What power rating do I need for my resistor?

Calculate the actual power dissipation then choose a resistor rated at least twice that value. Running at 50 percent or less of the rating ensures long term reliability and keeps the resistor temperature low.

Using a resistor well below its maximum rating for reliability. The standard rule is to derate to 50 percent of rated power at 70 degrees C ambient. At higher ambient temperatures derate further per the manufacturer datasheet.

Do SMD resistors have different power ratings than through-hole?

Yes. Common SMD ratings are 0201 at 1/20 watt, 0402 at 1/16 watt, 0603 at 1/10 watt, 0805 at 1/8 watt, and 1206 at 1/4 watt. Through-hole starts at 1/8 watt for small carbon and goes to several watts for wirewound types.

How does temperature affect power rating?

Power rating is specified at a reference temperature, typically 70 degrees C. Above that temperature the rating decreases linearly to zero at the maximum temperature. This is shown on the derating curve in the resistor datasheet.

Resistor Power Dissipation Calculator – Power Rating & Derating Check

P = I²R = V²/R = V×I

Enter R + V, R + I, or V + I. Optionally add power rating to check safety.

R Resistance

V Voltage Across Resistor

I Current Through Resistor

P_r Power Rating (optional — for safety check)

Power Dissipation Analysis

Power Dissipated
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Voltage

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Current

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Resistance

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Resistor Power Dissipation

Every resistor converts electrical energy to heat. The power dissipated must stay within the resistor’s power rating, ideally below 50% for long-term reliability. Exceeding the rating causes overheating, resistance drift, and eventual failure.

R — Resistance. The component converting electrical energy to heat.
I — Current through the resistor. Power scales with I².
V — Voltage across the resistor. Power scales with V².
P (heat) — Power dissipated as heat. Must stay within the resistor’s power rating.
Derating — Run at ≤50% of rated power for long life. 50–75% is acceptable. Above 75% the resistor runs hot.

Resistor Power Dissipation Calculator

Every resistor converts electrical energy into heat. The power dissipated must stay within the resistor’s power rating — and ideally well below it — or the resistor overheats, drifts in value, and eventually fails. This calculator computes the power from any two of resistance, voltage, and current, then checks it against the rated power with a colour-coded safety assessment.

The Three Formulas

P = V² / R — from voltage and resistance
P = I² × R — from current and resistance
P = V × I — from voltage and current

P_loss = all dissipated as heat

These are the power forms of Ohm’s Law. Enter any two known values and the calculator derives the third plus the power. The key insight: power scales with the square of voltage or current. Doubling the voltage across a resistor quadruples the power. Doubling the current through it also quadruples the power.

The 50% Derating Rule

Best practice: Choose a resistor rated at least 2× the calculated power dissipation. Running at ≤50% of rated power keeps the resistor cool, extends its life, and provides margin for voltage spikes and ambient temperature increases.

≤50% — Safe. Long life, low temperature rise.
50–75% — Acceptable for most applications. Ensure adequate airflow.
75–100% — Near limit. Resistor runs hot. Consider a higher rating.
>100% — Exceeds rating. Will overheat and fail.

LED Current-Limiting Resistor (220Ω / 3V)

P = V² / R = 3² / 220 = 9 / 220 = 40.9 mW
I = V / R = 3 / 220 = 13.6 mA

Rating: 0.25 W (1/4 W standard)
Utilisation: 40.9 mW / 250 mW = 16.4% — safe

40.9 mW in a 1/4 W resistor — well within limits at 16% utilisation. A standard 0805 SMD (1/8 W = 125 mW) would also work. For the full LED resistor calculation including forward voltage drop, see the LED Resistor Calculator.

Current Sense Resistor (0.1Ω / 2A)

P = I² × R = 2² × 0.1 = 4 × 0.1 = 0.4 W
V = I × R = 2 × 0.1 = 0.2 V

Rating: 1 W
Utilisation: 0.4 W / 1 W = 40% — safe

0.4 W needs a 1 W rated resistor for proper derating. A standard 1/4 W resistor would be at 160% — instant failure territory. Current sense resistors must be sized for the peak current, not just the average. For dedicated current sensing design, see the Current Sense Resistor Calculator.

