Enter a value and press Convert.
Quick memory check: −40° is the one temperature where Celsius and Fahrenheit agree exactly (−40°C = −40°F) — a handy way to sanity-check your own mental conversions.
Celsius (°C) sets 0° at water's freezing point and 100° at its boiling point (at standard atmospheric pressure) — the standard scale used by most of the world and in science. Fahrenheit (°F), still standard in the United States, sets water's freezing point at 32° and boiling at 212°, a 180-degree span instead of Celsius's 100-degree span, which is why the conversion involves both a scaling factor (9/5) and an offset (+32). Kelvin (K) is the SI scientific unit, using the same degree size as Celsius but starting at absolute zero (the coldest physically possible temperature, where all thermal motion stops) instead of water's freezing point — which is why Kelvin conversion is just a fixed +273.15 offset from Celsius, with no scaling needed.
| Reference point | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Water freezes | 0 | 32 | 273.15 |
| Room temperature | ∼20-25 | ∼68-77 | ∼293-298 |
| Body temperature | 37 | 98.6 | 310.15 |
| Water boils | 100 | 212 | 373.15 |
A subtlety that trips people up: a temperature difference of 1 °C is exactly 1 K, because the Celsius and Kelvin scales share the same degree size and differ only in where zero sits. That is why thermal resistance is quoted interchangeably as °C/W or K/W — the two are identical. The same is not true of Fahrenheit: a 1 °C difference is 1.8 °F.
Semiconductor datasheets give a thermal resistance θ in °C/W, which converts dissipated power into a temperature rise:
Tjunction = Tambient + P × θ
Suppose a device dissipates 5 W with a junction-to-ambient thermal resistance of 25 °C/W in a 40 °C enclosure. The junction sits at 40 + 5 × 25 = 165 °C — above the 150 °C limit typical of silicon, so that design fails without a heatsink, regardless of how comfortable the ambient temperature sounds.
Aluminium electrolytic capacitor life follows an Arrhenius relationship, and the working approximation is that rated life halves for every 10 °C rise in core temperature. A capacitor rated 2,000 hours at 105 °C lasts roughly 4,000 hours at 95 °C and about 16,000 hours at 75 °C. This single rule explains most premature power-supply failures: the capacitor did not fail because it was faulty, it failed because it ran hot.
°F = °C×9/5+32. For example, 25°C = 25×9/5+32 = 77°F.
K = °C+273.15. For example, 25°C = 298.15 K. Unlike the Fahrenheit conversion, there is no scaling factor — just a fixed offset.
°C = (°F−32)×5/9. For example, 98.6°F = (98.6−32)×5/9 = 37°C.
Absolute zero (0 K = −273.15°C) is the theoretical coldest possible temperature, where all thermal molecular motion stops. Kelvin is defined starting from this physical limit rather than an arbitrary reference like water's freezing point, which is why it is the standard scale for scientific calculations involving absolute temperature.
Many physical laws (like the ideal gas law, PV=nRT, or radiated power calculations) depend on absolute temperature, where "twice as hot" only makes physical sense measured from absolute zero. Celsius and Fahrenheit have arbitrary zero points, so ratios and proportional relationships in these formulas only work correctly with Kelvin (or Rankine).
−40° — both scales read exactly −40 at this one point, since it is where the linear conversion formula's scaling and offset happen to exactly cancel out.
It is primarily a matter of historical convention — Fahrenheit was the dominant scale globally before most countries transitioned to the metric system (including Celsius) in the 20th century, but the United States (along with a few other countries) retained Fahrenheit for everyday/weather use.
Consumer-grade components are commonly rated 0°C to 70°C; industrial-grade parts often extend to −40°C to 85°C; automotive and military-grade parts can extend further, sometimes to −55°C to 125°C or beyond — always check the specific component's datasheet.
Yes — the MPPT String Sizing calculator on this site uses exactly this kind of temperature figure (in °C) to compute cold-weather open-circuit voltage; make sure any temperature values you enter there are consistently in the same unit (typically Celsius) as the panel datasheet specifies.
For everyday and most engineering purposes, 1-2 decimal places is more than sufficient; only specialized scientific or metrology applications typically need higher precision than the standard conversion formulas already provide.
Yes, exactly. The Celsius and Kelvin degree are the same size, so a 10 °C rise is a 10 K rise. This is why thermal resistance is written as either °C/W or K/W. A Fahrenheit difference is not the same: 1 °C equals 1.8 °F.
Tj = Tambient + P × θ, where P is dissipated power in watts and θ the thermal resistance in °C/W. A 5 W device with θ = 25 °C/W in 40 °C air reaches 165 °C.
Because the Kelvin scale places zero at the point where thermal motion is minimal, while the Celsius zero is fixed at the freezing point of water. The offset between the two scales is 273.15 degrees.
As a working rule, rated life halves for each 10 °C of temperature rise. A 2,000-hour 105 °C part lasts roughly 16,000 hours if kept at 75 °C.
Design for the worst-case internal ambient, not room temperature. Air inside a sealed enclosure commonly sits 15–30 °C above the room, and that figure is the ambient every thermal calculation in the design should start from.
θJA is junction-to-ambient and assumes a specified board and still air, so it is only a rough guide. θJC is junction-to-case and is the figure to use when designing a heatsink, combined with the case-to-sink and sink-to-ambient resistances.
MPPT String Sizing (uses °C) • Junction Temperature Calculator • All Calculators