Apps like Flex Timer

Here are some great alternatives to Flex Timer by GymNext :
Sezzy Timer
Sezzy Timer
★3.7
Interval Timer: TRX, Tabata
Interval Timer: TRX, Tabata
★4.8
iCountTimer
iCountTimer
★4.3
iCountTimer Pro
iCountTimer Pro
★4.2
Interval Timer: Tabata Workout
Interval Timer: Tabata Workout
★4.9
Essential Interval Timer HIIT
Essential Interval Timer HIIT
★4.2
Interval Round Timer
Interval Round Timer
★4.4
SmartWOD Timer - WOD timer
SmartWOD Timer - WOD timer
★4.9
Boxing Interval Timer
Boxing Interval Timer
★5.0
Hero Timer: WOD Timer
Hero Timer: WOD Timer
★4.7
9 Timer: HIIT & Tabata Workout
9 Timer: HIIT & Tabata Workout
★4.3
Interval Timer: Custom Workout
Interval Timer: Custom Workout
★4.7
Tabata timer - HIIT Workout
Tabata timer - HIIT Workout
★4.4
Interval Timer: Tabata Timer
Interval Timer: Tabata Timer
★4.5
Custom Interval Timer: Workout
Custom Interval Timer: Workout
★4.4
Interval Timer: Tabata Timer
Interval Timer: Tabata Timer
★4.1