To minimize health injuries in mass-casualty nuclear events, personal-level preparedness, including a simple, universal, and cost-effective dosimetry tool, is crucial. In this study, we propose an inexpensive dosimetry system using a radiochromic film, which can immediately detect high-dose radiation exposure by the naked eye, and a portable scanner coupled with a smartphone as a practical tool for radiological emergency preparedness. Small pieces of Gafchromic EBT4 films were irradiated with X-rays (160 kVp, 6.3 mA) at 2.5, 5.0, and 10 Gy (for water) and scanned on an LED light table of a commercially available portable scanner (PictoScanner ApS, Denmark) using four different smartphones: Samsung Galaxy A23 5G SC-56C, Samsung Galaxy Note8, iPhone Xs Max, and iPhone 13 Pro Max.