Spanx Founder Sara Blakely, 43, stands atop a three-story construction site in Atlanta's uptown Buckhead district, surveying the city's skyline and blinking in the hot afternoon sun. By February, this will be the view from the company's rooftop garden, and the cavernous floors below—comprising 86,000 square feet—will be the site of Spanx's new offices, selected to accommodate Blakely's staff of 160. Men in hard hats and yellow vests scale the scaffolding, consult blueprints, maneuver cranes. She turns and smiles. "All this because I didn't like how my butt looked in white pants," she says.

Blakely's success store began in 1998, when, as a door-to-door fax-machine salesman fed up with bunching pantyhose, she created a proprietary slimming version designed to lie smoothly under pants. Oprah hailed it as one of her favorite products of the year in 2000, causing demand to explode among middle-aged housewives and red-carpet starlets alike, and leading to the creation of a full line that now encompasses more than 200 garments, from body shapers and bras to menswear and, beginning this fall, jeans.

Though she leads a company valued at more than a billion dollars, Blakely still primarily identifies as an inventor. Since she does her best thinking in the car (the idea to call her product Spanx came to her while she was stopped at a red light), she often extends her normal six-minute morning commute to an hour, driving aimlessly to allow her mind to wander. Wherever she goes, she carries a large spiral-bound Mead notebook, compulsively filling it with scribbles, notes and inspirations.

The mother of three has a salesman's facility with first names and an almost clinical ease about assessing other women's bodies, fixing an unabashed stare on the behind of a model as she walks by. "I look at butts all day," she says. "That's normal. I don't get in trouble with HR." The company channeled its founder's easygoing relatability, leading customers to develop an emotional attachment to the product and eliminating the need for advertising. Blakely estimates that roughly three times a day a woman on the street will recognize her and flash her Spanx, a gesture that has become a kind of secret handshake among the brand's adherents. It's a testament to the fact that in America, a little winking vanity can go a long way.

BLAKELY BY THE NUMBERS

11 questions asked by her son's prekindergarten classmates before they correctly guessed her identity (she was wearing a disguise).

$250 to trademark the word Spanx in 2000. She changed the ks to an x at the last minute.

200 people waiting in line at 10 a.m. for the opening of the stand-alone Spanx store (of which there are eight nationally) in Tampa, FL, this summer.

4 days Time Blakely's husband, the rapper and entrepreneur Jesse Itzler, spent in a wheelchair after running more than 100 consecutive miles for charity.

93 Age of Blakely's grandmother, who still wears Spanx and is especially a fan of the "Bra-llelujah."

60 countries where Spanx are sold worldwide, including China, Italy and United Arab Emirates.

8 minutes The typical length of Blakely's stand-up routine when she was an aspiring comedian before she started Spanx.

$24 million Amount the Sara Blakely Foundation has donated to support education and female entrepreneurship.

2 ideas for her next inventions: lipstick that stays on and comfortable high-heeled shoes.