40 C in a Child - When to Worry?

    A child's forehead feels warm but the thermometer reads 37.4. Is that a fever? Enter the reading, measurement site and age - the calculator corrects to rectal equivalent, classifies the fever grade and suggests approximate paracetamol and ibuprofen doses by weight.

    The thermometer reads 40 C on your child. Is this a fever? It depends on where you measured. Under the arm, 40 C may be normal, but on the forehead the threshold is different. Fever starts at 38.0 C rectally, 37.5 C on the forehead, and 37.2 C under the arm. Enter the child age and measurement site for a proper assessment.

    Parameters

    Enter data for calculations

    Age affects fever interpretation and medication dosing.

    Raw thermometer reading - the calculator will correct it based on measurement site.

    Rectal is the gold standard. Underarm reads lowest, forehead varies by device.

    Optional - needed to calculate medication doses.

    Form progress0 / 3 fields

    💡 Fill in all required fields to unlock the calculate button

    Two thresholds, three responses - rectal vs axillary and when to call the doctor

    A thermometer reading means nothing without context. 37.8 C under the arm is a fever (corrected to 38.3 C rectal). 37.8 C rectally is not. And a 38.2 C in a 2-month-old demands an ER visit, while the same number in a 4-year-old usually just needs fluids and monitoring.

    This calculator takes the raw thermometer reading, applies the correct site-specific adjustment, classifies the result against pediatric thresholds, and - if you enter the child's weight - provides approximate paracetamol and ibuprofen doses in milligrams and milliliters of standard syrup.

    How to use this calculator - step by step

    1. Select your child's age bracket - age determines the urgency of a fever. Infants under 3 months with any fever above 38.0 C need immediate medical attention.
    2. Enter the thermometer reading - type the exact number shown on the display. Do not adjust it yourself.
    3. Choose the measurement site - rectal, ear, forehead, underarm or oral. Each has a different correction factor.
    4. Optionally enter the child's weight - this unlocks medication dosing (paracetamol 15 mg/kg, ibuprofen 10 mg/kg per dose).
    5. Read the results - corrected temperature, fever classification, recommendation, and (if weight entered) dosing table.

    Measurement site correction table

    Every site except rectal underestimates core body temperature. The correction factors below convert any reading to its rectal equivalent.

    Measurement site Correction to rectal Notes
    Rectal +0.0 C Gold standard, most accurate
    Ear (tympanic) +0.2 C Fast but earwax can distort
    Oral +0.3 C Good for children 5+, wait 15 min after hot drinks
    Forehead (infrared) +0.4 C Convenient but sensitive to room temperature and sweat
    Underarm (axillary) +0.5 C Least accurate, requires 3-5 min hold, non-invasive

    Fever classification in children (rectal equivalent)

    Temperature (rectal) Classification Action
    Below 36.0 C Hypothermia Warm the child, recheck
    36.0-37.4 C Normal No intervention needed
    37.5-37.9 C Low-grade fever Monitor, push fluids
    38.0-38.9 C Moderate fever Medicate if child is uncomfortable
    39.0-39.9 C High fever Antipyretic + doctor if not improving
    40.0 C and above Very high fever Antipyretic + urgent medical attention

    Real-world correction examples

    Underarm reading: 37.2 C, child age 2 years
    Corrected: 37.7 C (low-grade). No medication needed, just monitor and keep hydrated.
    Forehead reading: 38.1 C, child age 8 months, weight 9 kg
    Corrected: 38.5 C (moderate fever). Paracetamol dose: 135 mg (5.6 ml syrup). Ibuprofen: 90 mg (4.5 ml).
    Ear reading: 39.3 C, child age 4 years, weight 16 kg
    Corrected: 39.5 C (high fever). Paracetamol: 240 mg (10 ml). Ibuprofen: 160 mg (8 ml). Contact doctor if no improvement in 1 hour.
    Rectal reading: 38.2 C, infant age 2 months
    Corrected: 38.2 C (no correction needed). URGENT: Any fever in an infant under 3 months requires immediate medical evaluation.
    Oral reading: 37.5 C, child age 10 years
    Corrected: 37.8 C (low-grade). Borderline - recheck in 30-60 minutes. No medication unless the child feels unwell.

    Frequently asked questions

    What temperature is considered a fever in children?
    38.0 C measured rectally is the standard threshold. For axillary (underarm) readings, 37.5 C indicates fever after adding the +0.5 C correction. For forehead: 37.6 C (+0.4 C correction). The important number is always the corrected rectal equivalent.
    When should I give fever medication?
    Not based on the number alone. Give medication when the child is uncomfortable, irritable, refusing fluids, or clearly in pain. A child at 38.5 C who is playing normally does not need medication. A child at 38.1 C who is lethargic and refusing drinks probably does. Treat the child, not the thermometer.
    How much paracetamol per kilogram of body weight?
    10-15 mg/kg per dose, every 4-6 hours, maximum 4 times per day. This calculator uses 15 mg/kg (upper single-dose limit). For a 12 kg child: 180 mg = approximately 7.5 ml of standard 120 mg/5 ml syrup. Maximum daily dose: 60 mg/kg.
    From what age can children take ibuprofen?
    From 6 months of age (and body weight above 7 kg). Dose: 5-10 mg/kg every 6-8 hours, max 3 times daily. This calculator uses 10 mg/kg. Do not give ibuprofen to children with chickenpox, dehydration, or known kidney issues. Avoid in infants under 6 months.
    Is a high fever more dangerous than a moderate one?
    Not necessarily. Fever height does not correlate with disease severity. A common cold can push temperature to 39.5 C, while a serious bacterial infection might only produce 38.3 C. What matters more: how the child looks and behaves (energy, hydration, responsiveness), not the exact number on the thermometer.
    Can teething cause a fever?
    Teething can raise temperature by 0.1-0.2 C, but it does not cause fever above 38.0 C. If a teething baby has a temperature of 38.5 C, look for another cause (infection, virus). The teething-fever myth persists because the teething window (6-24 months) overlaps with the age when children get their first round of common infections.

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    Ready-made calculations

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    See also

    Child Temperature 36.5 C - Fever or Normal?Child Temperature 37 C - Fever or Normal?Child Temperature 37.2 C - Fever or Normal?Child Temperature 37.5 C - Fever or Normal?Child Temperature 37.8 C - Fever or Normal?Child Temperature 38 C - Fever or Normal?Child Temperature 38.3 C - Fever or Normal?Child Temperature 38.5 C - Fever or Normal?Child Temperature 39 C - Fever or Normal?Child Temperature 39.5 C - Fever or Normal?

    Calculator verified by the LiczGrupa.pl team

    Content, formulas and results have been reviewed for accuracy and relevance by our team of specialists.

    Patryk Matyjasik

    Reviewed by: Patryk Matyjasik

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