Most are very effective at blocking the sun, usually having a shade factor between 85 – 100%, where 100% means that no visible light passes through (you cannot see through it). However, that is not the full story. The shade factor is different to UV Block. The amount of dangerous suns rays that pass through the sail can actually be different to the shade factor and different manufacturer’s designs and material will offer different protection. When selecting your sun shade you then need to decide on your preferred balance between these factors. Eg A shade offering 100% shade block that might make it dark underneath and not drain when it rains. A light-colored shade that is aesthetically pleasing but is too bright to sit under etc For further information the check out Planning for a Sun Shade- what you need to be thinking about or more specifically the section Shade Factor