It's time to get fit fast.

Short workouts -- think 15 or 30 minutes tops -- are popping up at gyms all across the city, geared towards the health conscious who don't have a lot of time on their hands.

Of course, there's nothing new about speedy workouts -- remember "8 Minute Abs"? But today's fitness classes are based on the latest in exercise science, incorporating high-intensity interval training that pushes your body to the max while keeping your heart-rate up for an efficient, effective workout.

If you're short on time but want big results, here's where to work out in 30 minutes or fewer.

If you have ... 15 minutes

NYSC

Just last week, the sports club launched its FIIT15 Series at one of its midtown locations. The series is geared towards the work crowd looking to get fit on their lunch break and is comprised of three different 15-minute workouts that incorporate HIIT cardio and target either the lower body, core or upper body while using NuBells weights. Gym goers can take one, two or all three circuits, depending on how much time they have. Thursdays at noon, FREE for members and nonmembers; 575 Lexington Ave., 212-317-9400, register at mysportsclubs.com.

Equinox

The boutique fitness chain offers a range of group fitness classes, from yoga and Pilates to martial arts, dance and athletic training. Those looking to tone their abs specifically should head to the Ab Lab, an intense 15-minute class that utilizes abdominal and lower back exercises to improve your balance, posture and core strength. Membership required; multiple locations, equinox.com.

If you have ...30 minutes

Crunch

Belly, butt, thighs, abs -- whatever your focus area, you can find a class that targets it in 30 minutes at Crunch. In Absolution, for instance, you'll work to strengthen your core, while the Belly, Butt and Thighs Bootcamp is all about your lower half. Or work your entire body in Overdrive, a HIIT class that incorporates tabata, sprint training and more. Open to members and first-timers with a daypass; sign up at crunch.com.

DavidBartonGym

This upscale fitness center offers a variety of efficient and effective 30-minute classes. But its most popular is its Guns class. The group class targets your biceps and triceps through a variety of muscle-toning, strength-building exercises. Membership required; offered Mondays at 7:30 p.m. at Limelight, 20th and Sixth Avenue, and Tuesdays at 6:30 p.m. at Astor Place, 4 Astor Place, 877-579-1300, learn more at davidbartongym.com.

Evolvefitness NYC

This fitness studio is all about its 30-minute full-body workout, which is developed to build strength in your core, lower body and upper body, increase muscle and cardiovascular endurance and burn fat through non-stop exercises. Its signature class is offered throughout the week and is geared towards small groups, upwards of 8 clients per trainer. $30/class; 109 E. 50th St., 316 E. 53rd St., 917-302-0319, sign up at evolvefitnessnyc.com.