To read a glass thermometer, hold it at eye level and slowly rotate it until you can see the liquid column inside the glass tube. The top of the liquid column lines up with a number on the printed scale, and that number is your temperature reading. It sounds simple, but the thin column can be surprisingly hard to spot, and the scale markings vary between Fahrenheit and Celsius models. Here’s everything you need to get an accurate reading.
Types of Glass Thermometers
Most glass thermometers sold today contain either a silver-colored liquid called galinstan (a gallium alloy) or red-dyed alcohol. Mercury thermometers, which also contain a silver liquid, have been banned for sale in numerous U.S. states and phased out of clinical use since the early 2000s. If you have an older silver-liquid thermometer, it may contain mercury. Newer ones are labeled “mercury-free” on the packaging.
The colored cap at the end of the thermometer sometimes tells you what it’s designed for. A blue or green cap typically indicates an oral or axillary (underarm) thermometer, while a red cap often marks a rectal thermometer. The scale and reading technique are the same regardless of type.
Shake It Down First
Glass thermometers work because the liquid expands with heat and stays in place after you remove the thermometer. That means the reading from your last use is still showing. Before every measurement, you need to shake the liquid back down into the bulb at the tip.
Grip the end opposite the bulb, between your fingers and thumb. Stand somewhere clear of furniture and hard surfaces, since the glass will shatter if it hits something. Use quick, sharp, downward flicks of your wrist, similar to cracking a small whip. Check the reading after several flicks. You want the liquid column below 94°F (34.4°C) before you start. If it’s still above that mark, keep shaking.
Understanding the Scale Markings
This is the part most people struggle with. The lines printed on a glass thermometer are tiny, and they mean different things depending on whether your thermometer reads in Fahrenheit or Celsius.
Fahrenheit Scale
Each long line represents one whole degree. Between each pair of long lines, there are four shorter lines. Each short line equals 0.2°F (two-tenths of a degree). So the short lines between 98°F and 99°F represent 98.2, 98.4, 98.6, and 98.8. If the liquid stops at the second short line past 98, your reading is 98.4°F.
Celsius Scale
Each long line also represents one whole degree. But between each pair of long lines, there are nine shorter lines, each representing 0.1°C (one-tenth of a degree). So between 37°C and 38°C, the short lines mark 37.1, 37.2, 37.3, and so on up to 37.9. If the liquid stops at the fifth short line past 37, your reading is 37.5°C.
How to See the Liquid Column
Hold the thermometer horizontally at eye level in good lighting. The liquid column is very thin and can be nearly invisible if you look at the thermometer straight on. Slowly rotate the thermometer back and forth between your fingers until the column catches the light and appears as a bright line against the scale. A white background, like a sheet of paper held behind the thermometer, makes it much easier to spot.
Read the temperature at the point where the top of the liquid column aligns with the scale. If it falls between two lines, round to the nearest line.
Where to Place the Thermometer
Where you measure matters, because each body site produces a different temperature and requires a different wait time.
- Oral: Place the tip under the tongue, close to the center of the mouth, and keep your lips sealed. Hold in place for 3 minutes. Avoid eating, drinking, or smoking for 15 minutes beforehand, as these can skew the reading.
- Underarm (axillary): Tuck the tip into the armpit so it touches only skin, not clothing. Press the upper arm snugly against the chest. Hold in place for 7 to 10 minutes. This method takes longer because the armpit is cooler and less insulated than the mouth.
- Rectal: Apply a small amount of lubricant to the tip. For infants under 3 months, insert only half an inch. For older children, about 1 inch. Hold in place for 3 minutes. Never force the thermometer.
What Your Reading Means
Normal body temperature isn’t a single number. It varies by measurement site, time of day, and individual. Here are the accepted normal ranges:
- Oral: 96.4°F to 99.1°F (35.8°C to 37.3°C)
- Underarm: 96.4°F to 97.3°F (34.8°C to 36.3°C)
- Rectal: 98.2°F to 100.8°F (36.8°C to 38.2°C)
Rectal readings run about 1°F higher than oral readings, and underarm readings run about 1°F lower. This is why you should always note which method you used. A reading of 99.5°F orally suggests a low-grade fever, but 99.5°F under the arm could indicate a more significant one. Generally, an oral temperature above 99.1°F, an underarm reading above 97.3°F, or a rectal reading above 100.8°F is considered elevated.
Cleaning Before and After Use
Clean the thermometer before and after every use. Rinse it under cool running water to remove any debris, then wipe the entire length of the glass with a cotton ball or pad dampened with rubbing alcohol (at least 60% concentration). Let the alcohol evaporate completely before placing the thermometer against skin. After measuring, repeat the process. Store the thermometer in its protective case once it’s fully dry.
Avoid using hot water, which can crack the glass or affect the liquid inside. Never boil a glass thermometer.
If a Mercury Thermometer Breaks
If you drop an older mercury thermometer and it shatters, the cleanup requires care. Mercury is a neurotoxin, and the silvery beads that scatter across the floor can release vapor at room temperature.
Get everyone, including pets, out of the room immediately. Open windows and exterior doors, but close interior doors to keep vapors from spreading through the house. Do not vacuum or sweep the mercury. A vacuum aerosolizes the droplets, and a broom breaks them into smaller pieces that spread further. Instead, use stiff cardboard to push the beads together, then pick them up with an eyedropper or sticky tape. Place everything, including gloves and cleanup materials, into a sealed plastic bag. Contact your local health department or municipal waste authority for disposal instructions, as mercury cannot go into regular household trash.
If you still have a working mercury thermometer, many pharmacies and local hazardous waste programs accept them for safe disposal and can replace them with a mercury-free alternative. Galinstan thermometers perform just as accurately. A study comparing the three types found that galinstan models actually had slightly better accuracy than both mercury and digital thermometers when measuring children’s body temperatures.