Apparatus, circuits and methods for calibrating time to digital converters (TDCs) are disclosed herein. In some embodiments, a circuit for calibrating a TDC is disclosed. The circuit includes a multi-bit delay circuit, a counter, and a register. The multi-bit delay circuit is configured for delaying a clock signal by a total delay time. The counter is configured for counting rising edges of the clock signal within the total delay time to generate a counted output. The register is configured for controlling the total delay time of the multi-bit delay circuit based on the counted output.