Pediatric Fever Threshold Calculator by Age
Check pediatric fever thresholds by age, including 100.4 F / 38 C infant cutoffs, high-fever bands, and urgent-care red flags.
Last reviewed: February 26, 2026
Clinical disclaimer
Fever threshold uses 38.0 C / 100.4 F. High fever is 39.0 C for under 24 months, 40.0 C for older children.
How fever is defined
Fever is typically defined as a core temperature of 38.0 C (100.4 F) or higher. In infants, rectal temperature is the most accurate method.
- Use rectal measurements for infants under 3 months whenever possible.
- Axillary readings can be lower than core temperature.
- High fever thresholds are used to guide more urgent evaluation in older infants and children.
Age-based thresholds
- Under 3 months: any fever >=38.0 C (100.4 F) needs urgent medical evaluation.
- 3-24 months: high fever >=39.0 C (102.2 F) should prompt same-day clinician contact.
- 2 years and older: very high fever >=40.0 C (104 F) warrants prompt evaluation.
Always seek care sooner for trouble breathing, dehydration, or altered alertness.
Red flags at any age
- Difficulty breathing, grunting, or severe work of breathing.
- Blue or gray lips, persistent vomiting, or signs of dehydration.
- Lethargy, inconsolable crying, or new seizure.
- Fever with rash that does not blanch or rapidly worsening symptoms.
Reassessment cadence
- For infants under 3 months with fever: immediate urgent evaluation pathway without delayed home observation.
- For older children with stable appearance: repeat temperature and hydration/perfusion checks at defined intervals.
- If red flags emerge or symptoms worsen: immediate reassessment and escalation regardless of prior threshold category.
Shift handoff checklist
- Temperature value, method, and timing of most recent measurement.
- Age-band threshold context and current risk interpretation.
- Hydration, respiratory status, and behavior trend since last assessment.
- Clear return precautions and next reassessment/follow-up timing.
Related calculators
References
Frequently asked questions
Fever is generally defined as a core temperature of 38.0 C (100.4 F) or higher. Rectal measurements are most accurate in young infants.
Infants under 3 months have higher risk of serious infection, so any fever in this age group needs urgent evaluation.
Different methods can read slightly differently. Rectal temperatures are preferred for infants under 3 months; axillary readings can underestimate fever.
Not always, but high fevers can signal more serious illness or dehydration. Use this tool to guide when to seek care, and consider the child's overall condition.
Document temperature method, age-based threshold interpretation, exam context, and explicit return precautions. Clear documentation improves safety when symptoms evolve after discharge.