Resistance tells you how a resistor behaves in a circuit; power rating tells you whether it survives. A 100 Ω resistor rated at 1/8 W and one rated at 2 W behave identically until the heat builds up, and then only one of them keeps working. Checking power takes one multiplication, so it is worth doing for every resistor that carries more than a trickle of current.
Three ways to calculate power
The basic relation is P = V × I: power in watts equals the voltage across the resistor times the current through it. Substituting Ohm's law gives two more forms, so you can use whichever two quantities you already know:
- P = V × I when you know the voltage across the resistor and the current.
- P = I² × R when you know the current and the resistance.
- P = V² / R when you know the voltage across it and the resistance.
A 9 V battery across a 1 kΩ resistor gives 81 / 1,000 = 0.081 W. An LED current of 20 mA through 150 Ω gives 0.020² × 150 = 0.06 W. The most common mistake is using the supply voltage instead of the voltage actually across the resistor. In a 5 V LED circuit, the resistor sees about 3 V, not 5 V, because the LED takes the rest. The Ohm's law calculator handles all three forms.
Standard through-hole ratings and sizes
| Rating | Watts | Typical body (length × diameter) | Typical use |
|---|---|---|---|
| 1/8 W | 0.125 | about 3.5 × 1.8 mm | compact signal boards |
| 1/4 W | 0.25 | about 6 × 2.3 mm | the default in kits and on breadboards |
| 1/2 W | 0.5 | about 9 × 3.2 mm | small loads, LED strings on 12 V |
| 1 W | 1 | about 11 × 4.5 mm | current sensing, droppers |
| 2 W | 2 | about 15 × 5 mm | dummy loads, power circuits |
Treat these sizes as rough. Metal film resistors are often smaller than carbon film parts of the same rating, and some "mini" 1/2 W parts are the size of an ordinary 1/4 W resistor. You cannot reliably judge a rating by eye, so keep resistors labeled by rating as well as value. Above 2 W you will mostly find metal oxide, wirewound or cement-bodied resistors in rectangular ceramic cases, rated 5 W, 10 W and up.
Derating and the 2× rule
A power rating is only valid under the conditions the datasheet assumes: free air, standard mounting and an ambient temperature up to a stated point, commonly 70 °C. Above that, the allowed power falls in a straight line to zero at the maximum operating temperature, often somewhere between 125 °C and 155 °C depending on the type.
Take a 1/4 W resistor rated to 70 °C and derated to zero at 155 °C, sitting inside an enclosure next to a voltage regulator at 100 °C. It may dissipate only (155 − 100) / (155 − 70) = 65% of its rating, or about 0.16 W. Even at room temperature, a resistor running at its full rating is very hot, well over 100 °C on the surface for many types, which is hard on nearby plastic, wire insulation and electrolytic capacitors.
The practical rule is to choose a resistor rated for at least twice the power you calculate. Running at half its rating keeps a resistor cooler, slows the drift of its value over time, and leaves room for supply variation. That last point matters more than it seems, because power scales with the square of voltage: a supply 10% high delivers 21% more power into the same resistor.
Worked examples
5 V across 100 Ω
I = 5 / 100 = 50 mA, and P = 5² / 100 = 0.25 W. That is exactly the rating of a 1/4 W resistor, with no margin at all. A USB supply at 5.25 V would push it to 0.28 W, already over. Using the 2× rule, choose a 1/2 W part, or 1 W if it sits in a closed box.
The 100 Ω resistor page lists other common jobs for this value.
A current-sense resistor
Current sensing uses a small resistance so the voltage drop stays low. A 0.1 Ω resistor carrying 2 A drops 0.2 V and dissipates 2² × 0.1 = 0.4 W, so a 1 W part, such as a 2512 chip or a 1 W axial resistor, is appropriate. Using 1 Ω instead would waste 2 V and dissipate 4 W, which is why sense resistors are kept as small as the measuring circuit allows.
A resistor as a voltage dropper
Feeding a 5 V, 100 mA load from 12 V through a resistor needs (12 − 5) / 0.1 = 70 Ω, so a 68 Ω part. It dissipates about 6.8 × 0.1 = 0.68 W, which calls for a 2 W resistor under the 2× rule. Worse, the output voltage depends on the load: if the load drops to 50 mA, the resistor drops only 3.4 V and the load sees 8.6 V. For anything other than a fixed load, a voltage regulator is the right part.
Sharing the heat
If you need 100 Ω at 0.4 W and only have 1/4 W parts, arrange four 100 Ω resistors as two parallel pairs in series. The total is still 100 Ω, each resistor takes a quarter of the power (0.1 W), and the group handles 1 W. The series and parallel calculator checks such combinations.
Pulses, surges and voltage limits
A power rating describes average heat over time. A resistor has some thermal mass, so brief pulses above the rating are often fine, and many datasheets include a pulse-load chart showing how far you can go for a given pulse width. PWM is the everyday case: at a 25% duty cycle and a frequency of a few kilohertz, the average power is 25% of the on-state power.
Inrush is the case people forget. Charging a 1,000 µF capacitor to 12 V through a 10 Ω resistor starts with 1.2 A and a peak power of 12² / 10 = 14.4 W. The time constant is 10 ms, and the energy dissipated in the resistor is ½CV² = 0.5 × 0.001 × 12² = 72 mJ, regardless of the resistor's value. A small film resistor may survive this once and fail after many repetitions. For repeated surges, choose a part with a published pulse rating, such as a wirewound or a dedicated anti-surge resistor.
There is also a voltage limit, independent of power. Many 1/4 W film resistors are specified for a maximum working voltage around 250 V. A 1 MΩ resistor across 400 V dissipates only 0.16 W, but the voltage exceeds its rating and it can arc or fail. High-voltage dividers use several resistors in series to share the voltage.
What a burnt resistor looks like
An overheated resistor usually shows it: darkened or brown-looking bands, a cracked or blistered coating, a scorch mark on the board underneath, and often a distinctive smell. Its value typically drifts upward and it may go open circuit. Some damaged parts look normal, so measure anything near a hot spot out of circuit.
Before replacing it, find out why it burned. A resistor rarely fails on its own; a shorted capacitor, transistor or diode downstream often pulled the extra current. Fitting the same part without fixing the cause just repeats the failure. Do not trust the color bands on a heat-damaged part either; get the value from the schematic or an identical resistor, as described in the color code reading guide.
SMD resistor power by package
| Package (imperial) | Typical rating | Example limit at 5 V across the part |
|---|---|---|
| 0201 | 1/20 W | 500 Ω or more |
| 0402 | 1/16 W | 400 Ω or more |
| 0603 | 1/10 W | 250 Ω or more |
| 0805 | 1/8 W | 200 Ω or more |
| 1206 | 1/4 W | 100 Ω or more |
| 2512 | 1 W | 25 Ω or more |
The last column is V² / P with the full voltage across the resistor and no margin; apply the 2× rule on top. Ratings differ between manufacturers and product lines, and they assume the recommended copper pads, because a chip resistor loses most of its heat through its terminations into the board. The SMD resistor code guide covers package sizes and markings in more detail.