How to Read a Thermometer: A Clear Step-by-Step Guide

Reading a thermometer means interpreting the temperature shown on its display or scale, in the units it uses. The basic process is straightforward, but placement and timing matter: a thermometer measures the temperature at its sensing point, not necessarily the temperature of everything around it. Before starting, decide what you are measuring—such as room air, outdoor air, a liquid, or food—and use a thermometer designed for that purpose. For food, use a food thermometer and follow safe internal-temperature guidance.

What You’ll Need

A thermometer suited to the material or environment you are measuring.
The thermometer’s instructions, especially for correct placement and stabilization time.
A suitable surface or probe access, if measuring a liquid, object, or food.
Good light, if you are reading a printed or liquid-filled scale.

Thermometers may show Fahrenheit (°F), Celsius (°C), or both. If the unit is not obvious, look for the symbol beside the number or check the device instructions. Do not assume a bare number is in the unit you expect.

Step-by-Step

Check the thermometer type. Digital probe thermometers, infrared thermometers, indoor/outdoor thermometers, and glass thermometers work differently. Use one intended for the measurement: for example, a surface-reading infrared model does not measure the internal temperature of food. Inspect the device for damage and confirm it is clean if it will touch food or liquid.
Find the scale and units. Look for °F or °C on the screen, dial, or printed markings. On a dual-scale thermometer, identify which row or side belongs to each unit. If there is a unit button, use it only as described in the manual; switching units changes how the temperature is displayed, not the actual temperature.
Prepare the measurement. Place the thermometer where its sensor can measure the target without being affected by something else. For air temperature, keep it away from direct sunlight, heaters, air-conditioning vents, and warm electronics. For a liquid, put the sensing area into the liquid without touching the container. For food, use the probe in the thickest or otherwise appropriate part, following the food thermometer’s directions.
Position it correctly. A probe thermometer senses at or near its tip, so make sure the sensing portion—not just the handle—is in the right spot. An infrared thermometer measures a surface; aim it at the target from the distance and angle stated in its instructions. A wall thermometer should be mounted or placed where air can circulate around it, rather than pressed against a warm or cold surface.
Wait for a stable result. Digital models may take a few seconds, while other instruments can take longer to adjust. Follow the manual’s timing instructions. If the number is still changing, wait until it settles before recording it. Moving the thermometer or removing a probe too early can produce a reading that reflects the previous location or the air around it.
Read the display or scale. For a digital model, read the full number and its unit. On a marked scale, look straight at the liquid column or pointer to avoid parallax, the apparent shift caused by viewing from above or below. Read the mark where the liquid’s top or pointer aligns, according to the instrument’s design.
Estimate between markings only when appropriate. If the liquid column falls between two labeled marks, use the smaller graduation marks to judge its position. For example, if each small division represents one degree, estimate the position between those divisions rather than treating the nearest large number as exact. Do not imply more precision than the scale supports.
Record the value with its unit. Write the number and °F or °C together, and note what was measured if you may need the reading later. If it is a food-safety check, compare the reading with the safe minimum for that food rather than judging doneness by appearance or time alone.

For food safety, insert a food thermometer into the thickest part without touching bone, fat, or the cooking vessel. Check more than one spot in large or uneven foods. USDA safe minimum internal temperatures are 165°F for poultry; 160°F for ground meat; and 145°F for whole cuts of beef, pork, veal, and lamb, followed by a 3-minute rest. Fish should reach 145°F, and leftovers should be reheated to 165°F. Use a food thermometer to verify these temperatures.

Common Mistakes

Mixing up Fahrenheit and Celsius: A number without its unit can be misleading. Always read and record the symbol, and convert only when you know which scale you started with.
Reading from an angle: Looking down at a glass scale or dial can make the level appear higher or lower. Bring your eyes level with the reading.
Using the wrong thermometer: An air thermometer is not a substitute for a food probe, and an infrared surface reading does not tell you the center temperature of food.
Measuring in the wrong place: A sensor touching a pan, sitting in sunlight, or placed near a vent can report that local influence instead of the intended temperature.
Reading too soon: A changing number is not a settled measurement. Wait for stabilization and follow the device’s instructions.
Assuming a reading is exact: Thermometers have limits, and their markings or displays show only the precision the device supports. Do not add decimal places that are not displayed or justified.

Tips

Check the thermometer’s range before using it; some models cannot measure very hot, very cold, or rapidly changing targets accurately.
Keep the sensor clean and dry when the instructions require it. For food use, clean the probe before and after each use to avoid transferring contamination.
For outdoor air, shade the thermometer and allow air to circulate around it. A sunlit window or hot wall can skew the result.
When comparing readings, measure in the same location and use the same method each time.
If accuracy matters, check the device’s manual for calibration or accuracy-check instructions. Not every thermometer can be adjusted by the user.
For a rough unit conversion, °C = (°F − 32) × 5/9; °F = (°C × 9/5) + 32. Keep the original reading available in case you need to verify the conversion.

Troubleshooting

The display is blank: Check that the thermometer is switched on and that its batteries are installed correctly or charged. Replace batteries only as directed. If the screen remains blank, consult the manual or stop using a damaged device.

The number keeps changing: The sensor may not have stabilized, or the target may genuinely be changing temperature. Keep the thermometer in place, avoid touching the sensing area, and allow more time. If it never settles, check the battery and instructions.

The reading seems implausible: Confirm the units, placement, and thermometer range. Repeat the measurement with the sensor correctly positioned, then compare with a suitable second thermometer if available. Do not rely on a reading from a visibly damaged instrument.

A glass thermometer is hard to read: Improve the light, hold it securely, and bring your eyes level with the scale. Do not shake or handle it in a way the instructions do not recommend. If the glass is cracked or the liquid column is separated, do not use it; follow local guidance for safe cleanup and disposal.

An infrared reading varies: Aim at a clean, unobstructed surface and use the specified distance. Reflections, steam, a small target, or measuring through glass can affect results. Remember that infrared models measure surface temperature, not temperature inside an object.

Frequently Asked Questions

How do I tell whether a thermometer is in Fahrenheit or Celsius?

Look for °F or °C on the display or scale. If neither appears, check the instructions rather than guessing from the number.

Do I read a thermometer at the top or bottom of the liquid?

Follow the thermometer’s markings and instructions. Many liquid-in-glass thermometers are read at the top of the liquid column, but designs can differ; read at eye level.

How long should I wait before reading it?

There is no single time for every thermometer. Wait until the display settles or the instrument’s instructions say it is ready.

Can an infrared thermometer measure the inside of food?

No. It measures the surface temperature. Use a food thermometer with a probe to check internal temperature.

Why does my thermometer show a different temperature from another one?

They may be in different locations, use different sensing methods, or have different accuracy limits. Place both correctly, allow them to stabilize, and compare like with like.

Can I use a thermometer with a cracked screen or glass?

No. Stop using damaged equipment. Follow the manufacturer’s instructions for disposal; if glass or thermometer liquid has spilled, follow local safety guidance.

Further reading: USDA FoodSafety.gov. Details can change, so check the official page for anything that matters to you. Spotted a mistake? Tell us. How we write an answer.