Resistor Wattage Calculator (Power Rating and Derating)

Background.

A 1 W resistor carrying 0.6 W looks comfortably sized. Seal it in a plastic box beside a linear regulator, where the air inside reaches 100 °C, and it is not: at that temperature a typical film resistor can shed only about two-thirds of its nameplate rating, so the part is running at 93% of what it can really handle, and the margin the designer counted on has gone.

The reason is printed on the datasheet. Power ratings apply at a stated ambient temperature, often written as a subscript: Vishay rates its MRS16 and MRS25 leaded resistors at 0.4 W and 0.6 W as P70, meaning in 70 °C air, and uses the same convention for its CRCW thick-film chips. Above that temperature the allowable power falls in a straight line to zero at the maximum film temperature, 155 °C for both families. Vishay states the condition directly: the rated dissipation applies only if the permitted film temperature is not exceeded.

Choosing a part therefore takes three steps. Work out the dissipation from whichever pair of quantities you know. Multiply by a design margin. Divide by the share of the rating that survives at your ambient temperature, which gives the nameplate figure to shop for. The calculator then picks the smallest common rating, from 1/16 W up to 10 W, that covers it, and reports how much margin you actually end up with.

Wattage is not the only limit. Every resistor also has a maximum operating voltage, and on high-value parts it is usually the one that bites: a 10 MΩ resistor with 400 V across it dissipates only 16 mW, far inside a 1/16 W rating, yet 400 V is double the 200 V limit of a 1206 chip or an MRS16. A voltage check runs alongside every result. Pulse and surge loads are a different specification altogether and are not covered.

What is resistor wattage calculator?

A resistor's power rating is the steady dissipation it can carry at a stated ambient temperature without its resistive film exceeding its permitted temperature. It is a thermal limit in electrical clothing. The heat produced, P = V × I, lifts the film above the surrounding air by an amount set by the package and the copper it is soldered to, and the rating holds only while the film stays under its ceiling — which is why datasheets publish a rating temperature and a derating curve rather than one number. The maximum operating voltage is a separate specification with a separate cause, breakdown across the resistive element rather than heating, so a part can sit inside one limit and outside the other.

How to use this calculator.

1. 01Choose the pair you know: current and resistance for a current-limiting or sense resistor, voltage and resistance for a divider leg or bleeder, or voltage and current if you measured both. The unused field is ignored.
2. 02Enter the voltage across this resistor alone. For an LED's series resistor on a 12 V rail, that is 12 V minus the LED's forward voltage, not the full rail.
3. 03Set the headroom factor. Two is the usual rule of thumb; use 3 or more inside sealed enclosures, for long service lives, or beside parts that dislike heat.
4. 04Enter the air temperature where the resistor actually sits, then check the rating temperature and maximum film temperature against your part's datasheet. The defaults, 70 °C and 155 °C, are Vishay's figures for its MRS and CRCW families.
5. 05Read the recommended rating with the real headroom beside it, then the voltage check. A requirement above 10 W calls for a wirewound or aluminium-housed power resistor chosen from its own datasheet and heatsink curve.

The formula.

Dissipation is P = V × I; substituting Ohm's law gives I²R and V²/R, so all three input pairs produce the same watts. The derating factor k is 1 when the ambient T is at or below the rating temperature T_rated, and (T_film − T) ÷ (T_film − T_rated) above it — a straight line from the full rating down to nothing at the maximum film temperature. With 70 °C and 155 °C, k is exactly 50% at 112.5 °C. The nameplate rating required is P_needed = P × h ÷ k, where h is the headroom factor: multiplying by h adds the margin, and dividing by k converts what you need at your ambient into the figure a datasheet prints at its own rating temperature. The recommendation is the first rung of the ladder 1/16, 1/10, 1/8, 1/4, 1/2, 1, 2, 3, 5 and 10 W at or above P_needed, and the headroom you actually get is that rung × k ÷ P. An ambient at or above the film limit is rejected, since the part could then dissipate nothing. The straight line is a model of the published curve; mounting, airflow and neighbouring heat sources all move the real film temperature, and none of them is in the formula.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Vishay BCcomponents, "MRS16, MRS25 Professional Thin Film Leaded Resistors", document 28724, revision 07-Mar-16. Read directly from the PDF on 2026-07-29. Source of the leaded-part figures on this page: rated dissipation P70 of 0.4 W (MRS16) and 0.6 W (MRS25); operating voltage Umax AC/DC of 200 V and 350 V; operating temperature range −55 °C to 155 °C; peak permissible film temperature 155 °C. The APPLICATION INFORMATION section states that "the rated dissipation applies only if the permitted film temperature is not exceeded" and that drift over operating time, rather than a fixed lifetime, is what eventually limits a part.
2. [2]Vishay, "D/CRCW e3 Standard Thick Film Chip Resistors", document 20035, revision 14-Apr-2026. Read directly from the PDF on 2026-07-29. Source of the surface-mount figures: rated dissipation P70 by size — 0.063/0.10 W (0402), 0.10/0.125 W (0603), 0.125/0.25 W (0805), 0.25 W (1206), 0.5 W (1210), 1.0 W (1218), 0.75 W (2010), 1.0 W (2512) — and maximum operating voltage Umax of 75 V, 75 V, 150 V, 200 V, 200 V, 200 V, 400 V and 500 V respectively. Permissible film temperature 155 °C; operating range −55 °C to +155 °C. States that the temperature rise "depend[s] on the thermal resistance of the assembled resistor together with the printed circuit board".
3. [3]OpenStax (Rice University), "University Physics Volume 2", §10.2 "Resistors in Series and Parallel" and the surrounding treatment of electric power. Source of the equivalence P = V x I = I²R = V²/R used by all three input modes, and of the statement that the total power dissipated by a network is the sum of the individual dissipations. Retrieved and read 2026-07-29.
4. [4]Vishay, "CRCW-HP e3 Pulse Proof, High Power Thick Film Chip Resistors". Cited as the counter-example that keeps this page's scope honest: pulse and surge handling is a separate specification from continuous dissipation, with its own load diagrams and its own part families, and nothing on this page should be used to size a part for pulse duty. Retrieved 2026-07-29.