37.8 degrees on the forehead thermometer. The toddler is cranky but playing. Is that a fever?
It depends. A forehead reading of 37.8 C corrects to 38.2 C rectal equivalent - which crosses the fever threshold. An underarm reading of 37.8 would correct to 38.3 C. But the same 37.8 taken rectally is just normal temperature at the high end of the range. The number alone tells you almost nothing. The measurement site changes the answer.
Meanwhile, outside, it is 32 C and you are wondering whether to go for your usual run. Your baseline pace is 5:30 min/km, but in this heat, that same effort will only produce 6:06 min/km. And if you do a walk-run session instead - 20 minutes of easy running mixed with 10 minutes of walking - you will burn 317 kcal and cover 4.33 km. Not as dramatic as a pure run, but sustainable in the heat. Three scenarios, three calculators, three sets of numbers that actually change what you do next.
The forehead says 37.8 - now what?
Every thermometer site reads differently because none of them measure core body temperature directly. Rectal is the reference standard. Everything else is a proxy that reads lower by a predictable margin.
| Site | Correction to rectal | Practical note |
|---|---|---|
| Rectal | +0.0 C | Gold standard, most accurate |
| Ear (tympanic) | +0.2 C | Quick, but earwax can throw it off |
| Oral | +0.3 C | Good for kids 5+, wait after hot drinks |
| Forehead (infrared) | +0.4 C | Convenient, affected by sweat and room temp |
| Underarm (axillary) | +0.5 C | Least accurate, takes 3-5 min, non-invasive |
So that 37.8 on the forehead becomes 38.2 after correction. That is a fever, technically - but a mild one. The question is not whether to medicate but whether the child is uncomfortable. A toddler with 38.5 who is playing happily does not need paracetamol. A toddler with 38.1 who is limp and refusing water probably does.
The dosing math is straightforward once you have the weight. Paracetamol: 15 mg per kilogram. Ibuprofen: 10 mg per kilogram. A 12 kg child gets 180 mg of paracetamol (7.5 ml of standard 120 mg/5 ml syrup) or 120 mg of ibuprofen (6.0 ml of 100 mg/5 ml syrup). But ibuprofen is off limits under 6 months.
One number deserves a red flag: any fever in an infant under 3 months. Even 38.1 C rectally in a 2-month-old means a call to the pediatrician or a trip to the ER. The immune system is too immature for the usual "wait and see" approach. The Child Fever Calculator shows an explicit warning for this age bracket.
Walking burns more than you think - if you count it right
The problem with most calorie trackers is binary classification. Your Garmin or Apple Watch sees either "running" or "walking" and applies a single MET value to the entire session. But a Galloway-style walk-run workout has two distinct phases with very different energy costs.
Running at 6:00 min/km has a MET value of 10.5. Normal walking at 10:00 min/km has a MET of 4.3. If you blur them together, you get the wrong number.
The correct approach: calculate each phase separately. For a 75 kg person doing 20 minutes of easy running + 10 minutes of normal walking:
- Running phase: 10.5 x 75 x (20/60) = 263 kcal, distance 3.33 km
- Walking phase: 4.3 x 75 x (10/60) = 54 kcal, distance 1.00 km
- Total: 317 kcal, 4.33 km, average pace 6:55 min/km
That is roughly 75-85% of what 30 minutes of continuous running would burn. The trade-off is sustainability. A beginner who cannot run 30 minutes straight will finish a 20/10 walk-run session and come back tomorrow. The runner who pushed through continuous running at the same fitness level might take two days to recover.
| Scenario (75 kg) | Running | Walking | Total kcal | Distance |
|---|---|---|---|---|
| 20 min run (6:00) + 10 min walk (10:00) | 263 | 54 | 317 | 4.33 km |
| 15 min jog (7:00) + 15 min brisk walk (8:30) | 150 | 94 | 244 | 3.93 km |
| 30 min run (5:30) + 0 walk | 431 | 0 | 431 | 5.45 km |
| 10 min run (6:00) + 20 min walk (10:00) | 131 | 108 | 239 | 3.67 km |
The Walk-Run Calories Calculator handles the split automatically. Enter weight, running time and pace, walking time and pace, and it returns the exact breakdown plus average pace and per-kilometer burn.
