How this instrument works
Q = C·V is the defining relationship of a capacitor stated the other way round: capacitance is by definition the ratio of charge to voltage, so multiplying that ratio back through voltage returns the charge sitting on one plate at that instant. Nothing here is squared or inverted — charge tracks voltage in a straight line. Push the voltage from 6 V to 12 V on a fixed capacitor and the charge exactly doubles, a proportionality that stored energy, which climbs with the square of voltage, does not share.
A coulomb is a large amount of charge, roughly 6.242 x 10^18 electrons, so most components deal in fractions of one. A 10 pF trimmer capacitor at 5 V holds only 50 picocoulombs, while a 1 F supercapacitor at 2.7 V holds 2.7 coulombs — a difference of nearly eleven orders of magnitude for a component obeying exactly the same formula. That range is why the result field carries a unit menu running from nanocoulombs up to whole coulombs rather than forcing every answer onto one scale.
The formula gives the charge once the capacitor has finished settling at a fixed voltage, not the charge at some mid-transient instant. Charging through a resistor, plate charge climbs as q(t) = C·V·(1 minus e to the minus t/RC) and only equals C·V after several time constants have passed. Even settled, real dielectrics leak: an electrolytic left disconnected slowly loses charge through its own internal resistance, so a part measured hours later reads lower than C·V predicted, which is exactly why bench technicians re-check with a meter rather than trusting a stale calculation.
- Enter the part's rating into Capacitance, choosing pF, nF, uF, mF or F to match the label — no need to convert a 100 uF electrolytic by hand.
- Enter the working Voltage across the plates — the voltage the capacitor actually sits at right now, not a printed maximum rating.
- Read Stored charge, switching its unit between nC, uC, mC and C to match the scale of your component.
- Recompute at a second voltage, such as a safe discharge threshold, and subtract the two charges to see how much actually moved.
Worked example — bench-checking a 100 uF filter capacitor
A technician about to resolder a connector on a 12 V DC power supply board wants to know how much charge the board's 100 uF filter capacitor is still holding before touching its leads. Capacitance is 0.0001 F (100 uF) and the rail sits at 12 V, so Q = C · V = 0.0001 x 12 = 0.0012 coulombs — 1.2 millicoulombs, or 1200 microcoulombs on the finer scale.
1.2 mC sounds small next to a car battery's amp-hours, but it is a few thousand times the charge in a typical electrostatic discharge from a person walking across carpet, which stores on the order of a few hundred nanocoulombs at a few kilovolts, the same regime JEDEC's human-body-model ESD test standard is built around. That gap is exactly why service manuals specify a bleed resistor across filter capacitors: left floating, 1.2 mC across two fingers is enough to be felt, and on higher-voltage rails the same arithmetic scales to genuinely dangerous levels.
Questions
Does doubling the voltage double the stored charge?
Yes. Q = C·V is linear in V, so for a fixed capacitance, twice the voltage always means exactly twice the charge — 6 V to 12 V on the same capacitor doubles the coulombs on its plates. That contrasts with stored energy, E = half C V squared, which quadruples over the same change, since energy scales with the square of voltage rather than voltage itself.
How is stored charge different from stored energy?
Charge is what actually sits on the plates, a straight multiple of voltage, Q = C·V. Energy is the work it took to put it there, E = half C V squared, because early charge arrived cheaply at low voltage while later charge pushed against whatever voltage the earlier charge had already built. A capacitor holding twice the charge of another, at the same capacitance, holds four times the energy, not two.
Why does a disconnected capacitor still shock people?
Because charge has nowhere to go once the circuit is open, and the plates keep the voltage, and the charge Q = C·V represents, until something bleeds it away. Large filter and motor-run capacitors can hold a dangerous voltage for minutes after power is removed, which is exactly why equipment manuals specify a bleed resistor and a minimum wait time, or a direct discharge check, before anyone touches the terminals.
What does this formula have to do with static-electricity shocks?
The same arithmetic applies to your own body. Walking across carpet in dry air can charge your body's self-capacitance, roughly 100 to 300 picofarads, to several kilovolts, storing charge on the order of a few hundred nanocoulombs — the regime the JEDEC human-body-model ESD test standard is built around. That is far smaller than a filter capacitor typically holds, but concentrated into a shorter, sharper discharge, which is what makes the spark noticeable.
Can I use Q = C·V while the capacitor is still charging?
No, the formula gives the settled value once voltage has stopped changing, not the charge at some instant mid-transient. While charging through a resistor, plate charge follows q(t) = C·V·(1 minus e to the minus t/RC), climbing toward C·V but only reaching it after roughly five time constants. Use this calculator for the before-and-after states, not for a snapshot taken partway through a charge or discharge.
How does stored charge relate to current in a circuit?
Current is charge in motion — one amp is one coulomb per second. A switched-capacitor circuit that moves Q = C·V coulombs every switching cycle at frequency f draws an average current of I = f · C · V, which is exactly how charge-pump and switched-capacitor converter datasheets predict output current from a chosen capacitor and switching frequency.