When Zach Korzyk spent a couple of days creating an online tool to teach his 11th-grade students to solve a specific math problem, he never imagined the website would expand far beyond his Manhattan classroom.

Five years later, his DeltaMath website has 600 different types of math problems – each with countless variations – and was used by more than 20,000 students in December 2014 alone.

“I feel kind of enthralled by the fact that what I make is used by so many students,” said Korzyk, 30, who has been teaching Algebra II and trigonometry at Manhattan Village Academy for eight years.

Teacher-created online tools have proliferated in recent years. But Korzyk’s site has caught on in the city, with about 1,600 city teachers using the tool and the total number of teacher accounts now approaching 2,700.

DeltaMath allows teachers to create free online accounts and assign their students automatically-graded, interactive problems from a long list of modules sorted by level – from middle school Common-Core-aligned math to AP Calculus. Teachers can assign as many problems as they want, pick a due date, determine how many consecutive correct answers are required to finish the assignment, view students’ scores, and see where they might have struggled.

And because each problem type generates hundreds of variations, students can’t copy each other, which Korzyk said was one of his “main motivations for creating the site.”

The site began to take off in September 2010, after Korzyk scrapped an early version of the website and built a more user-friendly interface that he started sharing with a network of fellow math teachers.

Now, the entire math department at the Urban Assembly School for Law & Justice in Brooklyn has incorporated DeltaMath into its curriculum. Chris Luzniak, a pre-calculus and college algebra teacher at the school, noted that the site allows students to know right away if they’re doing something wrong and gives struggling students additional practice — without teachers having to create and grade extra work.

“It just raises the students’ skill levels mostly across the board,” said Luzniak, who assigns his senior students two DeltaMath homework assignments per week and said he often uses it during class. “Struggling students, more than anyone, need that instant feedback.”

At Manhattan Village Academy, students vouched for the site on their own. Yahayra Colon, a junior, said DeltaMath boosted her confidence after she almost failed her geometry course last year.

“When I’m not sure of something, it shows a problem and it goes step-by-step with everything we need to do,” she said before heading home on a recent school day.

Korzyk said six of every 10 teachers using the site are based in New York City and 80 percent are from within the state. But there are also accounts originating in Salt Lake City, Washington, D.C., Boston, Los Angeles and Chicago, and a handful from Turkey and China.

“It does put pressure on me,” he said, noting that he’s been adding problem types to the site as teachers request new functions. “This year I’ve been looking at a lot of geometry and middle-school [content], which is not pertinent to what I teach, but it’s what teachers are asking for.”

And while Korzyk is always looking to improve the website — a mobile app is another dream — he knows his limitations as a one-man operation. Korzyk can spend up to 10 hours creating one new math problem, but he’s also not making any money from the website.

“It’s a constant internal conflict,” he said. “I really enjoy programming and creating content for DeltaMath, and I really enjoy teaching and interacting with students, so it’s not really a drag when it adds up to a lot of hours … But I think that other pathways that I could take the website would require that I quit my job.”