Ever Wonder Why Kettles Have Minimums and Maximums?

If you have a kettle, you must’ve noticed that there’s always a line that says “minimum” and “maximum” outside the pot. Most people know that it’s crucial to use these guides any time you boil water, but have you ever wondered why that is?

Kettles have minimums and maximums to prevent safety hazards. Underfilling or overfilling your electric kettle could cause electrical brownouts, melted plastic, electrocution, or fires. Underfilling or overfilling a stovetop kettle could melt plastic components, start a fire, or break the kettle.

Now let’s talk about how kettles work. We’ll go into detail so that you can finally understand why it’s so important to fill your kettle to the right level every time you boil up a pot of tea or coffee. 

Types of Kettles

There are two types of kettles:

  • Stovetop kettles are classic tea kettles that require a stove or hot plate to heat up.
  • Electric kettles contain an independent heating element, which means that all you have to do is plug them in and turn them on to heat your water. 

How Do Electric Kettles Work?

Before we explain why there are minimum and maximum levels on every electric kettle, it would be best to know how electric kettles work. 

At the bottom of an electric kettle is a metal coil, much like a coil on a range stove. This coil is what heats your water.

When you turn on your kettle, an electric current runs through the coil, but it also creates electrical resistance, which converts electrical energy into thermal energy. When there’s water in your kettle, this energy will heat your water until it boils. 

Some electric kettles have a metal plate at the bottom. In this case, the coil is housed underneath it. The plate doesn’t improve the kettle’s design, but it’s just there to make it look sleeker. 

Electric kettles that turn off independently when the water boils use a thermostat, not unlike one of those plastic pop-up turkey timers that come with your thanksgiving turkey.

When the heat inside the kettle gets to over 100° F (37.778° C), the boiling temperature of the water, the kettle blocks off the electrical current. When the thermostat turns the kettle off, it disengages the coil at the bottom of your kettle to stop heating your water.

This thermostat is most often near the on or off switch of the kettle.

Many kettles place the on/off switch near the top of the pot so that it measures the temperature of your water’s steam, ensuring that the water has had enough time to come to a roaring boil.

When you use the right amount of water for your kettle, this kind of thermostat works well, but if you fill your pot with too much or too little water, it can cause problems. 

What Happens if You Boil Your Electric Kettle Below the Minimum?

So, do you have to fill up your kettle to the “minimum” line every time you boil water, and what will happen if you don’t?

The short answer is yes. You should always fill up your kettle over the “minimum” line to prevent any dangerous issues and make your kettle last as long as possible. If there isn’t enough water in your kettle, one of a few things could happen depending on how much water you fill it with.

Your Kettle Could Overheat

If the water in your kettle doesn’t entirely cover the plate or coil in the bottom, or if you turn on your kettle with no water in it, it could overheat.

Overheating can break the kettle, melt plastic components, damage the metal components like copper wires, burst the coil, or create an electrical short or brownout.

That’s because the kettle’s coil is a resistor. If it builds up too much heat, the electrical circuit may be uncontainable, causing one of many hazardous issues. 

If you keep your electric kettle’s coil or bottom plate covered but don’t fill it to the “minimum” line, you can expect that your kettle may not last as long as it should. When you only use a small amount of water, like just enough to fill up your favorite coffee mug, the coil can become overstressed. That’s because the metal components of your kettle will get much hotter before it turns itself off. 

When the metal gets overheated, it can crack, flake off, or deteriorate. When that happens, you may see a black film on the plate or coil of your kettle, or you might see small flakes of black metal in your coffee or tea.

Using a kettle with deteriorating metal components can be dangerous. The weaker the metal gets, the more likely it’ll be that your kettle will have an electrical problem and potentially explode or start a fire in the future. 

Also, if you don’t use enough water and if the thermostat is at the top of the kettle, it may not turn the kettle off since there will be little to no steam. If you don’t catch the problem right away and leave it on with no water in it, your peaceful tea-time may turn into fire-fighting time. 

So, there’s a reason why there’s a “minimum” line on your kettle. Always keep it filled above that line if you don’t want to create a potentially dangerous problem. 

What Happens if You Boil Your Electric Kettle Above the Maximum?

When you heat anything, it expands. A kettle turns electrical energy into thermal energy to heat water. When the thermal energy heats the heating element in your kettle or electric kettle, it’s transformed into kinetic energy, meaning that the water molecules inside start to vibrate.

The hotter something is, the more kinetic energy it has, and the faster its molecules will vibrate. That’s why boiling water bubbles and spurts. 

Your Kettle May Boil Over

These particles need plenty of room to vibrate. If you fill your kettle up above the “maximum” line, the kettle may boil over and send hot water shooting out of its spout. While that may sound like a burn risk, it’s also an electrical hazard.

If your kettle’s wiring gets wet, the electrical current will carry through the water. Just like dropping a hairdryer into a bathtub, damp wiring on an electric kettle can electrocute you or start a fire in your home when left plugged in. 

Sometimes, when the thermostat is at the top of your electric kettle, it won’t turn it off automatically if water is covering the sensor. This buildup of hot water and pressure can melt the plastic components of your kettle, potentially breaking it permanently. Also, when the plastic melts, it can leach toxic chemicals into your hot water. 

How Do Stovetop Kettles Work?

Compared to electric kettles, stovetop kettles are very simple. They’re most often made of stainless steel or copper, and they use a flame or an electric coil on your stove to transfer heat through their metal base to the water inside. As the water rises in temperature, pressure from steam builds up, causing the whistle on your kettle spout to sound. 

Stovetop kettles aren’t nearly as dangerous as electric kettles can be when you underfill or overfill them. However, there are still some hazardous consequences of not using your stovetop kettle correctly. 

What Happens if You Underfill a Stovetop Kettle?

When you don’t put enough water in a stovetop kettle, nothing wrong will happen until the kettle gets very hot. If there’s no water in the kettle, but it’s left on, it could start a fire, just like if you leave the oven on for a long time.

When a stovetop kettle boils, it releases steam, which pushes all of the air in the kettle out, creating a high-pressure vacuum, just like a pressure cooker. If you leave your kettle long enough, all of the water will escape it in steam form as it gets hotter and hotter.

Once the water is gone, there’s no air left in the absurdly hot kettle, which may melt the plastic on your kettle, crack the metal, or start a fire, depending on how long you leave it. 

So, when you use a stovetop kettle, the more water you fill it with, the more time you have before you have to turn the stove off and remove it from heat. If you leave it for too long, you could end up dealing with some problematic issues. 

What Happens if You Overfill a Stovetop Kettle?

If you overfill a stovetop kettle, it’ll probably spit out water through its spout, which could burn you, or it could melt a plastic spout or handle on your kettle.

When using a gas stove, allowing your kettle to boil over could also put out the flame, which might mean that dangerous gas will leak into your home. 


It’s crucial to always fill up your kettle between the “minimum” and “maximum” lines to prevent safety hazards like fires and electrical issues and, at the very least, to keep your kettle functioning properly for as long as possible.