Diamondback Terrapin Size: How Big Are Diamondback Terrapins?

The information is current and up-to-date in accordance with the latest veterinarian research.

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Are you thinking of getting a diamondback terrapin? I suggest you research more on the basics of the species. For example, how big are diamondback terrapins? Or what do they eat? I am answering the first one for your convenience.

An adult diamondback terrapin ranges from 5 to 11 inches. The females grow larger, about 6 – 9 inches, while the males reach the carapace of up to 6.5 inches.

What parameters decide the growth of a diamondback terrapin? Or how can you ensure a full carapace length of your pet terrapin? I have explained everything in the following article.

How Big Are Diamondback Terrapins?

Diamondback terrapins fall under the medium category. Generally, their size ranges from 5 to 11 inches. But some turtles might not grow more than 4 inches, even when they are adults. Turtles of this size range are easy to handle, and hence, the diamondback terrapins attract hobbyists.

The terrapin hatchlings are born small, about 1 to 1.5 inches. However, these babies have an excellent and fast growth rate compared to other turtle species.

The growth of the diamondback terrapins varies depending on the gender of the turtles and their subspecies.

Usually, the female terrapins are larger and grow up to 11 inches. However, their average size range is 6 – 9 inches.

On the other hand, the male diamond terrapins gain a carapace length of up to 6.5 inches. The average growth of males is 4 to 5.5 inches.

Again, diamondback terrapin has 7 subspecies. In most cases, these turtles share a close size range, and it is hard to differentiate them based on carapace length. For your convenience, I have included a size chart of diamondback terrapin below,

Diamondback Terrapin Size Chart

Diamondback Terrapin SubspeciesDiamondback Terrapin Male SizeDiamondback Terrapin Female Size
Carolina Diamondback Terrapin5 inches (12.7 centimeters)9 inches (22.9 centimeters)
Texas Diamonback Terrapin4 – 5.5 inches (10.2 – 14 centimeters)6 – 9 inches (15.3 – 22.9 centimeters)
Ornate Diamondback Terrapin4 – 6.5 inches (10.2 – 16.5 centimeters)6 – 9 inches (15.3 – 22.9 centimeters)
Mississippi Diamondback Terrapin5.5 inches (14 centimeters)9 inches (22.9 centimeters)
Mangrove Diamondback Terrapin4 – 6.5 inches (10.2 – 16.5 centimeters)6 – 9 inches (15.3 – 22.9 centimeters)
East Florida Diamondback Terrapin4 – 6.5 inches (10.2 – 16.5 centimeters)6 – 9 inches (15.3 – 22.9 centimeters)
Northern Diamondback Terrapin4 – 5.5 inches (10.2 – 14 centimeters)6 – 9 inches (15.3 – 22.9 centimeters)

N.B. The chart is prepared from research and available online data. Your diamondback terrapin might grow shorter and larger than this range. In such cases, you need not worry if your turtle is healthy, active, and eating well. However, consulting a vet is always appreciable.

How Big Is The Largest Diamondback Terrapin?

The largest diamondback terrapin is reportedly 9.1 inches long. However, there is news of terrapins growing 9.4 inches. According to many sources, the captive diamondback terrapin can develop a carapace up to 11 inches.

Usually, the female terrapins are the ones to achieve these record-breaking growths. The males grow up to 6.5 inches and not more than that.

Growth Rate Of Diamondback Terrapins

The medium-sized diamondback terrapins have an excellent growth rate. Hatchlings are born 1 – 1.5 inches long and weigh only a few grams. At the end of the first year, these babies grow 2.5 inches, and in some cases, they might develop a carapace length of 3 – 4 inches.

Diamond terrapin babies turn 4 – 5 inches within 4 to 5 years. So don’t they have fast growth?

The male terrapins gain mature size at the age of 3 to 4 years. These turtles weigh about 300 grams or more when they are fully grown.

On the other hand, the female diamondback terrapins reach the breeding age really slow. These turtles become sexually mature at the age of 5 – 7 years. The age range can be 8 – 9 years for the Northern diamondback terrapins.

An adult female terrapin grows up to 11 inches and weighs around 500 to 750 grams. The largest terrapin can gain a weight of more than 1 kg.

As you can see, the male diamondback terrapins experience faster growth than the females. It is because adult males have to focus on their somatic growth only.

On the contrary, concentrating on the somatic and reproductive growth might be why the females gain the full carapace size late.

Do Female Diamondback Terrapins Grow Larger?

Diamondback terrapins exhibit extreme sex dimorphism. The first that would help you sex your terrapin is its size. Generally, the healthy adult female diamondback terrapin is larger than the male.

Usually, a male diamondback terrapin is 4 – 6.5 inches big, while the female grows up to 11 inches. So you might not notice any apparent size difference between a male and female terrapin at an early age. But in the long run, the female diamondback terrapin will surely outrun the male in the carapace length.

However, size alone is not enough to sex a diamondback terrapin. Other characteristics are also crucial to separate the genders.

