Mount Everest - how tall (from base)?

This question was asked previously and the issue is simply one of how you define the base and its elevation. That is not a precisely defined idea. What, exactly, is the "base" of a mountain?

This is why we use a standard reference elevation (mean sea level), and even that fairly specific term has some discussion of how to define its exact level.

Depends where you call "the base"! The foothills of the Himalayas are up to 500 metres (1600 feet) high.

All mountains are given a height "from sea level", and Everest is 8,848 metres (just over 29,000 feet) from sea level to peak.

It depends upon what you call the "base". Any measurement is valid, as long as you say where the starting point is. Actually, its REAL "base" is probably as deep in the mantle as the peak is above the surface.

Tibetan plateau is 4,500 meters above sea Level and Mt Everest sits on top of this.

Measurements for mountains are measured from Sea Level to the top of the Mountain = Method 1...

Method 2 is to measure from the Base of the Mountain, and Method 3 is to measure from the Center of the Earth. Actually, if measured from the sea level, Mt. Everest is the highest point on earth. But several other mountains are sometimes claimed to be the "tallest mountains on earth".

E.g., Mauna Kea in Hawaii is tallest when measured from its base with height over 10,200 m (33,464.6 ft), but only attains 4,205 m (13,796 ft) above sea level.

By the same measure of base to summit, Denali, in Alaska, also known as Mount McKinley, is taller than Everest as well. But its height above sea level is only 6,190 m (20,308 ft). Denali sits atop a sloping plain with elevations from 300 to 900 m (980 to 2,950 ft), yielding a height above base in the range of 5,300 to 5,900 m (17,400 to 19,400 ft).

In addition, the summit of Chimborazo in Ecuador is 2,168 m (7,113 ft) farther from earth's centre (6,384.4 km (3,967.1 mi)) than that of Everest (6,382.3 km (3,965.8 mi)), because the earth bulges at the equator. But its height above sea level is only 6,268 m (20,564.3 ft) while Mount Everest's if 8,848 m (29,028.9 ft).