You’re standing in the bathroom, feeling like your forehead is radiating enough heat to fry an egg. You stick the digital thermometer under your tongue, wait for the beep, and look down. It says 38.2. For a split second, you panic because your brain is hardwired for 98.6. Unless you’re a scientist or living literally anywhere else in the world besides the U.S., you probably need to know how to change thermometer from c to f right now.
It’s frustrating. Most modern digital thermometers—whether they are the cheap ones from the drugstore or the fancy infrared "no-touch" guns—are built for a global market. They default to Celsius because the metric system is the world's standard. But when you’re sick, you don't want to do math. You don't want to multiply by 1.8 and add 32. You just want the familiar numbers.
The Secret is Usually in the Long Press
Most people think their thermometer is broken or "stuck" in Celsius. It’s not. It’s just waiting for a specific sequence of touches. Manufacturers like Braun, Vicks, and iProven love to hide the toggle feature inside the power button.
Here is the most common trick: With the device turned off, press and hold the start or power button. Don't let go when it beeps. Keep holding it for about five to ten seconds. Usually, the screen will start flashing, and you’ll see the "C" change to an "F." Once you see that beautiful F, let go immediately. If you wait too long, it might cycle back.
Why the "Off" State Matters
It feels counterintuitive, right? You’d think you should change settings while the thing is actually on and working. But for 90% of handheld digital thermometers, the "On" state is for measuring, and the "Off" state is for configuring.
Take the Braun ThermoScan series, for example. These are the ones doctors usually use. If you want to switch scales on a ThermoScan 5 or 7, you have to start with the unit powered down. Press the "start" button and hold it. The display will cycle through "C / SET / F / SET." You release the button when the one you want is showing. It’s a bit like those old digital watches where you had to hold a tiny recessed button with a paperclip just to change the time.
What About the No-Touch Infrared Guns?
Infrared thermometers—the ones that look like little plastic phasers—are a different beast. These often have a dedicated "Mode" button. If you see a button labeled "Mode" or "Set," give it a quick tap while the device is on.
Sometimes, though, it’s a physical switch. Check the battery compartment. Seriously. Pop that plastic cover off and look next to the batteries. Some brands, especially the generic ones sold on Amazon during the last few years, have a tiny micro-switch hidden inside the casing. You might need a fingernail or a toothpick to slide it from C to F. It’s a "set it and forget it" design choice that drives people crazy because nobody thinks to look next to the AAA batteries for a UI setting.
The Weird Ones: Kinsa and Smart Thermometers
If you’re using a smart thermometer like a Kinsa, which plugs into your phone or connects via Bluetooth, the physical button won't do anything. You have to go into the app.
Go to settings. Look for "Profile" or "Device Settings." Changing it there pushes the update to the hardware. It's actually easier in the long run, but if your phone is dead and you’re trying to check a toddler’s fever at 3 a.m., it’s a nightmare. Always make sure your app settings are locked into Fahrenheit before the flu season actually hits.
Why Do We Even Use Fahrenheit Anyway?
Honestly, Fahrenheit is more "human." Celsius is great for water—0 is freezing, 100 is boiling. Simple. But for human bodies, Fahrenheit offers a more granular scale. A 100-degree day feels significantly different than a 90-degree day. In Celsius, that’s just the difference between 37 and 32. When it comes to a fever, every tenth of a degree matters. A 102.5 °F fever sounds a lot more urgent than a 39.2 °C, even though they’re the same thing.
Identifying Your Specific Model's Quirk
If the long-press didn't work, we have to get specific.
- Vicks SpeedRead: These usually require you to hold the button down while the unit is off. The display will toggle. Release when you see the "F."
- iProven: Look for a "S" (Set) button. If it doesn't have one, it’s the power button hold.
- Omron: These are notorious for being tricky. Often, you have to hold the "Measure" button and the "Power" button simultaneously.
- Exergen Temporal Scanner: These actually usually don't have a toggle button on the outside. You often have to find the tiny switch in the battery neck.
If you’ve tried holding the button for 30 seconds and nothing happens, your thermometer might be a "single scale" model. They are rare now, but some super-cheap, old-school digital sticks are hardcoded to one or the other. If you bought it at a pharmacy in Paris, it might just be a Celsius device forever.
Troubleshooting the "Lo" and "Err" Messages
While you’re trying to how to change thermometer from c to f, you might see "Lo" pop up. Most people think this means "Low Battery." It doesn't.
"Lo" usually means "Low Temperature." Because the thermometer is sitting in the room air (which is maybe 72 degrees) and it’s designed to measure body heat (98 degrees), it’s telling you the ambient temperature is below its reading range. It’s ready to go. Just put it in your mouth or on your forehead, and the "Lo" will turn into a number. If you see "Err," that’s usually a sensor issue or you moved too much during the reading.
Real Talk on Accuracy
Switching from C to F doesn't change the accuracy. It’s just a display change. However, if you find yourself constantly switching back and forth because you’re sharing the device with someone who prefers the other scale, you might be wearing out the membrane button. These things are built cheaply.
If you are a parent, just buy a dedicated Fahrenheit one and hide it. Keep it in a drawer where it won’t get its settings bumped.
Dealing with the Math if All Else Fails
If the button is stuck or you simply cannot figure out the toggle, keep this "cheat sheet" in your head. It’s faster than a calculator.
37 °C is 98.6 °F (Normal).
38 °C is 100.4 °F (The start of a "real" fever).
39 °C is 102.2 °F (High).
40 °C is 104 °F (Call a doctor).
Basically, every 1 degree Celsius is about 1.8 degrees Fahrenheit. If you see it go up by a full degree in C, you're looking at nearly a two-degree jump in the numbers you're used to.
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Actionable Next Steps for Success
Stop pressing buttons randomly. You’ll just end up in a calibration mode you can't get out of.
First, turn the device completely off. Wait five seconds.
Second, perform the "Long Press." Hold the main power button for a full 10 seconds. Watch the screen like a hawk. The moment you see the "F" or "deg F" icon flash, let go.
Third, if that fails, open the battery door. Look for a tiny physical switch.
Finally, once you get it to Fahrenheit, take a piece of clear tape and put it over the button if it's one of those models that switches too easily.
If you're still stuck, Google the specific model number followed by the word "manual." Most brands like Welch Allyn or ADC have PDFs online because they know their interfaces are confusing. Once you have it set, do a test reading immediately to "lock" it in the memory. You'll feel a lot better knowing you're ready before the next fever hits.