Take the tail and head size of these turtles as examples.

The male diamondback terrapin has long tails. Their heads are torpedo-shaped, which means they are long and narrow. On the contrary, the female terrapins have shorter tails and large or boxy heads.

What Influences The Growth Of A Diamondback Terrapin?

Diamondback terrapins have a generous growth. They can reach the carapace length of up to 11 inches, depending on their gender, subspecies, and several other factors.

Have you ever thought about why turtles of the same age experience different growth? Well, those less popular factors might be the answers. Let’s discuss those growth parameters of diamondback terrapins in brief,


Diamondback terrapin has 7 subspecies, each with different characteristics. Most of these subspecies have an average growth rate of 4 – 6.5 inches for males and 6 – 9 inches for females. However, a few subspecies might have a slightly different growth pace than the others, for example, Carolina diamondback terrapin and Mississippi diamondback terrapin.


Diamondback terrapins exhibit extreme sex dimorphism. The female terrapins grow larger than the males.

Geographical Location

A study indicates that the geographical location of the turtle might decide the size of a diamondback terrapin. However, the native habitat only influences the growth of the males, not the females.


Diamondback terrapins are supposed to grow at least 4 to 7 inches. Some turtles might not reach the carapace length due to their genes. A shorter growth due to genetics is totally okay if the pet is doing fine.

Care Level

A captive diamondback terrapin is totally dependent on its owner. Hence, if the keeper lacks care, the pet might now reach the desired carapace length.

The terrapin requires a fully furnished tank, a scheduled balanced diet, and emergency medical attention for healthy growth. If the owner neglects these things, the pet turtle will suffer. You can skim through this article to understand the basic needs of a diamondback terrapin.

My Diamondback Terrapin Is Not Growing: Why?

It is a common issue for turtles not growing as expected. Worry nothing as long as the pet is eating well and performing all the daily activities. But if the turtle avoids the tasks and shows fatigue, something is off.

Here are the probable reasons why your diamondback terrapin might not be growing,

1. Your Tank Arrangement Is Not Up To Mark

The diamondback terrapins are medium-sized. But you can not expect them to adjust in a small tank. Though the babies can live in a 10 – 25 gallon aquarium, the young or adult turtles require more space.

A 75-gallon tank is ideal for male terrapins and 125 gallons for females. You can accommodate two diamondback terrapins in a 150 – 175 gallon tank.

A bare aquarium filled with water is insufficient to grow the aquatic terrapins. You have to set up additional supplies. Such as,

If you fail to provide any of these materials, it will affect the healthy growth of the terrapins.

2. You Are Feeding The Turtles Garbage

You know the food plays a significant role in the growth of any animal, including turtles. So, if you do not feed the terrapins well, they will not flourish with life. In addition, the lack of nutrition might lead to unhealthy growth and other health issues.

Diamondback terrapins are primarily carnivorous and eat a small portion of vegetation to balance the diet. Their meals include crabs, mollusks, fishes, snails, worms, insects, leafy vegetables, seaweeds, etc.

Pellets make up 40 – 50% of the diet of the captive diamondback terrapins. Besides these items, supplements are a must to ensure the healthy growth of these turtles.

3. Re-check With The Basking Arrangements

Aquatic turtles also bask from time to time. If you do not let your turtles bask, they will lack UV rays. These exposures are essential sources for vitamin D and calcium production. Eventually, the pet will suffer from MBD and growth issues.

Install a spacious basking dock with a rampage inside the habitat for your diamondback terrapins. Also, set up a high-quality UV and heating light over the station. Keep the lamps on for 12 hours a day.

4. Something Is Fishy With The Water

Diamondback terrapins have specific requirements when it comes to water quality. They prefer brackish and salty water with a pH range from 6.8 – 7. Also, the water should be free from chlorine and other impurities.

Dirty water stresses out the terrapins and hinders their growth. Hence, you should really focus on maintaining the water quality.

5. Disease! Be Alert!

Diseases are more severe reasons why the diamondback terrapin is not growing. Generally, lethargy, runny nose, puffy eyes, abnormal movements, frequent basking, sneezing, coughing, etc., are the signs that your turtle is sick. Again, any untreated or prolonged illness can impact the growth of the terrapin.

Hence, it would be best to take your diamondback terrapins for a regular health check-up. Besides, seek instant medical help if you notice any abnormal behavior in the turtle.

You can solve any of these above issues as soon as they arise. However, sometimes, no trick will work. In those cases, genetics might be playing the master role behind the abnormal growth of your diamondback terrapin.


Diamondback terrapins grow up to 11 inches. If you look at the size distribution chart, the males are gathered around 5.5 inches, and females are in between 7 – 7.5 inches. A proper care sheet can help raise your terrapin to the desired carapace length.

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About Author

Muntaseer Rahman started keeping pet turtles back in 2013. He also owns the largest Turtle & Tortoise Facebook community in Bangladesh. These days he is mostly active on Facebook.


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