In the context of a modular synthesizer, the format is a set of parameters which specify the physical and electrical compatibility of a particular manufacturer's modules. Important mechanical parameters include the module height, the width of one horizontal unit (modules are usually designed in multiples of a basic unit width, referred as "1U", in order to avoid gaps between modules), and the pattern of mounting holes used for the screws that attach the module to the case's mounting rails. Important electrical parameters include the power supply voltages required, the type of connector used for connecting to the power supply, the voltage level of a trigger signal, and the type of jacks used for patch cords. The three most commonly used types of jacks are the 1/4" phone jack, the 3.5mm phone jack (often incorrectly referred to as a 1/8" jack), and the banana jack. There are grounding considerations that must be addressed when a modular which uses banana jacks is to be interconnected with another synth, or other equipment, which uses some other type of jack.

The formats used by modular synthesizers currently in production are often divided into categories, referred to as "small format" and "large format". Small-format modulars take up less space (the amount of space required is often a consideration when considering the purchase of a modular); they gain this space in part by using smaller knobs and jacks, which some users find uncomfortable to use. Large-format modulars generally use larger, easier-to-use knobs and sturdier jacks and other components, but they take up more room and are harder to move. The categories, with the most common formats used by current modular systems, are:

Small format:

Large format:

Methods have been devised for intermixing the two small formats in one case; similarly, some users have successfully intermixed all three of the large formats in one case. See Frankensynth.

Since the revival of modular synthesis in the early 21st century, there has arisen some stylistic and cultural differences between the two groups of formats. Eurorack has become the most popular format since about 2010; many Euro manufacturers work at hitting a lower price point than the large-format manufacturers do. There also exists the perception that Euro manufacturers are targeting a younger group of customers; some Euro modules feature outlandish graphics and fonts, and rather random placement of controls. As Euro has become the most popular format, the greatest variety of available designs have appeared in that format. The large formats, arguably, offer a superior user interface, with larger knobs and sturdier panels, and these formats use a quasi-standard of white graphics on a black background, with very clean and simple graphical designs. Price-is-no-object designs tend to be more common in large format, although performers who are handy with a soldering iron can alleviate that difficulty by building some modules from kits.

Current manufacturers which use formats unique to their brand include Buchla, Wiard (the 300 series), Serge, and Technosaurus.