 => 260.5 bits of security.  $\sqrt[0.43]{260.5 / 4}$ = $\sqrt[0.43]{65.125}$ = $16520.3$, making $16528$ the equivalent, which is beyond most systems' maximum of 16384.