  We present an efficient method to extract the amount of true randomness that
can be obtained by a Quantum Random Number Generator (QRNG). By repeating the
measurements of a quantum system and by swapping between two mutually unbiased
bases, a lower bound of the achievable true randomness can be evaluated. The
bound is obtained thanks to the uncertainty principle of complementary
measurements applied to min- and max- entropies. We tested our method with two
different QRNGs, using a train of qubits or ququart, demonstrating the
scalability toward practical applications.
