Hike Distance: 8.2 miles (13.2 km) round trip to Upper or Lower Cathedral Lakes; 9.2 miles (14.8 km) round trip to see both
Trailhead Elevation: 8,500 feet (2,590 meters)
Lower Cathedral Lake Elevation: 9,290 feet (2,830 meters)
Upper Cathedral Lake Elevation: 9,600 feet (2,925 meters)
The red line shows the route from the trailhead at Tuolumne Meadows to Upper Cathedral Lake. This trail is part of the John Muir Trail and continues for miles to the south, past the Sunrise High Sierra Camp and Yosemite Valley all the way to Mt. Whitney, which, according to a sign at the top of Nevada Fall, is only another 215 miles (346 km) away. If you've already covered the four miles to Upper Cathedral Lake, it hardly seems right to turn around and go back when California's highest peak is beckoning from so nearby. Yet lots of people do.
The green line shows the offshoot from the main trail to Lower Cathedral Lake. This segment is half a mile (0.8 km) long and descends about a hundred feet (30 meters).
It's not entirely clear from the map, but most of the climbing takes place on the first half of the hike. By the time you're halfway round Cathedral Peak, you've done nearly all the work.
Note that this map currently omits the stub trail, roughly half a mile long, from the new parking area (next to the visitor center) to the trail; the little hiker icon in the map above marks the former location of the trailhead.