Running Interval Calculator
Interval session fast miles, clock time, and weekly volume.
What it does
Intervals pack speed into small doses. Short fast repeats with jog rest build pace faster than steady miles alone.
The tool above totals the session. Six quarters at 8 min pace with 90 s rest runs 1.5 mi fast inside 19 min 30 s, or 4.5 mi a week over 3 days. The metric match is 6 x 400 m at 5 min per km for 2.4 km in the same time.
Pace sets the rep clock. Eight minute milers run each quarter in 2 minutes flat, while 5 min per km runners run each 400 in the same 2 minutes.
Rest shapes the workout. Ninety seconds suits quarters, longer suits mile repeats, and walking early reps beats blowing up late.
Weekly volume guards progress. Fast miles near a tenth of the week suit most runners, with days easy between hard ones.
How to use
- Pick imperial (mi) or metric (m, km).
- Enter reps and repeat: 6 x 0.25 suits the default.
- Enter goal pace: 8 min suits the default.
- Enter rest seconds: 90 suits the default.
- Enter days per week.
- Read fast volume, session time, weekly.
Formula
- reps
- Repeat count.
- repeat
- Repeat length.
- pace
- Goal pace per mile or km.
- rest
- Jog seconds between.
- days
- Sessions per week.
Pace is minutes per mile imperial and per km metric. Rest counts between reps only, so the last rep ends the clock.
Worked example
Example: 6 x quarter mile at 8 min pace
Imperial defaults: 6 x 0.25 mi at 8 min pace with 90 s rest, 3 days a week. These are also the default values, so the page opens on this answer.
- Repeats: 6 x 0.25 mi.
- Fast: 6 x 0.25 = 1.5 mi.
- Pace: 8 min per mile, so each rep is 480 x 0.25 = 120 s.
- Work: 6 x 120 = 720 s. Rest: 5 x 90 = 450 s.
- Session: 720 + 450 = 1170 s, or 19 min 30 s.
- Week: 1.5 x 3 = 4.5 mi. Tool shows 1.5 mi fast, 19 min 30 s, 4.5 mi per week.
Reference table
| Workout | Fast | Session |
|---|---|---|
| 6 x 0.25 mi, 90 s | 1.5 mi | 19 min 30 s |
| 4 x 0.5 mi, 120 s | 2 mi | 22 min 00 s |
| 8 x 400 m, 60 s | 3.2 km | 23 min 00 s |
| 10 x 400 m, 90 s | 4 km | 33 min 30 s |
Limits
- Time math only, not coaching.
- Heat, hills, and altitude slow pace.
- Fast miles need base first.
- Pain needs care, not splits.
Common mistakes
- First rep fastest, last rep slowest.
- Rest too short to hold pace.
- Doubling volume and pace together.
- Racing workouts weekly.
- Skipping warmup and strides.
Try it yourself
Quick checks. Answers sit tucked under each question.
4 x 0.5 mi at 8 min pace, 120 s rest?
4 x 0.5 = 2 mi fast. Each rep is 480 x 0.5 = 240 s. Work is 960 s, rest is 3 x 120 = 360 s. Session is 1320 s, or 22 min 00 s.
8 x 400 m at 5 min per km, 60 s rest metric?
8 x 400 m = 3.2 km fast. Each rep is 300 x 0.4 = 120 s. Work is 960 s, rest is 7 x 60 = 420 s. Session is 1380 s, or 23 min 00 s.
Questions
Fast volume?
Multiply reps by repeat length. Here 6 x 0.25 = 1.5 mi fast.
Session time?
Add work plus rests between. Here 720 + 450 = 1170 s, or 19 min 30 s.
Weekly volume?
Multiply session fast by days. Here 1.5 x 3 = 4.5 mi per week.
How much intensity?
Near 5 to 10 % of weekly miles per session for most runners. More needs coaching and base.
Adjust for hills?
Yes. Hills, heat, and altitude slow goal pace; run effort, not the watch.
Does this clear training?
No. Chest pain and joint injury need care, not splits.