Over-exposure to ultra-violet radiation (UVR) in the sunlight is the major cause of skin cancer, while under-exposure may cause vitamin D deficiency. Therefore, precise monitoring of the invisible UVR is an important challenge. We developed a photo-active ink that can generate a unique colour response to specific UV wavelengths. We use this ink to fabricate low-cost wearable sensors on a piece of paper that can provide naked-eye monitoring of UVR, even at very low doses typically encountered during solar exposure. As people with different skin colours have different UV tolerance, our UV sensors can be customized to meet the specific needs of different skin phototypes. These low-cost personalised wearable UV sensors offer remarkable potential in managing the impact of UVR in our day-to-day life.