Which octave is middle C?

C4, which is MIDI note 60, and that is the convention used throughout. Some manufacturers number the same note C3 and others C5, because the MIDI specification fixes the note number and not the name. The MIDI number is listed alongside so there is no ambiguity about which note is meant.

A hundredth of a semitone, so 1200 to an octave. It is logarithmic, which is what makes it useful: five cents means the same amount of out of tune at the top of the piano as at the bottom, where five hertz would be an enormous interval low down and inaudible high up.

Can I get the frequencies of a chord?

Yes. Separate the notes with commas and each one is listed with its frequency. That is what you want for notching an EQ or setting up an additive patch, where the comparison between the notes matters more than any one of them.

Note to Frequency

Note names to hertz, at whatever pitch you tune to.

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Note or frequency

A4 tuned to

Note names to hertz, at whatever pitch you tune to.

How to use it

Type a note. The letter, an optional sharp or flat, and the octave: A4, C#3, Bb5.
Or type a frequency. A number is read as hertz and answered with the nearest note and the error in cents.
Change the reference if you need to. Leave it blank for 440. Set 415 for baroque pitch and the whole scale moves.

When you would use this

Equal temperament divides an octave into twelve equal ratios, so a note's frequency is the reference multiplied by two to the power of the semitone count over twelve. A4 at 440 Hz is the usual anchor and everything else follows from it. Most note frequency pages are a table of numbers somebody typed in. A table is that formula with the results written down, and it stops being right the moment you tune to anything other than 440. Baroque ensembles use 415, plenty of orchestras use 442 or 443, and 432 has its adherents. Change the reference here and the whole scale moves with it. Give it a frequency instead of a note and it goes the other way, naming the nearest note and how far off it sits in cents. Cents are logarithmic, a hundred to a semitone, which is what makes out of tune mean the same thing at the top and the bottom of the range. Separate several notes with commas to get a whole chord at once.

Questions

Which octave is middle C?: C4, which is MIDI note 60, and that is the convention used throughout. Some manufacturers number the same note C3 and others C5, because the MIDI specification fixes the note number and not the name. The MIDI number is listed alongside so there is no ambiguity about which note is meant.
What is a cent?: A hundredth of a semitone, so 1200 to an octave. It is logarithmic, which is what makes it useful: five cents means the same amount of out of tune at the top of the piano as at the bottom, where five hertz would be an enormous interval low down and inaudible high up.
Can I get the frequencies of a chord?: Yes. Separate the notes with commas and each one is listed with its frequency. That is what you want for notching an EQ or setting up an additive patch, where the comparison between the notes matters more than any one of them.