Resistor, Capacitor & Inductor Calculators

Using Resistor, Capacitor & Inductor Calculators

Passive parts only come in set values, and each one behaves differently with frequency. Resistor, Capacitor & Inductor Calculators handle both problems. They find the value a circuit needs, then the nearest part you can actually buy.

Resistance stays the same at any frequency. Capacitors and inductors do not, which is what makes timing, filtering, and tuning possible.

Reading and Combining Resistors

The Resistor Color Code Calculator decodes 4, 5, and 6-band parts. The first bands are digits, the next is the multiplier, and the last is tolerance. Brown-black-orange-gold reads as 10 kΩ at ±5%.

Values are not arbitrary. Each E-series divides a decade into fixed logarithmic steps, and tolerance decides which series a part comes from.

E12 is the common stock series: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, then the same numbers a decade up. Each step is about 21% above the last, so ±10% parts overlap and no gap is left uncovered.

The Parallel Resistor Calculator combines a network and returns the equivalent value. Two 10 kΩ in parallel give 5 kΩ. A 10 kΩ with a 4.7 kΩ gives about 3.2 kΩ, which is one way to reach a value that no series stocks.

Parallel resistance is always below the smallest resistor in the group. Series resistance is the plain sum. Capacitors work the opposite way: they add in parallel and combine like parallel resistors in series.

Dividers, LEDs, and Bridges

The Voltage Divider Calculator gives the output of two resistors across a supply. Output is the input times the bottom resistor over the total. So 12 V across 10 kΩ and 4.7 kΩ gives about 3.84 V at the tap.

A divider only holds that voltage under light load. Draw real current from the tap and it sags, so dividers suit reference and sense points, not power.

The Series LED Resistor Calculator subtracts LED forward voltage from the supply, then divides by the target current. A 9 V supply, a 2.0 V LED, and 20 mA calls for 350 Ω.

That exact value is not stocked, so the tool moves to the nearest standard part. At 390 Ω the current settles near 17.9 mA and the resistor dissipates about 0.13 W, which a 1/4 W part handles comfortably.

The Wheatstone Bridge Calculator finds the differential output across a four-resistor bridge. The bridge balances, and reads zero, when the two arm ratios match. Strain gauges and RTDs use that null point to turn a tiny resistance change into a measurable voltage.

Capacitors: Charge, Energy, and Reactance

The Capacitance Calculator works from plate area, spacing, and dielectric for parallel-plate geometry, and returns stored charge and field strength.

Stored energy is half the capacitance times voltage squared. A 1,000 µF capacitor at 12 V holds 0.072 joules. Doubling the voltage quadruples the energy, which is why a charged bank stays dangerous after power is removed.

The Capacitor Impedance Calculator gives reactance at a frequency. Capacitive reactance is 1 divided by 2π times frequency times capacitance, so it falls as frequency rises. A 100 nF capacitor is about 1.6 kΩ at 1 kHz and only 160 Ω at 10 kHz.

That slope is why the same capacitor blocks DC, passes audio, and looks like a short at radio frequencies.

Time Constants

The RC Time Constant Calculator and Capacitor Charge and Time Constant Calculator cover charging, discharging, and filter cutoff.

One time constant is resistance times capacitance. In that period the capacitor reaches about 63.2% of the applied voltage. Five time constants put it above 99%, which is the usual settling figure.

Take 10 kΩ with 100 nF. The time constant is 1 ms, so the capacitor settles in about 5 ms. The same pair sets a filter corner at about 159 Hz, since cutoff is 1 divided by 2π times R times C.

Timing and filtering are the same maths seen from two directions. A slower charge is a lower cutoff frequency.

Resonance and Inductance

The LC Resonant Frequency Calculator finds where an inductor and capacitor cancel. At resonance their reactances are equal and opposite, and the frequency is 1 divided by 2π times the square root of L times C.

A 10 µH coil with a 220 pF capacitor resonates near 3.39 MHz. Tank circuits like that set the frequency in oscillators, tuners, and bandpass filters.

Inductance itself comes from geometry. The Coil Inductance Calculator works from turns, diameter, length, and core permeability, and also reports Q factor and stored energy. Turns count matters most, since inductance rises with the square of the turns.

The Wire Self Inductance Calculator covers a single straight conductor, which still has inductance and matters at high frequency. The Parallel Wire Inductance Calculator handles a two-wire line, and also returns capacitance, impedance, and propagation delay.

Common Mistakes

Designing to an unavailable value. A 350 Ω answer means picking 330 Ω or 390 Ω, then rechecking the current.

Ignoring resistor wattage. The value can be right while the part cooks. Check power before choosing a package.

Loading a voltage divider. Current drawn at the tap pulls the output below the calculated value.

Mixing up prefixes. Nano and micro are a thousand apart, and that error moves a time constant or a cutoff by the same factor.

Component FAQs

How do I read a 4-band resistor?

The first two bands are digits, the third is the number of zeros, and the fourth is tolerance. Brown-black-orange-gold is 1, then 0, then three zeros, so 10,000 Ω at ±5%. Gold means 5% and silver means 10%, while 1% parts use a brown tolerance band and usually five bands instead of four.

What resistor does a 9 V LED circuit need?

Subtract the LED’s forward voltage, then divide by the current you want. With a 2.0 V red LED at 20 mA, that is 7 V ÷ 0.02, or 350 Ω. Standard parts jump from 330 Ω to 390 Ω, and 390 Ω gives about 17.9 mA with 0.13 W in the resistor, so a 1/4 W part is fine.

Why are resistor values like 4.7k and 47k?

They come from the E-series, which splits each decade into logarithmic steps. E12 has 12 values per decade, each about 21% above the one before, which is what makes ±10% parts overlap without gaps. Tighter tolerances use more values, so E96 parts cover 96 steps per decade at ±1%.

How long does a capacitor take to charge?

One time constant, resistance times capacitance, gets it to about 63.2%. Five time constants put it past 99%, which most designs treat as fully charged. With 10 kΩ and 100 nF the time constant is 1 ms, so settling takes roughly 5 ms. Larger resistance or capacitance stretches both figures.

How do I find the resonant frequency of an LC circuit?

Take 1 divided by 2π times the square root of inductance times capacitance, in henries and farads. A 10 µH coil with 220 pF lands near 3.39 MHz. To move the frequency, changing either part shifts it by the square root, so four times the capacitance halves the frequency.

Do capacitors add like resistors?

They work in reverse. Capacitors in parallel add up, so two 100 nF parts give 200 nF. In series they combine the way parallel resistors do, so two 100 nF parts give 50 nF. Inductors follow the resistor rules: they add in series and combine inversely in parallel.