Danica Mae McKellar (born January 3, 1975)^[1] is an American actress, mathematics writer, and education advocate. She is best known for playing Winnie Cooper in the television series The Wonder Years.

McKellar has appeared in various television films for the Hallmark Channel. She has also done voice acting, including Frieda Goren in Static Shock, Miss Martian in Young Justice, and Killer Frost in DC Super Hero Girls. In 2015, McKellar joined part of the main cast in the Netflix original series Project Mc².

In addition to her acting work, McKellar later wrote seven non-fiction books, all dealing with mathematics: Math Doesn't Suck, Kiss My Math, Hot X: Algebra Exposed, Girls Get Curves: Geometry Takes Shape, which encourage middle-school and high-school girls to have confidence and succeed in mathematics, Goodnight, Numbers, and Do Not Open This Math Book.^[2][3][4]

McKellar was born in La Jolla, California.^[5] She moved with her family to Los Angeles when she was eight. Her mother Mahaila McKellar (née Tello) was a homemaker; her father Christopher McKellar is a real estate developer; her younger sister Crystal (b. 1976) is a lawyer.^[6] She is of paternal Scottish, French, German, Spanish, and Dutch descent and her mother is of Portuguese origin via the Azores and Madeira islands.^[7]

McKellar studied at the University of California, Los Angeles where she was a member of the Alpha Delta Pi sorority^[8] and earned a Bachelor of Science degree summa cum laude in Mathematics in 1998.^[9] As an undergraduate, she coauthored a scientific paper with Professor Lincoln Chayes and fellow student Brandy Winn titled "Percolation and Gibbs states multiplicity for ferromagnetic Ashkin–Teller models on ${\displaystyle \mathbb {Z} ^{2}}$."^[10] Their results are termed the "Chayes–McKellar–Winn theorem".^[11][12] Later, when Chayes was asked to comment about the mathematical abilities of his student coauthors, he was quoted in The New York Times, "I thought that the two were really, really first-rate."^[13] For her past collaborative work on research papers, McKellar is currently assigned the Erdős number four, and her Erdős–Bacon number is six.^[14]

McKellar has authored several mathematics-related books primarily targeting adolescent readers interested in succeeding at the study of mathematics:

• McKellar, Danica; Mary Lynn Blasutta (2008). Math Doesn't Suck: How to Survive Middle School Math without Losing Your Mind or Breaking a Nail. New York: Plume. ISBN 9780452289499.
• McKellar, Danica (2009). Kiss My Math: Showing Pre-Algebra Who's Boss. New York: Plume. ISBN 9780452295407.
• McKellar, Danica (2010). Hot X: Algebra Exposed. New York: Plume. ISBN 9780452297197.
• McKellar, Danica (2012). Girls Get Curves: Geometry Takes Shape. New York: Hudson Street Press. ISBN 9781594630941.
• McKellar, Danica (2018). Ten Magic Butterflies. New York: Random House Children's Books. ISBN 9781101933824.
• McKellar, Danica (2018). Do Not Open This Math Book. New York: Random House Children's Books. ISBN 9781101933985.
• McKellar, Danica (2019). Bathtime Mathtime. New York: Random House Children's Books. ISBN 9781101933961.
• McKellar, Danica (2020). The Times Machine. New York: Random House Children's Books. ISBN 9781101934029.
• McKellar, Danica (2022). Double Puppy Trouble. New York: Random House Children's Books. ISBN 9781101933862.
• McKellar, Danica (2022). Goodnight, Numbers. New York: Random House Children's Books. ISBN 9780593643556.

Her first book, Math Doesn't Suck: How to Survive Middle School Math without Losing Your Mind or Breaking a Nail, was a New York Times bestseller,^[2] and was favorably reviewed by Tara C. Smith, the founder of Iowa Citizens for Science and a professor of epidemiology at the University of Iowa.^[28] The book also received a review from Anthony Jones, writing for the School Librarian journal, who described the book as "a trouble-shooting guide to help girls overcome their biggest maths challenges," noting what he described as "real-world examples of great mathematics in action."^[29] In an interview with Smith, McKellar said that she wrote the book "to show girls that math is accessible and relevant, and even a little glamorous" and to counteract "damaging social messages telling young girls that math and science aren't for them".^[30]

McKellar's second book, Kiss My Math: Showing Pre-Algebra Who's Boss,^[31] was released on August 5, 2008. The book's target audience is girls in the 7th through 9th grades. Her third book, Hot X: Algebra Exposed!^[32] covers algebra topics, while the previous two titles were intended as "algebra-readiness books."^[33] Hot X was published on August 3, 2010. Her fourth book, Girls Get Curves – Geometry Takes Shape,^[34] focuses on the subject of geometry, and attempts to make the subject more accessible.^[35]

Three of McKellar's books were listed in The New York Times children's bestseller list.^[36][37] She received Mathical Honors for Goodnight, Numbers.^[38]

Chayes, L; McKellar, D; Winn, B (1998). "Percolation and Gibbs states multiplicity for ferromagnetic Ashkin–Teller models on ${\displaystyle \mathbb {Z} ^{2}}$" (PDF). Journal of Physics A: Mathematical and General. 31 (45): 9055–9063. Bibcode:1998JPhA...31.9055C. doi:10.1088/0305-4470/31/45/005.

McKellar was named Person of the Week on World News with Charles Gibson for the week ending August 10, 2007. The news segment highlighted her book Math Doesn't Suck and her efforts to help girls develop an interest in mathematics, especially during the middle school years.^[39] In January 2014, she received the Joint Policy Board for Mathematics (JPBM) Communications Award. The citation credited her books, blog, and public appearances for encouraging "countless middle and high school students, especially girls, to be more interested in mathematics."^[40]

McKellar married composer Mike Verta on March 22, 2009, in La Jolla, California; the couple had dated since 2001.^[41] They had their first child, a son, in 2010.^[42][43] McKellar filed for divorce from Verta in June 2012.^[44]

On July 16, 2014, she became engaged to her boyfriend Scott Sveslosky, a partner in the Los Angeles legal firm Sheppard, Mullin, Richter & Hampton. On November 15, 2014, they married in Kauai, Hawaii.^[45]

McKellar is a Christian and regularly attends church services.^[46] She cites Candace Cameron Bure as being a major influence in her life after Bure gave her a copy of the Bible.^[46]

McKellar's notoriety for Hallmark mystery films was spoofed in the 2019 film Knives Out, complete with the parody title Deadly By Surprise.^[47]