This tool was originally built for friends and family in Spain who needed a fast and reliable way to calculate their DNI letter — especially when filling out government forms or doing online paperwork. I noticed that many existing sites were outdated or full of distractions, so I focused on making something clean and mobile-friendly. Right now, it's being used by students, public service applicants, and even HR departments for quick verification. I also received feedback from a few bloggers who featured it in their resource lists.