Digital, Forehead, Ear or Mercury Thermometer: Which One and How
For most homes the answer is a plain digital stick thermometer, used under the tongue for adults and older children and in the armpit for babies. It’s cheap, it’s accurate when used properly, and it has no mercury inside. Forehead and ear thermometers are faster, but they only read true if you use them the right way, and this guide shows you exactly how.
Quick facts
- Best all-rounder: digital stick thermometer (oral or armpit)
- Fastest: ear or forehead (infrared), a second or two
- Least reliable: forehead strips and the back of your hand
- Which method by age: under 6 months, armpit only; 6 months to 5 years, armpit or ear; over 5 and adults, under the tongue
- Normal body temperature: roughly 36.5 to 37.5 °C by mouth
- See a doctor: 39 °C or above, fever past 3 days, or any fever under 3 months of age
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Digital vs forehead vs ear vs mercury thermometer: the honest comparison
Every thermometer measures heat. The difference is where it measures and how much can go wrong on the way. A tool that sits inside the mouth for a minute has fewer ways to fail than one that reads the skin from a few centimetres away.
| Type | Where and how fast | How reliable | Best for |
|---|---|---|---|
| Digital stick | Mouth or armpit, 10 to 60 seconds | Good by mouth; armpit reads lower | Every home; cheapest |
| Ear (infrared) | Ear canal, 1 to 3 seconds | Good if aimed well; poor under 6 months | Children over 6 months, adults |
| Forehead gun (non-contact) | Forehead, 1 second | Fair; fooled by sweat, sun and room heat | Quick screening, sleeping child |
| Mercury glass | Mouth or armpit, 3 to 5 minutes | Accurate but breakable and toxic | Nobody now; retire it |
Batteries matter too. A digital stick runs for a year or more on one small button cell. Infrared ones eat batteries faster, and a weak battery can give odd readings before the screen warns you.
A cheap digital stick used properly beats an expensive gun used badly.
How to check fever with a digital thermometer
The mouth gives the truest home reading for anyone old enough to keep their lips closed round it, usually from about age 5. Here is the method that works first time.
- Wait 15 minutes after hot chai, cold water, food or brushing teeth. A sip of hot chai can add close to a degree for a few minutes.
- Wipe the tip with soap and water or an alcohol swab, then rinse with cool water.
- Press the button and wait for the ready signal on the screen.
- Slide the tip under the tongue, to one side at the back, into the pocket there.
- Close the lips gently. Breathe through the nose. Don’t talk or bite.
- Keep it there until it beeps, then another 10 seconds if your model beeps early (cheap ones often do).
- Read it, write down the number and the time, and clean the tip again.
Armpit method (babies and young children)
Put the tip high in a dry armpit, touching skin only, not clothing. Hold the arm snugly against the side until the beep. Armpit readings run lower than the mouth, often by about 0.5 °C, so a reading of 37.5 °C in the armpit may mean the core is nearer 38 °C. It’s the least exact method, but it’s safe, and it’s what most doctors ask parents to use for babies. For child fever by age, what to do and when to worry, see our fever by age guide.
Forehead thermometer vs ear thermometer: do they work?
Forehead guns
Yes, as a quick check, not as the final word. A forehead gun reads infrared heat coming off the skin over an artery. Skin is easily warmed or cooled by things that have nothing to do with fever.
- Distance: hold it at the gap the leaflet states, usually 1 to 5 cm. Too far and it reads low.
- Sweat: a damp forehead cools as the sweat dries, so the gun reads low. Wipe it dry first.
- Sun or a hot room: a child just in from the sun, or wrapped in a blanket near a heater, reads high. Wait 10 to 15 minutes indoors.
- The gun’s own temperature: carried from a cold car into a warm room, it needs 20 to 30 minutes to settle.
- Hair and caps: push the hair aside; a cap traps heat.
If the gun says fever, or if your child seems ill whatever it says, confirm with a digital stick.
Ear thermometers, used properly
An ear thermometer reads the eardrum, which shares blood supply with the brain’s temperature centre. Aimed well, it’s quick and good. Aimed at the canal wall, it reads low.
- Fit a fresh probe cover if your model uses them.
- For adults and children over 1, pull the top of the ear gently up and back. For babies 6 to 12 months, pull it straight back.
- Insert the tip snugly, pointing towards the eye on the other side of the head.
- Press and hold until it beeps, 1 to 3 seconds.
- Take two readings in the same ear and use the higher one.
Skip the ear under 6 months (the canal is too small and curved), and skip it in an ear with pain, discharge or heavy wax. A child who was lying on one ear will read warmer on that side for a few minutes.