32 degrees outside - your 5:30 is now 6:06
Heat does not just make running feel harder. It makes running physically slower at the same physiological effort. Your heart rate at 5:30 pace in 12 C might be 148 bpm. At 32 C, the same 148 bpm only produces 6:06. The effort is identical. The pace is not.
The correction is roughly +15 seconds per kilometer for 27-30 C and +25 seconds for 31-35 C. Add humidity and it gets worse: high humidity (60-80%) tacks on another +6 seconds because sweat cannot evaporate, and your body loses its primary cooling mechanism.
Wind adds drag too. A moderate headwind (10-25 km/h) costs +5 seconds per km. Strong wind (25-40 km/h) costs +12 seconds. And here is what most runners get wrong about out-and-back routes: the headwind penalty is always larger than the tailwind benefit. Air resistance scales with the square of relative wind speed. Running into 20 km/h wind at 12 km/h means 32 km/h relative (1024 drag units). Running with it: 8 km/h relative (64 drag units). The math is not even close.
Surface matters too. Grass adds +10 seconds per km. Sand adds +25 seconds. A summer beach run in 30 C heat, moderate wind and soft sand could add up to +50 seconds per kilometer - nearly a full minute slower. At that point you are essentially doing a different workout than your training plan prescribed.
| Conditions | Baseline 5:30 | Adjusted | Correction |
|---|---|---|---|
| 14 C, calm, asphalt | 5:30 | 5:30 | +0 sec |
| 28 C, moderate humidity, no wind | 5:30 | 5:47 | +17 sec |
| 32 C, high humidity, moderate wind | 5:30 | 6:06 | +36 sec |
| -5 C, strong wind, asphalt | 5:30 | 5:50 | +20 sec |
| 25 C, moderate humidity, grass | 5:30 | 5:48 | +18 sec |
The Weather Pace Adjustment Calculator breaks down each factor separately. Temperature, humidity, wind, surface - each gets its own correction in seconds, so you can see exactly where the slowdown comes from.
Heat acclimation helps. After 10-14 days of regular heat training, you can roughly halve the temperature correction. But the calculator uses non-acclimated values as a safe default. Better to run slower than planned than to DNF from heat exhaustion.
Three tools, one principle
All three calculators do the same thing at a fundamental level: they take a raw number and put it in context.
A thermometer reading needs correction by site. Calorie burn needs separate calculation by phase. Running pace needs adjustment by conditions. The raw number - 37.8, 317, 5:30 - only means something after you account for the variables around it.
Tools discussed in this article
Child Fever Calculator - Enter temperature, measurement site and age. Get the corrected rectal equivalent, fever classification and approximate paracetamol/ibuprofen doses by weight.
Walk-Run Calories Calculator - Calculate exact calorie burn for walk-run interval sessions. Separate MET values for each phase, with total distance, average pace and per-km burn.
Weather Pace Adjustment Calculator - Adjust your running pace for temperature, humidity, wind and surface. See each factor's impact in seconds per kilometer.
More tools from Health & Fitness
- BMI Calculator - Body Mass Index
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- Ideal Weight Calculator - Target weight by frame and formula
- WHR Calculator - Waist-to-hip ratio
- Body Fat Calculator - Body fat percentage
- One Rep Max Calculator - 1RM prediction
- Calories Burned Calculator - Calorie burn by activity
- Water Intake Calculator - Daily water needs
- Protein Intake Calculator - Daily protein by goal
- Training Heart Rate Calculator - Five training zones
Check for a specific temperature
- Child fever 37.5 degrees - low-grade
- Child fever 39 degrees - when to medicate
- Walk-run calories 75 kg - interval training