How do I calculate the power dissipated by a resistor?
P = V × I = V² ÷ R = I² × R, with V and I as DC or RMS values. For example, 5 V across 100 Ω is 5² ÷ 100 = 250 mW, and 20 mA through 150 Ω is 0.02² × 150 = 60 mW.
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Resistor power calculator
From the voltage across a resistor or the current through it, this resistor power calculator finds the power it dissipates, then the smallest chip size or axial resistor that carries that power with margin at your ambient temperature. Ratings, voltage limits and derating come from published datasheets.
You know
Examples
V
DC, or RMS for AC.
Ω
°C
Air around the part, inside the enclosure.
%
50 % leaves headroom for hot spots and drift.
Resistor type
Power dissipated
250 mW
- Current through it
- 50 mA
- Rating needed at 25 °C
- 500 mW P ÷ 50 %
- Equivalent rating at 70 °C
- 500 mW no derating below 70 °C
- Smallest chip size
- 1210 runs at 50 % of its 25 °C rating
- Smallest axial resistor
- 1/2 W (MFR-50) 50 % of rating
Ratings are typical general-purpose values: YAGEO RC_L thick film chips and YAGEO MFR metal film axials. The rated power assumes the datasheet's mounting conditions; small pads or a crowded, hot board run hotter. Check the part you buy.
1210 rating50 % of rating
Chip sizes at 25 °C ambient
| Part | Rated at 70 °C | Allowed at 25 °C | Load | Max voltage | Verdict |
|---|---|---|---|---|---|
| 0201 | 50 mW | 50 mW | 500 % | 25 V | Over rating |
| 0402 | 62.5 mW | 62.5 mW | 400 % | 50 V | Over rating |
| 0603 | 100 mW | 100 mW | 250 % | 75 V | Over rating |
| 0805 | 125 mW | 125 mW | 200 % | 150 V | Over rating |
| 1206 | 250 mW | 250 mW | 100 % | 200 V | Over your margin |
| 1210 | 500 mW | 500 mW | 50 % | 200 V | OK |
| 2010 | 750 mW | 750 mW | 33 % | 200 V | OK |
| 2512 | 1 W | 1 W | 25 % | 200 V | OK |
How do you find the power in a resistor and choose its rating?
Work out P = V² ÷ R or I² × R, then pick a part rated well above it at your ambient temperature. 5 V across 100 Ω dissipates 250 mW; at half the rating that needs 500 mW, so a 1210 chip (500 mW) or a 1/2 W (MFR-50) axial resistor. Above 70 °C the rating of a thick film chip falls in a straight line to zero at 155 °C.
Resistor power formula and derating formula
Power is what heats the resistor; the rating is how much heat its body and the board can shed before the resistive film reaches its temperature limit.
P = V × I = V² ÷ R = I² × R
- P
- power dissipated, W
- V, I
- voltage across and current through the resistor (DC or RMS)
- R
- resistance, Ω
The resistor derating curve used here is flat up to 70 °C ambient (Ta), then falls to zero at the limit Tmax (155 °C, or 125 °C for 0201):
Pallowed = P70 for Ta ≤ 70 °C; Pallowed = P70 × (Tmax − Ta) ÷ (Tmax − 70) above
- P70
- rated power at 70 °C from the datasheet
- Tmax
- ambient where the rating reaches zero
The calculator then asks for a rating of P ÷ (usage × derating factor) and checks the voltage limit, since the YAGEO datasheet gives the continuous working voltage as √(P70 × R) or the maximum working voltage, whichever is less.
Worked examples
Example 1: 5 V across 100 Ω
P = 5² ÷ 100 = 250 mW. A 1206 is rated exactly 250 mW, so it would run at 100 % of its rating. At 50 % usage the part needs 500 mW: the smallest chip is a 1210 (500 mW), or a 1/2 W (MFR-50) axial resistor.
