Capacitor Calculator - Code, Tolerance & C = Q / V

A capacitor calculator combines three jobs in one tool: decodes the 3-digit marking printed on a component, encodes a capacitance value back into a 3-digit code, and solves the C = Q / V relationship when you know two of the three values.

• • Decode a capacitor marking: Read the printed code such as 104 or 475 and turn it into a capacitance in pF, nF, or µF.
• • Pick the right capacitor: Translate a planned capacitance in µF back into the 3-digit code you need to look up on a parts distributor.
• • Check tolerance acceptance: Apply the EIA tolerance letter such as J, K, or Z and read the min and max accepted capacitance.
• • Solve C = Q / V: Leave the value you want to solve for at zero and read the third value from the result panel.

Real capacitors are rarely labelled with the long-form capacitance. The 3-digit code convention lets manufacturers print small values without writing out exponents. A 104 marking means 10 pF times 10 to the 4, which is 100 nF, and the same convention shows up as 220 (22 pF) or 475 (4.7 µF).

The calculator also applies the standard tolerance letter table from the EIA marking convention. Letters B through D use absolute picofarad ranges, F through M use percentage ranges, and Z uses +80% / -20% common on ceramic bypass capacitors.

When you need the capacitance of a parallel-plate geometry rather than the value printed on a component, the capacitance calculator applies the C = εA/d formula to plate area, plate separation, and the dielectric material.

The calculator runs three small formulas, one for each mode, and reports the result in a unit prefix that keeps the printed number between 1 and 999.

• Capacitance C: Capacitance in the selected unit. Converted to farads before applying C = Q / V.
• Stored charge Q: Charge stored on the plates in the selected unit. Converted to coulombs before applying C = Q / V.
• Voltage V: Voltage across the plates in volts.
• Capacitor code: Three-digit marking where the first two digits are the value in pF and the third is the power-of-ten multiplier (0 to 6).
• Tolerance letter: EIA tolerance marking: B, C, D use absolute pF ranges, F through M use percentage ranges, Z uses +80% / -20%.

In parameter mode the calculator converts capacitance and charge into SI base units and rearranges C = Q / V depending on which input is left at zero. The same iteration returns E = ½ C V² for the resulting combination.

In code-to-capacity mode the calculator pads the input to three digits and reads the value as mantissa × 10^exponent picofarads. The reverse direction picks the smallest exponent that leaves a two-digit mantissa and rounds to the nearest integer.

According to the Wikipedia (Capacitor) article, which itself summarises the BS EN 60062 (formerly BS 1852) and EIA standard for fixed capacitors, the capacitance equals the ratio of the stored electric charge to the voltage across the plates, written as C = Q / V.

According to NIST Guide for the SI (Chapter 4), the farad is the SI derived unit of capacitance equal to one coulomb per volt, supporting the dimension of C = Q / V used in parameter mode.

After you decode the capacitance from the marking, the capacitor charge time calculator takes the same C along with a series resistance and a voltage threshold to estimate the RC charge or discharge time to that target.

Four ideas show up in every capacitor marking and every C = Q / V calculation; understand these once and the calculator reads itself.

The convention assumes the marking on the part is a nominal value and the tolerance letter sets the accepted range, matching how manufacturers test and bin parts.

Capacitance is also temperature-sensitive, voltage-sensitive, and aging-sensitive. The tolerance letter only describes the as-shipped spread.

The same decoded capacitance sets the AC impedance of the part at a chosen frequency, so the capacitive reactance calculator applies Xc = 1 / (2π f C) to translate the marking value into ohms for an AC analysis.

Five quick steps take you from a printed marking or a circuit assumption to a complete capacitance, charge, voltage, energy, and tolerance range result.

1. 1 Pick the calculator mode: Choose Code to capacity, Capacity to code, or Solve C = Q / V.
2. 2 Enter the marking or the capacitance: Type the 3-digit code such as 104 for code-to-capacity mode, or the capacitance value and unit such as 4.7 µF.
3. 3 Choose the tolerance letter: Pick the EIA tolerance marking that matches the printed letter.
4. 4 Supply charge and voltage for parameter mode: Fill any two of capacitance, charge, and voltage. Leave the field to solve for at zero.
5. 5 Read the result panel: Check the auto-selected prefix for capacitance and charge, the voltage in volts, and the tolerance range.

A bench capacitor is marked 225K. Pick Code to capacity, type 225, choose tolerance K. The result panel shows 2.2 µF with tolerance range 1.98 µF to 2.42 µF.

If the surrounding circuit needs a series resistor to limit the inrush current, the Ohm's Law calculator covers V = IR and the related power and current combinations for any two of voltage, current, and resistance.

Six practical benefits explain why this calculator saves time compared with a separate code reference, a separate C = Q / V worksheet, and a separate tolerance table.

• • Three jobs on one screen: Combines code decoding, value encoding, and the C = Q / V relationship so you do not switch between three separate reference cards.
• • Auto-selected unit prefix: Reads the result in the prefix that keeps the printed value between 1 and 999.
• • Covers the full EIA tolerance table: Handles every standard tolerance letter from B (±0.1 pF) through Z (+80% / -20%).
• • Reports stored energy alongside capacitance: Computes E = ½ C V² from the same C and V you already entered.
• • Accepts any common prefix on input: Takes capacitance in F, mF, µF, nF, or pF and charge in C, mC, µC, nC, or pC, then converts internally to SI base units.
• • Encodes values the standard way: Rounds the input to two significant figures and writes a 3-digit code in the same convention a manufacturer would print.

When the decoded value lands in a prefix the rest of the design does not use, the capacitance conversion calculator converts F, mF, µF, nF, and pF between each other using the same power-of-ten factor table this calculator uses internally.

Four factors decide whether the calculator's result matches the part on the bench, and two limitations tell you when to reach for a more detailed analysis.

• • The calculator assumes the capacitor is ideal, with no leakage current, no equivalent series resistance, and no dielectric absorption.
• • A 3-digit code cannot encode values above about 99 µF because the third digit cannot exceed 6.

According to the Wikipedia (Capacitor markings) section, which traces the convention back to the BS EN 60062 and EIA standard for fixed capacitors, a 3-digit code uses the first two digits as the capacitance in picofarads and the third digit as the power-of-ten multiplier, so a code of 104 represents 10 pF multiplied by 10^4, which equals 100 nF.

Once you know the capacitance, the capacitor charge calculator applies Q = C V and E = ½ C V² to the same value and reports stored charge and stored energy with consistent units and prefixes.