Convert feet to km (ft to km)

One foot is exactly 0.0003048 kilometers. That is a definition rather than a measurement: the imperial and US customary units were fixed against metric ones by the 1959 international yard and pound agreement, so this factor does not drift and will not be revised.

Worked through: 10,000 ft x 0.0003048 = 3.048 km.

Going the other way, one kilometer is 3,280.839895 feet.

For mental arithmetic: Divide by 3,280. Or multiply by 0.3 and divide by 1,000, then add 1.6%.

Altitude is where feet meet kilometers. Aviation worldwide measures height in feet, so a cruising altitude of 35,000 ft is 10.67 km, and mountains are quoted in both: Everest's surveyed height of 8,848.86 m is 29,031.7 feet. Going up, the edge of space by the Karman line convention is 100 km, which is 328,084 feet. Over that kind of range a small error in the factor is large in the answer: using 3,000 feet to the kilometer instead of 3,280.84 puts an airliner more than 900 meters too high.

Precision is worth matching to the input. A measurement given as 100 ft carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.