Example 2: 12 V across 330 Ω in an 85 °C enclosure
P = 12² ÷ 330 = 436 mW. At 85 °C a part keeps (155 − 85) ÷ (155 − 70) = 82 % of its rating, so at 50 % usage it needs a 70 °C rating of 1.06 W. No single standard chip size does. A 2 W (MFR200) axial resistor works, or two chips of twice the resistance in parallel, each carrying 218 mW, fit in 2010 packages.
Example 3: a 230 V bleeder resistor, where voltage decides
230 V across 470 kΩ is only 113 mW, which a 1206 could carry on power alone. But 230 V is above the 200 V working voltage of every YAGEO RC_L size, so no single chip qualifies. Two 235 kΩ resistors in series each see 115 V and 56.3 mW: two 0805 chips do it. Voltage limits vary by maker (Vishay rates its CRCW2512 at 500 V), and mains circuits also need creepage distance on the board.
SMD resistor power rating chart by size
Standard thick film ratings from the YAGEO RC_L datasheet, with the allowed power at higher ambients from the derating line. Critical resistance is where the voltage limit takes over from power.
| Size | Body | Rated at 70 °C | Max voltage | At 85 °C | At 105 °C | At 125 °C | Critical R |
|---|---|---|---|---|---|---|---|
| 0201 | 0.6 × 0.3 mm | 50 mW | 25 V | 36.4 mW | 18.2 mW | 0 W | 12.5 kΩ |
| 0402 | 1.0 × 0.5 mm | 62.5 mW | 50 V | 51.5 mW | 36.8 mW | 22.1 mW | 40 kΩ |
| 0603 | 1.6 × 0.8 mm | 100 mW | 75 V | 82.4 mW | 58.8 mW | 35.3 mW | 56.3 kΩ |
| 0805 | 2.0 × 1.25 mm | 125 mW | 150 V | 103 mW | 73.5 mW | 44.1 mW | 180 kΩ |
| 1206 | 3.1 × 1.6 mm | 250 mW | 200 V | 206 mW | 147 mW | 88.2 mW | 160 kΩ |
| 1210 | 3.1 × 2.6 mm | 500 mW | 200 V | 412 mW | 294 mW | 176 mW | 80 kΩ |
| 2010 | 5.0 × 2.5 mm | 750 mW | 200 V | 618 mW | 441 mW | 265 mW | 53.3 kΩ |
| 2512 | 6.35 × 3.1 mm | 1 W | 200 V | 824 mW | 588 mW | 353 mW | 40 kΩ |
Axial resistor power rating
Normal-size metal film resistors from the YAGEO MFR datasheet. The body size is what a footprint is drawn for.
| Rating | Body (length × diameter) | Max voltage | At 85 °C | At 105 °C | At 125 °C | Critical R |
|---|---|---|---|---|---|---|
| 1/6 W (MFR-12) | 3.4 × ⌀1.9 mm | 200 V | 137 mW | 98 mW | 58.8 mW | 240 kΩ |
| 1/4 W (MFR-25) | 6.3 × ⌀2.4 mm | 250 V | 206 mW | 147 mW | 88.2 mW | 250 kΩ |
| 1/2 W (MFR-50) | 9.0 × ⌀3.3 mm | 350 V | 412 mW | 294 mW | 176 mW | 245 kΩ |
| 1 W (MFR100) | 11.5 × ⌀4.5 mm | 500 V | 824 mW | 588 mW | 353 mW | 250 kΩ |
| 2 W (MFR200) | 15.5 × ⌀5.0 mm | 500 V | 1.65 W | 1.18 W | 706 mW | 125 kΩ |
What the rating does not tell you
The board is part of the heatsink
Vishay states that the rated dissipation applies only if the permitted film temperature is not exceeded, and that the temperature rise depends on the thermal resistance of the resistor together with the board. Small pads, thin copper, crowded hot parts or a sealed enclosure all eat into the rating.