Myth: the back of the hand tells you a fever
Every mother does it, and it’s useful as a nudge. But a hand on the forehead can’t tell 37.8 from 38.8, and those two numbers lead to different decisions. Cold hands make everyone feel hot. Warm hands miss real fevers.
Forehead strips (the plastic strips that change colour) have the same problem. They read the skin, not the body, and are often wrong by a degree or more. Neither is a measurement. Use them to decide to fetch the thermometer, nothing more.
Why mercury thermometers are being phased out
A mercury thermometer reads well. The problem is what happens when it breaks, which glass does. The silver bead gives off vapour at room temperature, and in a closed room that vapour is what you breathe. Many countries have signed the Minamata Convention, which phases out mercury in thermometers, and most pharmacies in Pakistan and India now sell digital ones for less than the cost of a meal out.
If one breaks, don’t vacuum it or sweep it. Follow the step list in our broken mercury thermometer clean-up guide. If you still own an intact one, seal it in a jar and ask a hospital or pharmacy whether they take it back.
Check your digital thermometer in 2 minutes
Fill a glass with plenty of crushed ice and a little water, stir, and wait 2 minutes. Put the tip in the middle of the ice water, not touching the glass, and read it. A good one reads close to 0 °C, within about 0.2 °C. Some home sticks only display from 32 °C upwards and will show “L” here, which is normal for that model. In that case, compare it against a second thermometer in the same person’s mouth, a few minutes apart.
What “H”, “L” and “Err” mean, and when to replace it
- L or Lo: the reading is below the range it can show. Usually the tip wasn’t in place long enough, or the room is cold. Try again.
- H or Hi: above its range, often because the tip touched something hot. Retake after a few minutes. If a person genuinely reads “H”, treat it as an emergency.
- Err or a blinking battery: change the battery. If the error stays, the sensor is faulty.
Replace any thermometer that fails the ice test, gives readings more than half a degree apart on repeat checks in the same spot, has a cracked tip or casing, or has been dropped hard and now behaves oddly. Never boil a digital thermometer or put it in the washing machine, and keep the tip out of water above the line the leaflet marks unless it says waterproof.
Get a doctor’s view the same day for a temperature of 39 °C or above, a fever lasting more than 3 days, or any fever at all in a baby under 3 months. Go to emergency care now for a stiff neck, a fit, a rash that doesn’t fade when you press a glass on it, fast or difficult breathing, or someone who is drowsy and hard to wake, whatever the thermometer says. The number guides you; how the person looks guides you more. Home care for fever is in our fever home care guide.
More device and kitchen-shelf guides live in Desi Remedies.
Thermometer questions people ask
Which thermometer is best for home use?
A digital stick thermometer. It’s cheap, accurate by mouth, safe, and works in the armpit for babies. An ear or forehead thermometer is a useful extra for a sleeping or restless child, but keep a digital stick to confirm.
Is a digital thermometer as accurate as a mercury thermometer?
Yes, when it’s used properly and left in place until it beeps. Most errors come from technique, such as hot chai just before or a tip placed on top of the tongue, not from the device itself. Digital ones also can’t spill mercury.
How much should I add to an armpit temperature?
Armpit readings usually run about 0.5 °C lower than the mouth. Don’t add anything to the number you tell the doctor; tell them the reading and that it was taken in the armpit. They’ll interpret it.
Do forehead thermometers work for adults?
They work as a quick screen. Sweat, sun, a hot room and holding it too far away all throw them off. If the reading surprises you, check with a digital stick in the mouth.
What is a normal temperature on a digital thermometer?
By mouth, roughly 36.5 to 37.5 °C in most adults, a little lower in the morning and higher in the evening. Armpit readings run lower. A reading of 38 °C or more by mouth is usually counted as fever.
Why does my thermometer give different readings each time?
Usually the tip moved, the mouth opened, or the second reading was taken too soon after the first. Wait a few minutes, keep the same spot and technique, and hold it until the beep. Readings that still jump by more than half a degree mean it’s time to replace it.
Can I use an ear thermometer on a newborn?
No, not under 6 months. The canal is too narrow and curved for a true reading. Use a digital stick in the armpit, and remember that any fever under 3 months needs a doctor the same day.
How do you use a mercury thermometer?
You shake it down below 35 °C, hold it under the tongue or in the armpit for 3 to 5 minutes, then read it against the light. We’d rather you didn’t: a digital one is safer and just as accurate, and a broken glass one is a mercury spill.
How often should I replace a digital thermometer?
There’s no fixed date. Replace it when it fails the ice-water check, gives jumpy readings, shows an error after a fresh battery, or has a cracked casing.