UPF 50+ blocks about 98 percent of UV. What the rating measures, why it is not SPF, and why some shirts lose it in the wash.
UPF is a fabric rating, not a sunscreen rating
SPF measures how long treated skin takes to burn. UPF measures how much ultraviolet radiation a fabric lets through to the skin underneath. A UPF 50 rating means roughly one fiftieth of UV gets through, which is about 98 percent blocked. The plus sign means it tested above 50 and the scale stops there.
A plain cotton t-shirt is closer to UPF 5
This is the part that surprises people. An average white cotton tee tests somewhere around UPF 5 to 8 dry, and drops further when it gets wet, because water fills the gaps in the weave and lets more light through. Wearing a long sleeve is not the same as wearing sun protection.
Weave and colour do most of the work
Rating comes from how tightly the fabric is knit, what the fibre is, and how it is dyed or treated. Tight synthetic knits rate high without being heavy. Some shirts get there with a chemical finish instead, which is cheaper to make and washes out over a season.
Coverage beats rating past a point
The difference between UPF 30 and UPF 50 is small compared to the difference between a hood and no hood. Necks, ears and the backs of hands are where anglers actually burn, because they are the parts people forget to cover and the parts sunscreen sweats off first.