Body Temperature Calculator
Convert between Fahrenheit and Celsius instantly, or check where a reading falls on the hypothermia–fever spectrum. Includes route-of-measurement adjustment and reference ranges for adults and children.
| Category | °C range | °F range | Clinical notes |
|---|
| °C | °F | K | °R | Context |
|---|
| Site | Offset vs oral | Oral equivalent if you measured here | Notes |
|---|
Why two scales exist
Daniel Gabriel Fahrenheit set 0 °F as the freezing point of a brine solution and 96 °F as body temperature (~98.6 °F after recalibration). Anders Celsius set 0 °C as water's freezing point and 100 °C as its boiling point. Most of the world uses Celsius; the US, Bahamas, Cayman Islands, Palau, and Belize still use Fahrenheit for everyday and clinical temperatures.
Circadian variation
Core body temperature varies by 0.5–1.0 °C (0.9–1.8 °F) over 24 hours — lowest around 4–6 AM, highest around 4–6 PM. This is why a single afternoon reading of 37.5 °C may be normal, while the same value at 6 AM suggests fever.
Site matters
Rectal temperature is the closest to true core temperature, ~0.5 °C (0.9 °F) higher than oral. Axillary is ~0.5 °C lower than oral. Tympanic (ear) sits between but is sensitive to technique. Temporal artery is convenient but least accurate in extremes.
When to act
Seek urgent care for a temperature ≥ 40 °C (104 °F), any fever in an infant under 3 months, fever with stiff neck, confusion, rash, or difficulty breathing, or hypothermia below 35 °C (95 °F). Age and underlying conditions change the threshold.
How to Use This Calculator
Pick the scale you have, enter the value, and the other scales appear instantly alongside the clinical category and reference ranges.
1. Choose the scale you have
Select °F, °C, K, or °R. The other three are calculated automatically and highlighted in the result panel.
2. Enter the value
Type a number, drag the slider, or tap a preset. Presets cover hypothermia, low-normal, normal, fever, high fever and hyperpyrexia thresholds.
3. Read the result card
The converted value appears large, with the clinical category (hypothermia / normal / low-grade fever / fever / hyperpyrexia) shown in a colour-coded box below.
4. Check the charts
The clinical-zones tab shows where your value sits on a colour-coded scale. The conversion line shows the linear relationship. The circadian tab shows the normal 24-hour body-temperature curve.
5. Reference tables
Table 1 gives clinical categories with °C and °F ranges. Table 2 lists common conversions across all four scales. Table 3 (Advanced) adjusts between measurement sites.
6. Advanced mode
See sensitivity to the input, plus derived quantities like the site-adjusted oral equivalent and fever threshold comparison.
What This Tool Offers
A complete workbench for temperature conversion and clinical classification — the same calculation used by nurses, physicians, and patients worldwide.
Four-way conversion
Convert between Fahrenheit, Celsius, Kelvin, and Rankine instantly — solve for any one from the other three.
Three interactive views
Colour-coded clinical-zones bar, linear conversion plot, and the normal 24-hour circadian temperature curve.
Clinical reference tables
Temperature categories with clinical notes, common conversions, and measurement-site adjustment grid.
Category detection
Automatic classification as hypothermia, low-normal, normal, low-grade fever, fever, or hyperpyrexia.
Sensitivity check
See how a small change in the input shifts the result across all scales — useful for precision and error analysis.
Companion quantities
All four scales, clinical category, Fahrenheit fever threshold comparison, and site-adjusted oral equivalent.
What Is Body Temperature?
Body temperature is the balance between heat produced by metabolism and heat lost to the environment. In healthy adults the core temperature is tightly regulated by the hypothalamus to about 37 °C (98.6 °F), though normal values vary by 0.5–1.0 °C across the day, by measurement site, and by individual. Fever is a regulated rise in the hypothalamic set-point — usually triggered by infection — and is not the same as hyperthermia, which is an uncontrolled rise due to heat exposure or impaired dissipation. Hypothermia is a drop below 35 °C (95 °F) and is a medical emergency. Because different countries use different temperature scales, being able to convert quickly and accurately between Fahrenheit and Celsius is essential for interpreting clinical readings, comparing international guidelines, and communicating across healthcare settings.
