Ohm’s law and power
V = I × R I = V ÷ R R = V ÷ I P = V × I = I² × R = V² ÷ R
Voltage in volts, current in amps, resistance in ohms, power in watts. 5 V across 220 Ω drives 5 ÷ 220 = 22.7 mA through it and turns 5 × 0.0227 = 114 mW into heat. Work in base units, or let the calculator read the prefixes: 20m is 20 mA, 4k7 is 4.7 kΩ.
Resistors in series and in parallel
Series: R = R1 + R2 + R3 … Parallel: 1/R = 1/R1 + 1/R2 + 1/R3 … Two in parallel: R = R1 × R2 ÷ (R1 + R2)
In series the same current flows through every resistor and the voltages add up: the total is above the largest value. In parallel every resistor sees the same voltage and the currents add up: the total is below the smallest value. Two equal resistors in parallel give half the value and can handle twice the power of one.
Making a value that isn’t standard
Resistors come in the preferred values of IEC 60063: 12 per decade in the E12 series, 24 in E24 and 96 in E96. When the value you need isn’t one of them, two or three standard parts usually get within a fraction of a percent: 10 kΩ ∥ 10 kΩ is exactly 5 kΩ, and 4.7 kΩ + 330 Ω gives 5.03 kΩ. A combination is only as accurate as its parts: with ±1 % resistors the result is within ±1 % of its nominal value.
Power rating and temperature
A resistor’s power rating holds up to 70 °C ambient; above that the usable power falls in a straight line to zero at 155 °C, as the Yageo datasheets below show. Choose a part rated for at least twice the power it dissipates: it runs cooler and drifts less. NASA’s parts derating rules allow at most 60 % of the rating. Every resistor also has a maximum working voltage, which matters on mains and high-voltage circuits.
| Part | Power at 70 °C | Working voltage |
|---|---|---|
| Through-hole CFR-25 | 1/4 W | 250 V |
| Through-hole CFR-50 | 1/2 W | 350 V |
| Through-hole CFR-100 | 1 W | 500 V |
| Through-hole CFR2WS | 2 W | 500 V |
| Chip 0402 | 1/16 W | 50 V |
| Chip 0603 | 1/10 W | 75 V |
| Chip 0805 | 1/8 W | 150 V |
| Chip 1206 | 1/4 W | 200 V |
| Chip 1210 | 1/2 W | 200 V |
| Chip 2010 | 3/4 W | 200 V |
| Chip 2512 | 1 W | 200 V |
Standard power versions. Through-hole: Yageo CFR carbon film datasheet (2023). Chips: Yageo RC_L thick film datasheet (2020), with 1 % E24/E96 values from 1 Ω to 10 MΩ. Other makers’ parts are similar: check the datasheet of the one you buy.
For an LED, the LED resistor calculator also subtracts the forward voltage. To scale a voltage down for an ADC, use the voltage divider calculator, and to read colour bands or SMD codes, the resistor code calculator.
Frequently asked questions
How do I calculate the resistance from voltage and current?
Divide the voltage by the current: R = V ÷ I. For 12 V and 2 A that is 6 Ω, and the resistor dissipates 12 × 2 = 24 W, so it has to be a power resistor rated for about 50 W.
What power rating does my resistor need?
Work out P = I² × R or V² ÷ R and choose at least twice that. 5 V across 220 Ω is 114 mW, so a 1/4 W through-hole part or a 1206 chip. Above 70 °C ambient the usable power drops, down to zero at 155 °C.
How do I calculate resistors in parallel?
Add the reciprocals: 1/R = 1/R1 + 1/R2 + … For two resistors R = R1 × R2 ÷ (R1 + R2); for equal ones, divide the value by how many there are. Three 1 kΩ resistors in parallel give 333.3 Ω.
How do I get 5 kΩ, or another value that isn’t standard?
Combine standard values: 10 kΩ ∥ 10 kΩ gives exactly 5 kΩ, and 4.7 kΩ + 330 Ω gives 5.03 kΩ (+0.6 %). The calculator above tries every arrangement of one, two and three resistors from E12, E24 or E96.
Do tolerances add up when I combine resistors?
Not as a percentage: if every part is within ±1 %, any series or parallel combination of them is also within ±1 % of its nominal value. Add the nominal error of the combination to see how far from your target it can end up.
Which resistor gets the hottest?
In series the largest one, because the same current flows through all of them and P = I² × R. In parallel the smallest one, because they all see the same voltage and P = V² ÷ R.