- Higher-power versions exist in the same sizes: YAGEO RC_L offers double-power options (for example 0603 at 1/5 W and 1206 at 1/2 W), and Vishay CRCW has an extended mode for 0402 to 0805. Use their own derating curves.
- Thin film, metal strip and wirewound parts have different curves and limits; the 70 °C to 155 °C line here is for general-purpose thick film chips and metal film axials.
- Pulses are rated separately. A part that survives its continuous rating can still fail on a surge; check the single-pulse and continuous-pulse curves.
- Above the critical resistance the maximum working voltage is the limit. Split high voltages across resistors in series.
Glossary
- Rated dissipation (P70)
- The continuous power a resistor may dissipate at 70 °C ambient, under the mounting conditions the datasheet assumes.
- Derating curve
- Allowed power against ambient temperature. For the film resistors here: flat to 70 °C, then a straight line to zero at the film temperature limit.
- Permitted film temperature
- The highest temperature the resistive layer may reach, 155 °C for standard thick film chips.
- Maximum working voltage
- The highest continuous DC or AC RMS voltage across the part, whatever the power.
- Critical resistance
- Vmax² ÷ P70. Above it the voltage limit, not the power rating, sets the maximum load.
- Ambient temperature
- The air temperature around the part, inside the enclosure, not the room.
Related guides
- Ohm’s law calculator: voltage, current, resistance and power from any two.
- LED resistor calculator: the series resistor and its power.
- Standard resistor values: the E24 and E96 values you will round to.
Frequently asked questions
What wattage resistor do I need?
Work out P = V² ÷ R or I² × R, then pick a rating well above it. With this calculator’s default of 50 % usage, 5 V across 100 Ω (250 mW) needs 500 mW: a 1210 chip or a 1/2 W (MFR-50) axial resistor.
What is the power rating of a 0603 resistor?
A standard thick film 0603 is rated 100 mW (1/10 W) at 70 °C with a 75 V maximum working voltage (YAGEO RC0603; Vishay CRCW0603 in standard mode). Double-power versions rated 1/5 W exist, such as the YAGEO RC_L high-power option.
What are the power ratings of 0402, 0805 and 1206 resistors?
In standard thick film: 0402 62.5 mW (1/16 W), 0805 125 mW (1/8 W) and 1206 250 mW (1/4 W), each at 70 °C ambient. The chip table on this page lists every size from 0201 to 2512 with its voltage limit.
What is resistor power derating?
On the film resistors this calculator covers, the rating holds up to 70 °C ambient and then falls in a straight line to zero at 155 °C (125 °C for YAGEO 0201 chips), the film temperature limit. At 85 °C a part keeps 82 % of its rating; at 125 °C, 35 %.
Can I run a 1/4 W resistor at 0.25 W?
Up to 70 °C ambient it is within its rating, but the derating line ends at the permitted film temperature, so at full rating the film runs near its limit, and Vishay allows up to 1 % resistance change after 1000 h at that load (CRCW, standard mode). This calculator defaults to using 50 % of the derated rating; set your own margin if your design rules say otherwise.
Why does a high-value resistor need a bigger package even at low power?
Every size has a maximum working voltage. The continuous rating is √(P × R) or that voltage, whichever is less, so above R = Vmax² ÷ P a part is voltage-limited: 56.3 kΩ for a 75 V, 100 mW 0603.
Does AC change the calculation?
Use RMS values: P = Vrms² ÷ R gives the average power that heats the part. Datasheet working voltages are DC or AC RMS. Short pulses are rated separately, on the pulse-load curves in the datasheet.
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Sources
- YAGEO RC_L general purpose chip resistors, product specification V.14 (power, voltage, derating)
- YAGEO MFR metal film resistors, product specification V.4 (axial ratings and derating)
- Vishay D/CRCW e3 standard thick film chip resistors datasheet (document 20035)
- Vishay Beyschlag, Basics of Linear Fixed Resistors (application note 28771)
- IEC 60115-1:2020, Fixed resistors for use in electronic equipment, generic specification
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