What is a microfarad (µF)?
A microfarad is one millionth of a farad, the unit of capacitance — how much electrical charge a capacitor can hold at a given voltage. In HVAC it is the number that matters: it is stamped on every capacitor, it is what your meter reads, and it is the one specification that must match exactly when you replace one. You will also see it written MFD on older cans and on most meters; it means the same thing.
Common questions
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MFD, mfd, µF, uF — are these all the same?
Yes, all four. MFD is the older American convention and still appears on capacitor labels and meter dials; µF is the SI form; uF is what people type when they cannot find a µ. There is no difference. (Strictly, "m" is the SI prefix for milli-, so MFD is technically a thousand times what it says — but no HVAC capacitor has ever been rated in millifarads, and the trade has used MFD to mean microfarad for eighty years.)
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How far off is too far off?
Most run capacitors are rated ±6%, and that tolerance is the whole answer: a 45 µF section is healthy down to about 42.3 µF and failed at 40. A capacitor does not usually die outright — it drifts down over years, and the motor runs hotter and draws more current every season it drifts. That is why "it still reads something" is not a pass.
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Can I go up a size for a bit more starting power?
No. This is the most common well-intentioned mistake in the trade. The capacitance is matched to the motor's start winding, not to how hard you would like it to work — too much µF shifts the phase angle past optimal, and the motor draws more current and runs hotter while feeling perfectly fine. If a compressor needs more starting torque, the answer is a hard-start kit, not a bigger run capacitor.