Numbers have always defined Richard Semmler.

A 59-year-old mathematician, he teaches calculus and algebra at Northern Virginia Community College.

He can explain how to find the derivative of a polynomial and why a{+-}{+m} always equals 1/(a{+m}).

But in his private life, Semmler has reduced his existence to the simplest equation.

In the last 35 years, by working part-time jobs and forgoing such everyday comforts as a home telephone and vacations, by living in an efficiency apartment and driving an old car, Semmler has donated as much as half of his annual income or more to charity.

His goal: $1 million before he retires.

"If I didn't do all of the things I was doing, I would probably have a new car every two years and I would have a huge house with a huge pool," Semmler said this week as he took a break from pounding nails on a Habitat for Humanity house in Vienna. He donated $100,000 to this house, most of the money required to build it.

He stared determinedly up at the half-finished house, his T-shirt streaked with sweat and sawdust.

"But I would not do it that way," he said. "I want to do it this way."

Percentage-wise, Semmler's generosity is exceedingly rare among the middle-class -- or the rich, for that matter, say those who study philanthropy. Each year, U.S. households give away an average of 2 percent of their income to nonprofit and religious organizations, according to Giving USA, which tracks donation trends.

A household with Semmler's annual income, $100,000, donates an average of $2,000 annually to charity. Last year, Semmler gave away $60,000.

One beneficiary of his largesse: his employer. Since joining NVCC in 1974, Semmler has given $355,000 to fund scholarships as well as the school's distance-learning program, where he often works.

"He's headed toward half a million," said John J. Ruffino, executive director of the NVCC Educational Foundation, which puts Semmler in the top tier of its private donors.

"He's a great example not only for this college but for anybody," Ruffino said.

A list of Semmler's charitable contributions, which he prints by hand in neat columns, shows that he has also donated $200,000 to his alma mater, Plattsburgh State University of New York.

Other contributions over his lifetime include $140,000 to the Northern Virginia chapter of Habitat for Humanity; $30,000 to Ernest Angley Ministries, an evangelical Christian group, at $100 a month since 1979; $22,880 to Central Union Mission, a D.C. homeless shelter and soup kitchen, covering 10,400 meals; and $2,400 to the evangelical Christian group Crystal Cathedral Ministries.

By the end of this year, by his calculations, the total will be slightly more than $770,000.

Those are the numbers expected from the scion of a wealthy family, not the son of a Rochester electrician and a secretary who couldn't afford to send their son to college. Semmler, a top track athlete in high school, attended Plattsburgh State on scholarships.

In gratitude, he made his first contribution -- $25 -- to the school after graduating in 1968.

"That's the snowball that started rolling," Semmler recalled with a rich chuckle. "As it did, it got bigger and bigger and bigger."

Three decades later, he resembles a youthful, beardless Santa Claus -- with his rolling laugh, bushy white hair, rosy face and slight paunch. Those who work with him say he is always smiling.

He has never married, and he lives alone in a tiny condo in a faded apartment building on Scott Circle NW in the District. It is strewn with papers from the math textbooks he writes and edits to help fund his charitable ventures.

One day last week, he was in his office on the NVCC Annandale campus by 6 a.m. to correct papers before heading over to the Habitat site in the Briarwood Trace community in Vienna.

After three hours of sawing and hammering in the 90-degree heat, he stopped for a milkshake at Wendy's and then headed to his office again for more paperwork. He would go from there to a condo meeting and expected to be home by midnight.

This semester, he is teaching his classes via e-mail, videos and mail through NVCC's Extended Learning Institute to give him more time for his Habitat project.

On the phone at his cluttered desk amid a jumble of spider plants, he lobbied NVCC administration officials to offer scholarships to the children of the four households that will occupy the Habitat houses being built in Briarwood Trace.

"Do I need to stop by and see you on that?" he said cheerfully but firmly.

Acquaintances steered him to Habitat, to which he has been a contributor since the mid-1990s, and to the Central Union Mission, where he has been active since 1999. His religious pursuits sent him to the Christian groups.

But Semmler's approach isn't checkbook philanthropy. He stays involved with his money.

"Most of my dollars go to very specific projects, so I know what I'm funding," Semmler said. "I want to see my dollars at work."

Once a week and on holidays, he serves dinner at the Central Union Mission, and he offers math tutoring to clients seeking their high school equivalency diploma. His NVCC and Plattsburgh contributions go to scholarships and to specific programs at the schools. He serves on the board of the Habitat chapter in Northern Virginia and always assists in building the houses he helps fund.

"He puts his hands where his money goes," said Steve Greene, manager of volunteers for Habitat.

"He's one of a kind," said Don Thompson, who will own the Vienna house. "He doesn't just talk a good game. He's out there in the trenches."

By standard calculations, the personal price for Semmler has been high. Leisure activities are few. Aside from textbook work, he is also a part-time maintenance man at the Annandale building where he owns a condo that he rents out.

But even with extra work, he expects he'll have to sell his D.C. condo shortly to meet some of his charitable commitments.

Life isn't always about multiplying what you get, he explained. Sometimes, it's about subtraction.

Richard Semmler lives sparingly in order to donate. In 2004, he made $100,000 and gave away $60,000.Richard Semmler, center, offers muscle as well as funds in contributing to the Habitat for Humanity mission.