Most of us have moles somewhere on our body. And sometimes, one of those moles can turn cancerous. The more moles you have, the greater your risk for skin cancer.

Now scientists have found an easy way to guesstimate how many moles you have — and what your chances are of developing melanoma, the deadliest form of skin cancer.

The scientists looked at data from a United Kingdom study that tracked 3,594 female twins between January 1995 and December 2003, as well as other data. They discovered three things: First, the number of moles on the right arm was most predictive of the number of moles on the whole body. Second, having more than seven moles on your right arm means you are likely to have more than 50 moles over your whole body. (Having 50 or more moles increases the risk of melanoma, according to the Melanoma Research Foundation.) Third, having 11 moles on your right arm means you are likely to have more than 100 moles in total.

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Of course, having 7 or 11 or more moles on your right arm doesn't mean you'll get melanoma, only that your risk of developing it is higher. Todd Ridky, MD, PhD, an assistant professor of dermatology at Penn Medicine in Philadelphia, who was not involved in the study, told CBS News that while having more moles can put you at higher risk, “most moles fortunately don’t develop into cancer. There are plenty of moley people who have never a cancer, but your risk of developing a melanoma is greater.”

Melanoma is most common in men over age 50. It's the second most common cancer in teens and the most common type of cancer in young adults, according to the Melanoma Research Foundation, whose #GetNaked campaign encourages people to do skin self-checks.

Related: Beyond Sunscreen: Other Bright Ways to Protect Your Skin

Risk factors for melanoma include having fair skin, blonde or red hair and blue eyes (although dark skin and eyes does not protect you from it). Other risk factors include using a tanning bed, exposure to other sources of ultraviolet radiation, a family history of melanoma, sunburns at an early age, a weakened immune system and a previous skin cancer diagnosis.

According to the Melanoma Research Foundation, you can help ward off melanoma by protecting your skin from sun exposure, avoiding tanning salons, doing regular skin self-checks and seeing a dermatologist and ophthalmologist regularly. Signs of melanoma include a dark mole that is asymmetrical, more than one color, bigger than a pencil eraser or has irregular borders.

If you find any suspicious moles, see a doctor right away.

Related: 6 Powerful Ways to Reduce Your Skin Cancer Risk

Diana is an award-winning writer and editor with more than 20 years' experience in magazine, video, book and digital journalism, with a specialty in health coverage. She was a longtime writer and news editor at the Center for Investigative Reporting; has written for publications from the Washington Post to the Times of London syndicate; and has served as a senior and/or consulting editor at Time Inc. Health, Hippocrates, HealthDay News Service and Reporting on Health. She was also editor in chief of Consumer Health Interactive, a national health and medical web site, and has reported on finance for Blueshift Research and PBS Frontline. Before joining SafeBee, she was editor of Bioenergy Connection, a national magazine about bioenergy at UC Berkeley. Her favorite safety tip: Wear a bike helmet.