A thermometer is used to measure body temperature which indicates whether you have a fever, and can be helpful in diagnosing other conditions too.
There are many types of thermometers available. Which thermometer you use, and how you use it, will depend on the age of the person.
Types of thermometers
There are many types of thermometers available.
The most common thermometers are digital. You can use these for adults and children of any age. They are accurate, easy to use and are usually the cheapest.
Electronic ear thermometers
These are expensive and are less accurate in small babies.
The following thermometers are not recommended
Infrared forehead thermometers
Infrared forehead thermometers appear quick and easy to use, as you simply point them at the person's forehead. However, they are not accurate as they measure the forehead skin temperature which changes a lot with blood flow and room temperature.
Plastic strip thermometer
These are plastic strips that you place on the person's forehead – they are not accurate so it's best not to use them.
These old style thermometers are no longer available but some households still have them. Mercury vapour can be toxic if the thermometer breaks, so don't use these thermometers. Consider getting a digital thermometer instead.
How do I use a digital thermometer?
You can use a digital thermometer for adults and children of any age. A digital thermometer gives a digital reading. There are a number of brands. You need to read and follow the instructions that come with your thermometer. It is useful to keep a written record of temperature readings so that you can see if there is any change over time. It is also helpful information to give to your healthcare provider if you seek medical help.
Children under 5 years: use a digital thermometer under your child's arm
If your child is under 5 years, you can use the digital thermometer under your child's arm. If you measure the temperature under their arm, it records about 0.5–1 degree Celsius lower than the core temperature.
Follow these steps to use a digital thermometer under the arm (the axillary temperature).
- Turn it on (these thermometers usually have a button you press to turn on).
- Place the end in the armpit against the skin, and bring your child's arm down over the top of it – it often helps to hug your child to keep the arm down and the thermometer in place.
- Most thermometers beep when they have finished measuring your child's temperature so wait for the beep.
- Note that some thermometers also beep while measuring and the beep changes when the thermometer has finished measuring. To avoid confusion, it is worth keeping the thermometer in place for 2 minutes.
- Remove the thermometer and read the number on the side - the temperature you read is about 0.5–1 degree Celsius lower than your child's actual body or core temperature.
Adults and children 5 years and older: measure their temperature in their mouth
In adults and children 5 years of age or older you can measure the temperature in their mouth (the oral temperature). Make sure you only use a digital thermometer in your child's mouth.
Follow these steps to use a digital thermometer in the mouth for older children. It is important that your child can cooperate, which usually means they need to be aged 5 years or older.
- Turn the thermometer on.
- Place the end in the mouth under the side of the tongue – try to get your child to keep it there.
- Most thermometers beep when they have finished measuring so wait for the beep.
- Note that some thermometers also beep while they are measuring and the beep changes when the thermometer has finished measuring. To avoid confusion it is worth keeping the thermometer in place for 2 minutes.
- Remove the thermometer and read the number on the side – the temperature you read in in degrees Celsius is close to your child's actual body or core temperature.
How do I use an ear thermometer for my child?
The electronic or infrared ear thermometer is fast and accurate if used correctly. You can use it in older children but not in young babies. There are a number of brands. They are more expensive than digital thermometers.
Read the instructions for your thermometer to find out how to turn it on and take the reading. When placing the measuring end in the ear, be gentle. You do not have to push it far into the ear canal, just at the entrance.
Fever ranges and what to do
Around 37ºC is normal body temperature for children and adults.
- Mild fever: A mild fever is represented by a temperature higher than 38ºC.
- High fever: A high fever is represented by a temperature higher than 39ºC.
Most fevers aren't life-threatening, but sometimes you need to seek medical advice.
Babies – if your baby is under 3 months old and has a fever, you should always see a doctor.
Children – a viral infection (such as a cold) is usually the cause of a fever. Fever by itself does not tell you whether your child is seriously sick. If your child looks unwell and you are worried, take them to a doctor whether they have a fever or not. Learn more about fever in children.
Anyone – see your doctor or go to an emergency department immediately if you notice the following symptoms with a fever: severe headache, stiff neck and light hurts your eyes. Read more about meningitis.
Pregnant people – If you’re pregnant and have a fever, check with your midwife, doctor or nurse before you take any medicines. If your fever lasts for longer than a day, check with your midwife, doctor or nurse. Read more about pregnancy and body temperature.
People with weakened immunity (immunocompromised) – contact your doctor immediately if you have a fever and have a weakend immune system, eg, from having cancer or an organ transplant or if you are HIV positive. Read more about fever.
Content courtesy of KidsHealth NZ which has been created by a partnership between the Paediatric Society of New Zealand (PSNZ) and the Starship Foundation, supported and funded by the Ministry of Health.