One kilohertz is exactly 1,000 hertz. The pair spans the range of human hearing and mains electricity, and it is where the difference between a cycle you can feel and one you can hear falls.
About the units
The hertz is the SI unit of frequency, one cycle per second, named after Heinrich Hertz, who demonstrated the existence of electromagnetic waves in 1887. Before the unit was adopted internationally in 1960, frequency was written as cycles per second, and older equipment is marked c/s or cps. The kilohertz is a thousand cycles per second. Human hearing spans roughly 20 Hz to 20 kHz, which means this single pair covers the entire audible range — from the lowest note of a pipe organ to the upper limit of a young person's hearing.
The exact factor
The factor is exactly 1,000. Useful anchors across the range: mains electricity is 50 Hz in Europe and 60 Hz in North America; the A above middle C is 440 Hz; the lowest note on a piano is 27.5 Hz and the highest 4,186 Hz; and telephone audio is limited to 300 to 3,400 Hz, which is why voices sound thin on a phone call. Ultrasound begins above 20 kHz, and medical imaging uses 2 to 18 MHz.
Where you meet this conversion
Audio engineering throughout. Equaliser bands, crossover frequencies and sampling rates are all specified in this range — CD audio samples at 44.1 kHz, chosen to capture the full audible band with margin. Loudspeaker specifications give frequency response in hertz at the low end and kilohertz at the high. Electrical engineering uses hertz for mains and kilohertz for switching power supplies. Vibration analysis in machinery works at the low end, where bearing faults show as characteristic frequencies of a few hundred hertz.