Most current proximity sensing methods fail the stringent requirements of modern smartphones. A position-sensing device (PSD) requires a laser placed some distance away from the sensor, intensity-based solutions are sensitive to changes in reflectivity, and ultrasound-based sensors cannot measure small distances because of resonance. With modern transistors getting smaller and smaller, single-photon detectors have become feasible. Using a single-photon detector called a SPAD and a laser, the travel time of light can be measured. This technique, called time-of-flight, existed for several decades where radar and ultrasound are concerned but only recently includes single-photon detectors. Several products exist that use single-photon time of flight to measure proximity. However, they are limited in terms of maximum distance, resolution and ambient light tolerance. The question arises what the best possible performance of such a system is. For radar and ultrasound, this has been calculated long ago already, but for time of flight, no such analysis exists. This analysis is the main contribution of this thesis. A formula is calculated that takes all parameters of the system into account and produces an expected standard error. This formula is verified using a simulator. The effect of an increasing opening angle of laser and SPAD is analyzed, as well as different waveforms of the laser, using multiple SPADs in smart ways, and increasing the time of a single measurement. It is shown that when less than a thousand SPADs are used, no smart way of combining hits on different SPADs exists. The waveform emitted by a laser is typically a mix of a sine, a square wave and some effects resembling RC-behavior. The nearer to a square wave this is, the smaller the resulting standard error is. The most power-hungry aspect of such a proximity sensing solution is often the time discretization device. To obtain a high resolution in the order of millimeters, the time resolution should be in the order of picoseconds. Such an extremely high resolution, below the switching time of a single transistor, can typically only be obtained by trading trade area, power and read-out time for resolution. This thesis analyzes a solution using a low-resolution time-to-digital converter (TDC) and multiple sub-intervals for a shorter time to increase resolution.