In this paper, a set of variables which can be easily computed in the course of iterative decoding of turbo decoders called decoding metrics (DMs) are introduced. According to the measured DMs after each iteration, a lot of information other than signal-to-noise ratio (SNR) in the received bits, such as how good/bad the current block is and how close the current iteration of decoding is to convergence, can be obtained. Detailed discussions are provided regarding why these variables are chosen. Based on the measured DMs after the first iteration, an approximate SNR-related variable Lc can be obtained for MAP-based turbo decoders. Simulation results show that there is almost no performance degradation if approximated Lc values are used instead of exact values. It is also shown that adaptive decoding using DMs is more efficient than existing methods both in terms of hardware and latency. Other applications of DMs are pointed out at last.