What this interval timer does
This tool runs a work/rest cycle for as many rounds as you need, so you never have to glance at a phone or wall clock mid-set. Enter a work time, a rest time, and a round count; add an optional warm-up before round one and a cool-down after the last round. Press start and it counts down through every phase on its own, with a short tone marking each transition and a longer tone at the finish.
It is built for the training styles where the timing itself is part of the workout — Tabata-style HIIT, boxing or Muay Thai rounds, circuit stations, or any rest-based routine where getting the interval wrong changes the effect of the exercise.
How to use it
Set it once, then start — pause, resume or skip ahead any time mid-workout.
- Set work and rest.Enter how long each work interval lasts and how long you rest between them, in seconds. Set rest to 0 for continuous, back-to-back work.
- Set the rounds.Enter how many work intervals to complete. Rest only runs between rounds, not after the last one.
- Add a warm-up or cool-down.Both are optional — leave either at 0 to skip it, or give it a few seconds to ease in and out of the session.
- Press start.The countdown runs automatically. Use Pause to stop the clock, Skip to jump to the next phase, and Reset to start over.
How the total session length is calculated
The total is just each phase added together — but the rest count is one less than the round count, since there is nothing to rest for after the final work interval finishes.
160 secondsRest: (8 − 1) × 10 =
70 secondsWarm-up / cool-down:
0 secondsTotal = 160 + 70 = 230 seconds = 3:50
Why rest is skipped after the last round
Rest exists to recover before the next work interval. Once the final round finishes there is no next interval, so this timer moves straight from the last work phase into the cool-down (or finishes) instead of running an unnecessary rest that most people skip through anyway.
Countdown accuracy
Each phase's end time is calculated once from the browser's high-resolution clock, and the remaining time is recalculated from that fixed point rather than counted down one second at a time — so briefly switching tabs will not cause the display to drift. It cannot, however, keep running once a mobile browser fully suspends the page, which is typically when the screen locks.
Quick examples
A few common interval patterns and their total session length.
| Work | Rest | Rounds | Warm-up | Total |
|---|---|---|---|---|
| 20s | 10s | 8 | 0s | 3:50 |
| 30s | 15s | 10 | 30s | 7:45 |
| 3:00 | 1:00 | 12 | 0s | 47:00 |
| 40s | 20s | 6 | 60s | 7:40* |
| 45s | 15s | 20 | 0s | 19:45 |
*Includes a 60-second cool-down. Want the gap between a start and end time instead of a repeating interval? The time duration calculator handles that.
Who uses it, and for what
- HIIT and Tabata training — short, hard work intervals with a fixed rest, repeated for a set number of rounds.
- Boxing, Muay Thai and combat sports — a fixed round length with a corner-rest interval, run back to back.
- Circuit and station training — one timer covering every station, with the same work/rest applied throughout.
- Coaches and PE teachers — running a class or drill on a shared, hands-free countdown with audio cues.
- Rehab and stretching routines — hold/release patterns where the timing itself matters.
Timing a single stretch of focused work instead of a repeating cycle? The Pomodoro timer is built for that. Just need one plain countdown with no rounds, use the countdown timer, or the stopwatch to count up instead of down.
Frequently asked questions
What is an interval timer used for?
It runs repeating work and rest periods without you watching a clock or counting rounds — common for HIIT, Tabata, circuit training, boxing or Muay Thai rounds, and rest-based rehab exercises.
Does it add a rest period after the last round?
No. Rest only runs between rounds, so an 8-round session has 8 work intervals and 7 rest intervals, then moves straight to the cool-down or finishes. This avoids a wasted rest period nobody needs.
Can I add a warm-up and cool-down?
Yes, both are optional. Enter seconds in the warm-up or cool-down field and that phase is added before round one or after the final round. Leave either at 0 to skip it entirely.
Will the timer stay accurate if I switch tabs or lock my phone?
The countdown recalculates from the real elapsed time rather than just ticking a number down, so briefly switching tabs will not cause drift. However, most mobile browsers pause JavaScript entirely when the screen locks, so keep your screen on during a workout.
Is there sound on phase changes?
Yes, a short tone plays at the start of each phase and a longer tone plays when the workout finishes. Use the sound button to mute it. Some browsers require one tap on the page before audio is allowed to play.
Can I run continuous work with no rest?
Yes, set the rest field to 0 and the timer moves straight from one work interval to the next, which suits AMRAP-style training.
Is the interval timer free and private?
It is completely free and runs entirely in your browser. No account, no upload, nothing stored on a server.
Accuracy & sources
The total session length shown here is simple addition: every work, rest, warm-up and cool-down phase in seconds, summed exactly as shown in how the total is calculated. There is no proprietary formula, and every figure in the quick-examples table above has been independently verified by hand. The live countdown itself is driven by the browser's high-resolution timer rather than a naive one-second interval, so it stays accurate against real elapsed time rather than drifting.
This tool follows general interval-training conventions such as Tabata-style work/rest structuring, but is not tied to any single certifying body's protocol — treat the timer as a flexible stopwatch for whatever work/rest pattern your training plan calls for. Written and reviewed by Fletcher Langley, and re-checked whenever the tool changes — last reviewed August 2026.
About the author
Operations and workforce-systems specialist with 14+ years inside shift-based industries and payroll platforms, and a BS in Industrial Engineering. Fletcher writes and reviews the calculators, converters, and guides here — from overnight-shift math to decimal payroll hours — drawing on years spent translating between 12-hour, 24-hour, and military time for real timesheets.
Written & reviewed by Fletcher Langley · Last updated: August 2026