Formula and Variables
K = °C + 273.15 · °R = °F + 459.67
| Symbol | Meaning | Notes |
|---|---|---|
| °F | Fahrenheit | US customary scale; water freezes at 32 °F, boils at 212 °F |
| °C | Celsius (centigrade) | SI-preferred scale; water freezes at 0 °C, boils at 100 °C |
| K | Kelvin | SI absolute scale; 0 K = −273.15 °C |
| °R | Rankine | Absolute Fahrenheit scale; 0 °R = −459.67 °F |
| 32 | Offset between freezing points | 32 °F = 0 °C |
| 5/9 ≈ 0.5556 | Scale factor | One °F interval is 5/9 of one °C interval |
Clinical Temperature Categories (Oral)
| Category | °C range | °F range | Notes |
|---|---|---|---|
| Severe hypothermia | < 28 | < 82.4 | Loss of consciousness; resuscitation may succeed |
| Moderate hypothermia | 28–32 | 82.4–89.6 | Confusion, drowsiness, slowed pulse |
| Mild hypothermia | 32–35 | 89.6–95.0 | Shivering, cold skin |
| Low-normal | 35.0–36.1 | 95.0–97.0 | Common in the morning |
| Normal | 36.1–37.2 | 97.0–99.0 | Typical oral range in adults |
| Low-grade fever | 37.3–38.0 | 99.1–100.4 | Often viral, self-limiting |
| Fever | 38.1–39.0 | 100.5–102.2 | Investigate if persistent > 48 h |
| High fever | 39.1–40.0 | 102.3–104.0 | Antipyretics, hydration |
| Hyperpyrexia | > 40.0 | > 104.0 | Medical emergency |
Frequently Asked Questions
The number comes from a 19th-century study of 25,000 patients by Carl Reinhold August Wunderlich, who reported a mean oral temperature of 37 °C. Converting to Fahrenheit gives 98.6 °F. Modern studies using more precise thermometry suggest the true population mean is closer to 36.6 °C (97.9 °F), with a normal range of 35.5–37.5 °C (95.9–99.5 °F) depending on age, sex, time of day, and measurement site.
Yes, significantly. Rectal temperature is closest to true core and runs about 0.5 °C (0.9 °F) above oral. Axillary runs about 0.5 °C below oral. Tympanic readings depend heavily on technique and can vary by 0.3–0.5 °C. Temporal artery thermometers are convenient but less accurate in extremes. When you compare readings between sites, always adjust before deciding whether a value represents fever.
It depends on time of day, site, and the individual's normal baseline. If measured orally in the late afternoon, 37.5 °C is at the upper end of normal. If measured in the early morning, it suggests low-grade fever. Values above 38.0 °C (100.4 °F) are considered fever regardless of time. In immunocompromised patients or infants under 3 months, even a modest rise warrants medical attention.
Kelvin is the SI base unit of thermodynamic temperature and starts at absolute zero — the point at which molecular motion stops. This makes it ideal for thermodynamics, physics, and chemistry where ratios and rates depend on absolute rather than relative temperature. Body temperature 37 °C = 310.15 K. Rankine is the equivalent absolute scale in Fahrenheit degrees and is used in some US engineering applications.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| °C | Temperature in Celsius | °C |
| °F | Temperature in Fahrenheit | °F |
What it means
The conversion between Fahrenheit and Celsius is a routine but essential calculation in clinical practice, as body temperature is measured and documented using both scales depending on regional convention. The formula °C = (°F − 32) × 5/9 is derived from the linear relationship between the two scales, where water freezes at 32 °F (0 °C) and boils at 212 °F (100 °C). Accurate conversion is crucial for determining fever (≥38.0 °C or 100.4 °F), hypothermia (<35.0 °C), and for setting temperature thresholds in therapeutic protocols (e.g., targeted temperature management after cardiac arrest). In international settings, electronic medical records often default to Celsius, so clinicians must convert readings from thermometers that display Fahrenheit. Moreover, drug dosing for antipyretics may depend on temperature elevation, requiring correct interpretation. The inverse conversion (°F = °C × 9/5 + 32) is also used. Understanding this formula prevents miscommunication, enhances patient safety, and ensures consistency in monitoring during handovers and transfers. It is a fundamental skill taught early in medical and nursing education.
Worked example
Temperature Conversion – Two Examples
Real‑World| Parameter | Value |
|---|---|
| °F | 100.4 |
| Parameter | Value |
|---|---|
| °F | 98.6 |
Common mistakes
- Order of operations: Subtract 32 first, then multiply by 5/9 – not the reverse.
- Units: °F to °C – the result is in Celsius.
- Rounding: Clinical temperatures are often reported to one decimal place.
- Normal range: 36.1‑37.2°C (97‑99°F).
Applications
Body temperature conversion between Fahrenheit and Celsius is a routine but essential calculation in healthcare, as different countries use different temperature scales. The formula °C = (°F − 32) × 5/9 is used to convert Fahrenheit readings (common in the US) to Celsius (used in most other countries). Accurate conversion is vital for diagnosing fever, hypothermia, and monitoring treatment responses. Clinicians use it to communicate temperature data across international healthcare teams, to interpret patient records, and to set thresholds for interventions (e.g., antipyretic administration). By mastering this conversion, healthcare professionals ensure consistency in patient care and facilitate global collaboration.
- Daily patient temperature recording and interpretation
- Communication of temperature data in multinational settings
- Preparation of medical records for international patients
- Calculation of fever thresholds for clinical decision‑making
- Patient education on home thermometer use
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