What is the correct formula for converting Fahrenheit into centigrade?

Appeared in: PGIMS Rohtak NO - 2017

Explanation

The correct formula to convert degrees Fahrenheit (°F) to degrees Celsius (°C) is C = (F - 32) × 5/9.
This involves two steps: first subtracting 32 from the Fahrenheit value, and then multiplying the result by the fraction 5/9.
For example, to convert a normal body temperature of 98.6°F: (98.6 - 32) × 5/9 = 66.6 × 5/9 = 37°C.

Option A: This formula, (C × 9/5) + 32, converts Celsius to Fahrenheit, which is the reverse of what the question asks.
Option B: This formula is incorrect because it adds 32 instead of subtracting it, and the order of operations is wrong.
Option D: This formula uses the incorrect fraction (9/5). The fraction 9/5 is used when converting from Celsius to Fahrenheit, not the other way around.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Temperature conversion between Fahrenheit and Celsius scales as background academic context rather than a clinical decision trigger.
Accurate temperature conversion is a critical nursing skill for assessing fever, monitoring patient response to treatment, and preventing medication errors, especially with antipyretics.
In India, while Celsius is the standard, many patients may still refer to their temperature in Fahrenheit. Nurses must be able to convert and educate patients accurately.
What if? If a patient's temperature is 104°F and a nurse incorrectly uses the C-to-F formula, they might calculate (104 × 9/5) + 32 = 219.2°F, a nonsensical and dangerous miscalculation. The correct conversion is (104 - 32) × 5/9 = 40°C, indicating a high-grade fever requiring intervention.

How to Approach the Question

First, identify the core question: it asks for the formula to convert Fahrenheit (F) to Celsius (C).
Recall the key steps for F to C conversion: you must first subtract 32 from the Fahrenheit value.
This immediately eliminates any options that add 32.
Next, recall the correct fraction. When going from a larger number (Fahrenheit scale) to a smaller number (Celsius scale), you use the smaller fraction, which is 5/9.
Combine the steps: (F - 32) followed by multiplication by 5/9. Match this logic to the given options.
Mentally check with a known value, like the freezing point of water: (32°F - 32) × 5/9 = 0°C. This confirms the formula is correct.

Concept Tested & Keywords

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E6 Nursing Fundamentals Taylor p. 120-122

Practise the full PGIMS Rohtak NO - 2017

Attempt every question from this paper in a timed mock, then review the full solution for each one.