How Big Are Bed Bugs When They Bite?

Bed bugs are evil little vampires. They sneak out at night and drink your tasty, tasty blood. They do this so they can grow and reproduce. The bites may cause an allergic reaction, which may present as swelling, hives, and even blistering. 

How big are bed bugs when they bite? Do they start biting even as nymphs?

Bed bugs subsist entirely on blood, so even baby bed bugs, or nymphs, will go out in search of blood. They can be as small as a small grain of rice.

Bed bugs prefer sleeping in a warm environment, like a bed or sofa, as it is easier to find food and helps them stay hidden in dark spaces. They usually bite around the face, neck, hands, feet, or other body parts when there is less activity in the surroundings.

Bed bugs are attracted to factors such as warmth and carbon dioxide

They will often bite in the middle of the night when you exhale carbon dioxide while asleep. Here are some signs of bed bug bites:

  • Tiny welts or reddish bumps in a specific pattern
  • Notice blood stains on the sheets from scratching
  • Frequently itchy bumps appear with a dark center and swollen surroundings

Unfortunately, bed bug bites are similar in appearance to other types of insect bites. However, bed bugs differ from other pests in size, shape, and behavior. Bed bugs have been around for a long time, but they have become more prevalent in the last few decades due to increased travel and trade and increasing urbanization. 

In this article, we will emphasize the size of the bed bugs and how big they are when they feed on their host.

What Is the Actual Shape and Size of the Bed Bug?

It is important to familiarize yourself with the shape and size of the bed bug so you can differentiate it from other similar pests. The size of an adult bed bug can vary depending on its age, species, and location. Usually, they are smaller than a grain of rice and release a strong odor, excreting an oily substance called oleic acid to ward off predators or attackers. 

The average size of an adult bed bug is about 4-5 mm long. They have an oval-shaped body, two antennae, and no wings. Males and females look very similar, and are very difficult to tell apart without a microscope.

An adult bed bug is often compared with the look of an apple seed. The eggs are about the size of poppy seeds (though they look like tiny grains of rice) and hatch into nymphs that grow into adults in about 6-7 weeks.

An adult bed bug is often compared with the look of an apple seed.

The size of the adult bed bug ranges from 1/8 to 3/16 of an inch in length. These bugs get larger as they consume more blood and grow. Hence, the size of the bed bug depends on whether they have fed on its host.

Bed bugs are usually brown or reddish-brown in color, but maybe black or whitish-gray, depending on their age and type. 

The most common type of bed bug that infests humans is Cimex lectularius. It is generally found in the northern temperate climates of Central Asia, Europe, and North America. The wide flat body of C. lectularius significantly looks giant after the blood meal.

How Big Are Bed Bugs When They Bite?

Bed Bug Nymph

There are a total of three stages in the life cycle of a bed bug. The first stage is called the egg stage when bed bugs hatch from their eggs and begin to feed on their host. The second stage is called the nymph stage, which lasts for around two weeks, during which bed bugs molt and grow in size. 

There are a total of three stages in the life cycle of a bed bug.

Finally, the third stage is called the adult stage, where they live for around a year and more, depending on the environmental conditions.

The first nymph stage of bed bugs occurs around the fifth day after hatching. Ultimately, it is also known as the first feeding stage, and during this time, nymphs start feeding on human blood.

Nymphs are less than a millimeter long and can only survive for about two weeks after hatching from their eggs. 

Therefore, they desperately need to feed on human blood as it contains proteins that give them energy for growth and development during this short period. 

From Nymph to Adult Bed Bug

Bed bugs first bite their host as a nymph, and their size is approximately 1/8th inch long, but they can reach up to 3/8th of an inch after feeding. It is very difficult to see newly hatched nymphs with the naked eye. They are very small, and often transparent or white color. 

But as soon as they are fully fed, you may identify them as their body plumps up and looks like a raspberry seed. The blood turns their body a reddish-brown color. Eventually, it becomes easier to find nymphs after a blood meal than adult bed bugs. 

Moreover, there are various development stages of nymphs or instars. For instance, the first instar or nymph is around 1 mm, and after having a blood meal, it looks like a third instar or a juvenile of around 4mm.

Similarly, adult bed bugs change shape and size after the blood meal. 

They visibly turn from reddish brown to dark red color. When the adult bed bug initiates feeding and injects its proboscis into the human skin, it is around 4-5 mm long. However, it grows to 7 mm in size after its delicious meal of your blood. 

Bed bugs can consume a meal three times more than their body weight in a single feeding attempt. When they reach 6 or 7 mm cylindrical length, they quickly waddle away to hide without being noticed.


Bed bugs can bite humans throughout their life cycle, starting at the nymph stage. They can be as small as 1/8″ in size, and are very hard to see. Eventually, they grow into adults, continuing to feed on blood as they grow. They become the size of an apple seed.