High-Voltage Bleeder (10kΩ / 400V)

P = V² / R = 400² / 10000 = 160000 / 10000 = 16 W
I = V / R = 400 / 10000 = 40 mA

Rating: needs a 25–50 W wirewound resistor

16 watts — this is a serious heater. High-voltage bleeder resistors in power supplies routinely dissipate tens of watts. They need wirewound or ceramic power resistors mounted with adequate ventilation. For dedicated bleeder design with discharge time calculations, see the Bleeder Resistor Calculator.

Voltage Divider (4.7kΩ / 5V)

P = V² / R = 5² / 4700 = 25 / 4700 = 5.32 mW
I = V / R = 5 / 4700 = 1.06 mA

Rating: 0.125 W (1/8 W)
Utilisation: 5.32 mW / 125 mW = 4.3% — negligible

5.3 mW in a 1/8 W resistor — 4.3% utilisation. Voltage dividers at low voltages rarely have power problems. The concern shifts to high-voltage dividers where even large resistances dissipate significant power. For the divider ratio calculation, see the Voltage Divider Calculator.

Standard Power Ratings

Package / Type	Rating	Typical Use
0201 SMD	1/20 W (50 mW)	Ultra-compact, low-power logic
0402 SMD	1/16 W (63 mW)	Dense PCB layouts
0603 SMD	1/10 W (100 mW)	General purpose SMD
0805 SMD	1/8 W (125 mW)	Standard SMD
1206 SMD	1/4 W (250 mW)	Higher power SMD
2512 SMD	1 W	Power SMD, current sense
1/4 W axial	250 mW	Standard through-hole
1/2 W axial	500 mW	Medium power through-hole
1–5 W	1–5 W	Power resistors, metal oxide
Wirewound	5–100+ W	High power, braking, bleeder

Temperature and Derating

A resistor’s power rating is specified at a reference temperature, typically 70°C. Above that temperature, the maximum allowed power decreases linearly to zero at the maximum operating temperature (usually 155°C for film resistors, 275°C for wirewound). In a 70°C ambient, a 1/4 W resistor is still rated at full power. At 100°C ambient, it may be derated to only 60% of its rating.

The calculator’s 50% derating rule accounts for this — if you are already at 50% in a cool environment, you have margin for higher ambient temperatures without recalculating. For dedicated thermal management, see the Heat Sink Calculator.

Frequently Asked Questions

What power rating do I need?

Calculate the actual power, then choose a resistor rated at least 2× that value. For 40 mW dissipation, use a 1/8 W (125 mW) or 1/4 W (250 mW) resistor. For 0.4 W, use a 1 W resistor.

Why do resistors get hot?

Resistors convert all electrical energy passing through them into heat. P = I²R. The power dissipated is entirely thermal — there is no other output. A 1 W resistor generates 1 watt of heat, the same as a small LED bulb.

What happens if I exceed the power rating?

The resistor overheats. Short-term it may survive but the resistance value drifts. Sustained overload causes discolouration, cracking, desoldering from the PCB, or fire. SMD resistors can desolder themselves and tombstone off the pads.

Can I parallel resistors to increase power handling?

Yes. Two identical resistors in parallel share the current equally, so each dissipates half the total power. Two 1/4 W resistors in parallel handle 1/2 W total. This is a common technique when the exact power rating is not available in the right resistance value.

Do SMD resistors have different derating than through-hole?

SMD resistors are more sensitive to PCB layout because they rely on the copper pads and traces for heat dissipation. A 0805 rated at 1/8 W assumes standard pad sizes and reasonable copper area. Poor layout can effectively derate an SMD resistor by 30–50%.

How does this relate to the Power Calculator?

The Power Calculator finds total circuit power from voltage and current (P = VI). This calculator focuses specifically on the power dissipated by a single resistor and checks it against the component’s power rating — the critical step between circuit analysis and component selection.

Last updated: March 2026