Temperature conversion: Celsius, Fahrenheit and Kelvin explained

Converting between temperature scales comes down to two formulas: multiply Celsius by 1.8 and add 32 to get Fahrenheit, and add 273.15 to Celsius to get Kelvin. That covers the arithmetic, but in electronics and engineering work the harder part is usually knowing which figures matter. Datasheets specify everything in Celsius, US-authored application notes and legacy documentation often quote Fahrenheit, and thermal calculations use Kelvin.

This guide gives you the formulas, a quick-reference chart built around the temperatures that actually come up at the bench, component ratings, solder melting points, test conditions and advice on choosing the right instrument from the thermometry and thermography range when converting a reading isn't enough and you need to take one.

What are the temperature conversion formulas?

Two details worth noting. Kelvin takes no degree symbol, it's 298 K, not 298°K, because it's an absolute scale counting up from absolute zero. And a temperature difference of 1°C is exactly the same size as a difference of 1 K, which is why thermal resistance can be quoted as °C/W or K/W interchangeably on a datasheet.

Temperature conversion chart for electronics work

Most conversion charts are built around weather. This one is built around the figures that turn up in datasheets, on solder spools and in test specifications.

How do you convert Celsius to Fahrenheit in your head?

The quick approximation is double it and add 30. For 20°C that gives 70°F against a true value of 68°F - close enough for conversation. The shortcut is exact at 10°C (both methods give 50°F) and stays within a few degrees across the everyday range of roughly −5°C to 35°C.

Beyond that range the error grows steadily, because the true multiplier is 1.8, not 2. At 100°C the shortcut says 230°F when the real answer is 212°F. For anything that ends up in a specification, a test report or a purchase decision, use the full formula - the approximation is for weather, not engineering.

Why do electronics datasheets use 25°C?

Almost every parameter on a component datasheet, resistance, capacitance, gain, leakage current and output power, is specified at 25°C unless stated otherwise. It's the industry's agreed "room temperature" reference, and it matters because most parameters drift with temperature. A resistor's temperature coefficient, a thermistor's resistance curve and a power semiconductor's derating chart all describe behaviour relative to that 25°C baseline.

This is also where the commercial, industrial and automotive grades in the chart above come from. The same part number is often sold in several temperature grades, typically 0°C to 70°C for commercial, −40°C to 85°C for industrial and −40°C to 125°C for automotive and the price difference reflects the qualification testing, not just the silicon. Specifying the right grade for the real operating environment is cheaper than discovering the wrong one in the field.

When is Kelvin used?

Kelvin appears in electronics more often than people expect. Thermal calculations use it because temperature differences in Kelvin and Celsius are identical — a heatsink rated at 2.5 K/W and one rated at 2.5°C/W are the same thing. Noise calculations and semiconductor physics use absolute temperature directly.

The other everyday use is colour temperature. LED and lamp specifications quote it in Kelvin: around 2700 K for warm white, 4000 K for neutral and 6500 K for daylight. Despite the name, no conversion to Celsius is meaningful here - colour temperature describes the spectrum of the light, not how hot the fitting runs.

How do you measure temperature accurately?

Converting a figure is easy; trusting the figure is the real problem. The right instrument depends on the range, the required accuracy and whether you can touch the thing you're measuring.

• Thermocouples and probes cover the widest range - a general-purpose K-type spans roughly −200°C to over 1100°C - making them the default for ovens, soldering equipment and process work
• Thermistors trade range for sensitivity: over a span of roughly −50°C to 150°C they resolve small changes well, which is why they dominate onboard sensing and battery monitoring
• Temperature sensor ICs and modules output a calibrated digital or analogue signal directly, simplifying design work where the sensor talks to a microcontroller
• Thermal imaging cameras and infrared thermometers measure without contact - invaluable for live panels and moving machinery, with the caveat that readings depend on the surface's emissivity

For bench and laboratory work, dedicated thermometers and thermo-hygrometers add logging and humidity measurement where the application calls for it.

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Frequently asked questions

What is −40°C in Fahrenheit?

−40°F - it's the one point where the two scales meet. This is why so many industrial and automotive components are rated down to exactly −40°: the specification reads the same in both systems.

What is 25°C in Fahrenheit?

77°F. It's worth memorising because 25°C is the standard reference temperature for component datasheets - the condition under which almost all quoted parameters were measured.

What temperature does solder melt at?

Traditional Sn63/Pb37 leaded solder melts at 183°C (361°F). Common lead-free alloys melt higher: SAC305 at around 217–219°C (423–426°F), which is why lead-free work needs higher iron temperatures and reflow profiles peaking near 260°C.

Is a change of 1°C the same as a change of 1 K?

Yes - the scales have identical step sizes and differ only in where zero sits. That's why thermal resistance figures in °C/W and K/W are interchangeable, and why temperature coefficients can be quoted per Kelvin or per degree Celsius without conversion.

How accurate is the double-and-add-30 shortcut?

Within about 2–3°F across the everyday range of −5°C to 35°C, and exact at 10°C. The error grows outside that range because the true multiplier is 1.8, so use the full formula for anything technical.

Why doesn't Kelvin use a degree symbol?

Because Kelvin is an absolute unit rather than a scale offset from an arbitrary zero. The SI convention is 298 K, not 298°K. Celsius and Fahrenheit keep the degree symbol because their zero points are defined by convention rather than by absolute zero.

Measure it properly

Whether the job needs a contact probe, an onboard sensor or a non-contact reading from a safe distance, the right instrument turns a conversion chart into a number you can act on.