Temperature conversion: a reading is not a temperature difference
A temperature of 10°C is not the same thing as a rise of 10°C. Confusing a reading with a change makes a correct formula answer the wrong question.
A reading needs a scale factor and an offset
For a reading, F = C × 1.8 + 32; in the other direction, C = (F − 32) ÷ 1.8. The added 32 accounts for different zero points and is not optional.
Simply multiplying by 1.8 is therefore not enough for a weather reading or a device setting. Keep °C or °F beside the value so its meaning stays clear.
References: NIST — Conversion tables, including exact temperature equations
A difference does not need the added 32
Subtracting two readings cancels the zero-point offset. A rise of 10°C is a rise of 18°F, not 50°F. A reading of 10°C, however, is 50°F.
You can check by converting both readings before subtracting them. If converting the difference gives another answer, the reading formula was probably used where the difference formula was needed.
Kelvin notation and rounding
A Kelvin reading is Celsius + 273.15. Its symbol is K without a degree sign: write 295.15 K, not °K. One Celsius degree has the same size as 1 K, but their zero points differ.
Round according to the original measurement. A unit conversion does not improve a sensor’s accuracy, calibrate a device or by itself establish a safe setting for a particular use.
References: NIST — Conversion tables, including exact temperature equations · NIST — SI Guide, section 8.5: temperature intervals and differences
Worked example: a room reading and its later change
An illustrative reading of 22°C is 22 × 1.8 + 32 = 71.6°F, or 22 + 273.15 = 295.15 K. All three describe the same reading in different units.
A rise from 18°C to 26°C is a difference of 8°C, equivalent to 14.4°F. Check the endpoints: 18°C is 64.4°F and 26°C is 78.8°F. Their difference is 14.4°F.