TOOL / HP · U · MM
Eurorack HP and U calculator
1 HP = 5.08 mm of width and 1 U = 44.45 mm of height. Type a width in HP or in millimeters and pick a row: the calculator gives the size the panel is cut at, draws it to scale with its mounting holes, and works out the size of a case.
Calculator
How the numbers are worked out
Width, from HP to millimeters
1 HP is 5.08 mm, a fifth of an inch: the pitch of the threads on the rails. A panel is cut slightly under its pitch so that neighbours do not bind. Doepfer publishes the width of the common sizes, 30.00 mm for 6 HP and 60.60 mm for 12 HP, and the calculator takes 0.35 mm off the pitch for the others. The HP to mm table lists every width up to 42 HP.
Millimeters back to HP
Divide by 5.08 and round up: a 58 mm board takes 12 HP of rack space, since 11 HP would only give 55.88 mm. Rack space always comes in whole HP.
Height, from U to millimeters
1 U is 1.75 inches, 44.45 mm: the rack unit of 19-inch racks. A 3U Eurorack panel is 128.50 mm high, the 133.35 mm of its row less the 4.85 mm the rail lips take. 1U comes in two standards that do not fit each other's cases: Intellijel (39.65 mm) and Pulp Logic tiles (43.18 mm, in multiples of 6 HP). No brand publishes 2U or 4U, which are worked out the way 3U is. The panel dimensions give the mounting holes of each format.
Cases
A case is counted in rows and in HP. A 19-inch rack row holds 84 HP; two 3U rows make a 6U case, 266.70 mm of rack space, and a 1U row on top makes it 7U. The calculator gives the space the rows take: the case's own panels and rails come on top.
Rack units at a glance
| RACK UNITS | ROWS | HEIGHT (MM) | HEIGHT (IN) |
|---|---|---|---|
| 1U | One 1U row | 44.45 | 1.75 |
| 3U | One 3U row | 133.35 | 5.25 |
| 6U | Two 3U rows | 266.70 | 10.50 |
| 7U | Two 3U rows and a 1U row | 311.15 | 12.25 |
| 9U | Three 3U rows | 400.05 | 15.75 |
| 12U | Four 3U rows | 533.40 | 21.00 |
Questions
How do I convert HP to millimeters?
Multiply by 5.08: 12 HP is 60.96 mm of rail. The panel itself is cut a little narrower, 60.60 mm for 12 HP, so that neighbouring modules do not bind.
How do I convert millimeters to HP?
Divide by 5.08 and round up. A 58 mm circuit board takes 12 HP: 11 HP would only give 55.88 mm.
How tall is a 3U Eurorack module?
Its front panel is 128.50 mm high. 3U is 133.35 mm of rack space, and the rail lips take the other 4.85 mm.
What is the difference between HP and U?
HP measures width and U measures height. 1 HP is 5.08 mm, a fifth of an inch: the horizontal pitch of the rails. 1 U is 44.45 mm, 1.75 inches: the rack unit of 19-inch racks. Eurorack modules are 3U high and a whole number of HP wide.
How many HP fit in a 19-inch rack?
84 HP per row, the usable width Doepfer gives for a 19-inch rack.
How tall is a 6U or a 7U case?
A 6U case, two 3U rows, takes 266.70 mm of rack height. A 7U case adds a 1U row, for 311.15 mm. The case's own top and bottom come on top of that.
Sources
The pitch, the 3U height and the 84 HP of a 19-inch row come from the Doepfer A-100 construction details, 1U from the Intellijel 1U specifications and from Pulp Logic. The calculator uses the code of the editor: panelFormat.ts for the sizes and mountingHoles.ts for the holes.
Draw it instead of measuring it
Eurorack Panel Designer applies every number on this page automatically: pick a width in HP or millimeters, drop the parts, and export an STL, an SVG, a PNG or a KiCad Edge.Cuts outline. It runs in the browser, it is free, and it needs no account.
Open the editor