Capacitor Charge and Voltage Formula Explained

The basic capacitor formula is Q = C × V: charge equals capacitance multiplied by voltage. This calculator uses capacitance and voltage as inputs and returns the stored charge in coulombs, along with the equivalent charge in microcoulombs.

Formula

The charge-voltage relationship for a capacitor is Q = C × V.

• Q is electric charge, measured in coulombs (C).
• C is capacitance, measured in farads (F).
• V is the voltage across the capacitor, measured in volts (V).

The calculator accepts capacitance in microfarads (uF), so it converts that value to farads by multiplying by 0.000001 before applying the formula. One microcoulomb (uC) is one millionth of a coulomb. The second result is simply the first converted to microcoulombs.

Worked Example

For the default inputs of 100 uF and 12 V:

1. Convert capacitance: 100 uF × 0.000001 = 0.0001 F.
2. Calculate charge: Q = 0.0001 F × 12 V = 0.0012 C.
3. Convert to microcoulombs: 0.0012 C × 1,000,000 = 1,200 uC.

The results are 0.001200 C and 1,200 uC. The coulomb result is displayed to six decimal places; the microcoulomb result is displayed to two.

How Charge and Voltage Relate

Capacitance describes how much charge a capacitor stores per volt across its terminals. Rearranging the same relationship gives V = Q / C, which calculates voltage when charge and capacitance are known. It also gives C = Q / V, useful when charge and voltage are known.

For an ideal, linear capacitor, capacitance is constant, so doubling the voltage doubles the charge. For example, a 100 uF capacitor at 6 V holds half the charge it holds at 12 V: 0.0006 C rather than 0.0012 C. This proportional relationship is the central meaning of the formula.

Charge is not the same thing as current. Charge is an amount, measured in coulombs; current is the rate at which charge flows, measured in amperes. One ampere corresponds to one coulomb passing a point each second. The formula Q = C × V describes the charge associated with a capacitor at a particular voltage, not how quickly it charges.

Units and Conversions

Capacitance is commonly labeled in farads and smaller metric units because one farad is a large capacitance for many everyday circuits:

• 1 mF (millifarad) = 0.001 F.
• 1 uF (microfarad) = 0.000001 F.
• 1 nF (nanofarad) = 0.000000001 F.
• 1 pF (picofarad) = 0.000000000001 F.

Charge is often expressed in coulombs, millicoulombs (mC), or microcoulombs (uC). Voltage is measured in volts. The calculator’s capacitance input is specifically in uF; entering a value printed in nF or pF without converting it first produces a result off by a factor of 1,000 or 1,000,000, respectively. Convert to microfarads before entering: 10 nF is 0.01 uF, while 1,000 pF is also 0.001 uF.

Using the Result and Understanding Its Limits

Use Q = C × V when you know a capacitor’s capacitance and the voltage across it, and want the corresponding stored charge under the usual ideal-capacitor model. Check that the capacitance and voltage refer to the same capacitor and operating condition. The calculator assumes a fixed capacitance and does not model leakage, tolerance, temperature effects, or voltage-dependent capacitance.

Real components can differ from their labeled capacitance, and some capacitor types change capacitance with applied voltage. Therefore, a calculation based on a nominal label value is an estimate of charge, not a measurement. For a varying voltage, the instantaneous relationship still applies to an ideal constant-capacitance capacitor, but the voltage value must be the voltage at the time of interest.

The input range permits zero because zero voltage or zero capacitance gives zero charge in this model. A negative voltage has a polarity or reference direction; the formula can represent signed charge if a sign convention is chosen. This simple calculator uses nonnegative magnitudes, so it does not represent polarity.

Energy Is Not Charge

A frequent misconception is that Q = C × V gives the energy stored in a capacitor. It gives charge. The stored electrical energy is E = ½ C V², measured in joules. In terms of charge, the equivalent expression is E = Q² / (2C). These formulas assume an ideal capacitor and use capacitance in farads and voltage in volts.

For the worked example, energy would be ½ × 0.0001 F × (12 V)² = 0.0072 J. That is a separate quantity from the 0.0012 C of charge. Keeping coulombs and joules distinct prevents a common unit and interpretation error.

Frequently Asked Questions

How do you calculate charge on a capacitor?
Multiply capacitance in farads by voltage in volts: Q = C × V. The result is in coulombs.

How do you calculate voltage from charge and capacitance?
Divide charge by capacitance: V = Q / C. Use coulombs and farads to obtain volts.

What is the charge on a 100 uF capacitor at 12 V?
Convert 100 uF to 0.0001 F, then multiply by 12 V. The charge is 0.0012 C, or 1,200 uC.

Does capacitor charge increase with voltage?
Yes. For a fixed capacitance in the ideal linear model, charge is directly proportional to voltage. Twice the voltage means twice the charge.

Is Q = CV the energy stored in a capacitor?
No. Q = CV calculates charge. Energy is calculated with E = ½CV² and is measured in joules.

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in c | Capacitance | uF | 100 | 0 | 1000000000
in v | Voltage | V | 12 | 0 | 1000000000
out q | Charge | C | c * 0.000001 * v | 6
out qu | Charge | uC | q * 1000000 | 2