6.3 Temperature & Time Conversions

Temperature Conversions

The NEX tests the two clinical temperature scales: Celsius (°C) and Fahrenheit (°F). The two formulas are:

• C to F: °F = (°C x 9/5) + 32
• F to C: °C = (°F - 32) x 5/9

The factor 9/5 (or 1.8) appears in one direction and 5/9 in the other. A reliable way to remember which: when going from C to F, the Fahrenheit numbers get bigger (because 1 °C is wider than 1 °F), so you multiply by 1.8 and add 32. When going from F to C, the Celsius numbers get smaller, so you subtract 32 first, then multiply by 5/9.

Why Two Scales Matter Clinically

United States clinical practice still reports patient temperature in Fahrenheit at the bedside, while most electronic health records, pediatric units, intensive care units, and virtually every international guideline use Celsius. You will routinely read a Fahrenheit thermometer, chart a Celsius value, and interpret a provider order written in the other scale. That back-and-forth is exactly why the NEX requires fluency in both directions rather than memorizing one formula. Expect problem-solving items that describe a patient reading in one scale and ask for the equivalent in the other, sometimes embedded in a short clinical sentence about fever, hypothermia, or antipyretic effectiveness.

Worked Example 1: Body Temperature

Normal body temperature is about 37 °C. Convert to °F.

°F = (37 x 9/5) + 32
   = (37 x 1.8) + 32
   = 66.6 + 32
   = 98.6 °F

Answer: 98.6 °F. This is the canonical normal body temperature you will see throughout nursing school.

Worked Example 2: Fahrenheit to Celsius

A patient's temperature is 100.4 °F. Convert to °C.

°C = (100.4 - 32) x 5/9
   = 68.4 x 5/9
   = 342 / 9
   = 38.0 °C

Answer: about 38.0 °C. A Celsius temperature of 38 °C or above is a common clinical definition of fever. Notice the order of operations: you must subtract 32 before multiplying by 5/9. If you multiplied first you would get a different, wrong answer.

Worked Example 3: Freezing and Boiling

Water freezes at 32 °F and boils at 212 °F. In Celsius these are 0 °C and 100 °C — the two anchor points that define the Celsius scale. You can verify with the formula:

°C = (212 - 32) x 5/9 = 180 x 5/9 = 100 °C

The Kelvin scale exists but is not tested on the NEX. Do not spend time studying Kelvin conversions. The anchor points 0 °C and 100 °C are worth memorizing because they let you sanity-check any temperature answer: a reasonable body temperature in Celsius must sit between these two water phase-change values.

Time Unit Conversions

The time conversions you need are simple:

• 1 min = 60 s
• 1 h = 60 min
• 1 day = 24 h

Most NEX time items are problem-solving style: they give a start time and a duration, or two durations to combine, and ask for a normalized result in hours and minutes. The skill is not the conversion factor itself but normalizing the answer so minutes never exceed 59 in the final value.

Worked Example 4: Elapsed Time

A medication is given at 08:15 and the next dose is due 6 hours later. What time is the next dose?

Add 6 hours to 08:15: 08:15 + 6:00 = 14:15, or 2:15 PM.

Worked Example 5: Adding and Subtracting Time

A patient receives two IV boluses. The first runs over 45 min, the second over 38 min. What is the total time?

45 min + 38 min = 83 min. Convert: 83 min = 1 h 23 min (since 60 min = 1 h, 83 minus 60 = 23). The normalized answer is 1 h 23 min, not 83 min.

Worked Example 6: Subtracting Across Hours

A shift starts at 14:30 and ends at 22:45. How long is the shift in hours and minutes?

From 14:30 to 22:30 is 8 hours. From 22:30 to 22:45 is 15 minutes. Total: 8 h 15 min.

Alternatively, convert both to minutes since midnight: 14:30 = 14 x 60 + 30 = 870 min; 22:45 = 22 x 60 + 45 = 1,365 min. Difference = 495 min. Convert back: 495 / 60 = 8 remainder 15, so 8 h 15 min. The minutes-since-midnight method is the most reliable when a subtraction crosses an hour boundary and you want to avoid borrowing errors.

Worked Example 7: Elapsed Time Across Midnight

A medication is started at 22:20 and the infusion runs for 4 h 45 min. At what time does it finish?

Add hours first: 22:20 + 4 h = 26:20, which is past midnight, so subtract 24 to normalize the day: 26:20 - 24:00 = 02:20. Then add the 45 min: 02:20 + 45 min = 03:05. The infusion finishes at 03:05 the next calendar day. When elapsed time pushes the clock past 24:00, subtract 24 hours and roll into the next day — a common scenario for overnight infusions and on-call dosing schedules.

Military / 24-Hour Time

The 24-hour clock avoids AM/PM ambiguity and is standard in clinical documentation. Hours run 00:00 (midnight) through 23:59. To convert PM times to 24-hour, add 12 to the hour (except 12 PM which stays 12:00). To convert 24-hour back to PM, subtract 12 (except 12:00–12:59 which stays 12 PM). AM times keep their hour unchanged, with 12 AM becoming 00:00.

Examples:

• 2:30 PM becomes 14:30
• 10:15 AM stays 10:15
• 23:45 becomes 11:45 PM

A quick consistency check: a correct 24-hour time never has an hour above 23, and converting PM to 24-hour always produces an hour of 12 or greater (except the noon case). If your converted PM hour is single-digit, you forgot to add 12.

Common Traps

• Formula direction. C-to-F uses 9/5 and adds 32. F-to-C subtracts 32 first, then uses 5/9. Mixing these up is the single most common temperature error.
• 5/9 vs 9/5. A Celsius degree is 1.8 Fahrenheit degrees; a Fahrenheit degree is 5/9 of a Celsius degree. The fraction in the formula matches the direction of travel.
• Carrying minutes past 60. If elapsed minutes sum to 70, you have 1 h 10 min, not 70 min as a final answer. Always normalize.
• Forgetting 24 in day conversions. When converting days to hours, multiply by 24, not 60. (60 is for minutes and hours.)
• Order of operations in temperature. In F-to-C, the subtraction must happen before the multiplication. (°F - 32) x 5/9 is not °F x 5/9 - 32.
• Crossing midnight. When adding a duration that passes 24:00, subtract 24 hours and advance to the next day rather than reporting an hour like 26:20.