Eurorack case size is measured in HP (horizontal pitch, width) and 3U rows (height), and modules are sized in HP

The standard Eurorack sizing system uses two units: HP (horizontal pitch) for width and 3U for height. HP is measured in 0.2-inch increments, so a 10 HP module is 2 inches wide. A 3U row is a single tier of module slots. When buying a case, you choose its HP capacity and number of rows; when browsing modules, each module’s HP count tells you how much horizontal space it needs. A standard 84 HP case can hold a variety of modules whose HP widths add up to 84 or less. This measurement system is universal across the Eurorack standard, meaning a module from any manufacturer will fit any case of the same rack-unit height.

Examples

A Manis Iteritas from Noise Engineering takes 10 HP. A 3U x 84 HP case holds modules whose HP widths total 84 — roughly 8 to 15 modules depending on their size.

Assessment

Define HP and 3U in the context of Eurorack cases. If a case is 84 HP wide and you want to add a 10 HP module, how many HP remain for other modules?