What is the PHO file format?
A .pho file is a Gerber photoplot file - an ASCII text file encoding photoplotter instructions for printed circuit board (PCB) fabrication. The name refers to the photoplotter machines that historically exposed PCB film layers using this data. The .pho extension is the default output convention of PADS PCB Layout (now Siemens PADS); other EDA tools use extensions such as .gbr, .ger, or .gtl for the same format.
The file content follows the RS-274X ("Extended Gerber") standard: a sequence of aperture definitions (%ADD...%), format and unit statements (%FS...%, %MOIN% or %MOMM%), coordinate commands (D01 draw, D02 move, D03 flash), and an end-of-file marker (%M02%). RS-274X embeds aperture data directly in the file, replacing the earlier RS-274D standard that required a separate aperture list. One .pho file typically represents one fabrication layer - copper, silkscreen, solder mask, drill drawing, and so on.
Any Gerber viewer or CAM tool handles .pho files identically to .gbr files. Modern Gerber X2 attributes (%TF/%TA metadata commands) may also appear in .pho output. Ucamco, the format maintainer, recommends .gbr as the standard extension going forward.
Security & safety
RISK: LOWPHO/Gerber files are plain ASCII text and contain no executable code. They are safe to open with any Gerber viewer. No malware risk specific to this format.
Format details
in a nutshellPrograms that open PHO files
Windows5 apps
Altium Designer
Paid File → Import → Gerber, then select the .pho file to view or import into a PCB project. 
ViewMate (PentaLogix)
Free Open the .pho file directly in ViewMate - it opens Gerber files regardless of extension.
KiCad (Gerber Viewer)
Open-source Open KiCad's Gerber Viewer (GerbView), then File → Open → select the .pho file. 
macOS2 apps
KiCad (GerbView)
Open-source Open GerbView from the KiCad suite, then load the .pho file for viewing. 
Linux2 apps
KiCad (GerbView)
Open-source Open GerbView, load the .pho file - available in most Linux package managers.
Gerbv
Open-source Available in most Linux distros (apt install gerbv); opens .pho as standard Gerber.
PCBShopper Gerber Viewer (online)
Free Upload the .pho file (rename to .gbr if the site doesn't accept .pho extension) to view online. Technical details
deep spec| Format standard | RS-274X (Extended Gerber); compatible with RS-274D (obsolete predecessor) |
| Encoding | Plain ASCII text (7-bit) |
| Primary producing tool | PADS PCB Layout (Siemens PADS) - default output extension for photoplot layer files |
| Layer scope | One file per PCB fabrication layer (copper, silkscreen, solder mask, drill drawing, etc.) |
| Aperture definitions | %ADD...% commands embedded in file (RS-274X); no separate aperture list required |
| Coordinate commands | D01 (draw/linear), D02 (move), D03 (flash aperture); integer or decimal coordinates |
| Unit declaration | %MOIN% (inches) or %MOMM% (millimeters) |
| End-of-file marker | %M02% |
| Gerber X2 attribute support | %TF/%TA/%TD/%TO metadata attribute lines (optional; backward-compatible with RS-274X viewers) |
| Coordinate precision | Sub-micron resolution supported via integer/decimal digit configuration in format statement |
| Typical file size | 10 KB - 5 MB per layer; large copper pour layers can exceed this |
| Magic bytes | None; file begins with Gerber comment lines (G04) or format/aperture command blocks |
| Identity with .gbr | Content identical to .gbr, .ger, .gtl files; .pho is a vendor naming convention, not a distinct format |
| Released | 1960s (original Gerber format); RS-274X extended format 1991 |
| Latest version | Gerber X2 (2014) / X3 (2023) - see spec; .pho files typically RS-274D or RS-274X |
| Open standard | Yes · royalty-free |
| Specification | www.ucamco.com |
PHO conversions
Community Q&A
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