Do I need to install an app?

Not to make the font. The conversion runs in Safari. Installing the finished font system-wide on iOS does require a font manager app, because Apple installs fonts through configuration profiles.

Does it work without a signal?

Yes. The conversion runs on the device, so once the page has loaded it does not need the network. The page itself is cached for offline use after your first visit.

Can I use the font in Instagram or Notes?

Only if the app uses the iOS system font picker. Many social apps use their own text rendering and will not offer installed custom fonts.

Create a custom font on iPhone

You can go from a pen on paper to an OTF file without leaving your phone, and without installing anything. The conversion runs in Safari. What happens after you have the file is where iOS gets particular, and it is worth knowing before you start.

Capture the lettering

Write your characters on unlined white paper with a pen that lays down solid ink, leaving clear space between them. Then photograph the sheet rather than scanning it.

The camera is the weak link on a phone, so give it help. Find flat, indirect light — near a window on an overcast day is close to ideal. Hold the phone parallel to the page rather than at an angle, and check for your own shadow falling across the paper before you shoot. Turn the flash off; it puts a bright spot in the middle of the frame and darkens the corners.

Convert it in Safari

Open the converter and use the camera button to shoot directly, or the upload button to pick a photo you already took. The image is traced and built into a font on the phone itself — nothing is uploaded, which is also why it works with no signal.

01Tap Camera to shoot the sheet, or Upload to choose an existing photo.
02Wait for the trace. Check the character map and confirm each shape landed on the character you meant.
03Adjust spacing and baseline until a typed line looks right, then name the font.
04Download the OTF, or the full kit if you want the SVGs and CSS too.

What iOS does with the file

This is the part that surprises people, and it is an iOS design decision rather than anything to do with the conversion.

Downloading an OTF in Safari saves it to the Files app. iOS will not install a font by opening it there. System-wide font installation on iPhone and iPad goes through a configuration profile, which is what font manager apps from the App Store produce — they wrap the font in a profile and hand it to Settings to install. Once installed that way, the font appears in apps that use the system font picker, such as Pages, Keynote and Mail.

Apps that do not use the system picker will not see it. That is normal and not a fault in the font.

Where the font works without installing anything

You often do not need a system install at all. Transfer the OTF to a computer via AirDrop, iCloud Drive or email and install it there in the usual way. Upload it to a design tool that accepts custom font files. Or use the exported font.css and OTF directly on a website, where the browser loads the font and no installation is involved.

Questions

Do I need to install an app?: Not to make the font. The conversion runs in Safari. Installing the finished font system-wide on iOS does require a font manager app, because Apple installs fonts through configuration profiles.
Does it work without a signal?: Yes. The conversion runs on the device, so once the page has loaded it does not need the network. The page itself is cached for offline use after your first visit.
Can I use the font in Instagram or Notes?: Only if the app uses the iOS system font picker. Many social apps use their own text rendering and will not offer installed custom fonts.