Cron expression for every 3 hours

Field breakdown

Next runs

How this schedule works

Cron evaluates the five fields of 0 */3 * * * — minute, hour, day of month, month, day of week — against the clock once a minute and starts the job on any minute where all five match, which cronstrue reads as "On the hour, every 3 hours": 8 runs per day. The runs land at 00:00, 03:00, 06:00, 09:00, 12:00, 15:00, 18:00, 21:00. Twenty-four divides by 3 without a remainder, so the spacing survives midnight: the gap from the 21:00 run to the next day's 00:00 run is the same 3 hours as every other gap.

When to use every 3 hours

Eight runs a day fit incremental backups, third-party token refreshes with a multi-hour lifetime, and syncs where a three-hour gap is acceptable but a full day is not. The interval divides the day evenly, so the spacing holds across midnight and every run gets the same headroom as the last. It is a common landing spot for jobs that are expensive enough that hourly would be wasteful, yet important enough that a daily run would leave too much backlog to work through in a single invocation.

Common mistakes with every 3 hours

Three divides 24, so this schedule is well behaved — which is exactly why people generalise from it and get burned. Copy the pattern to */5 in the hour field, assume the same clean rollover, and you end up with a four-hour gap between the 20:00 run and midnight. Check the divisor before trusting a step in the hour field. The other thing to watch is the assumption inside the job: code that computes "the last three hours" from the current clock will double-count when a run is late and lose data when one is skipped. Read from a stored watermark instead.

Platform snippets

The same expression, ready to paste into the four schedulers that read five-field cron. The notes below each snippet are the ones that follow from this schedule; the cron expression generator lists every caveat for every platform.

name: Scheduled workflow
on:
  schedule:
    - cron: '0 */3 * * *'
  workflow_dispatch:

jobs:
  scheduled:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Run job
        run: echo "Running scheduled job"

apiVersion: batch/v1
kind: CronJob
metadata:
  name: my-cron-job
spec:
  schedule: "0 */3 * * *"
  jobTemplate:
    spec:
      template:
        spec:
          containers:
            - name: job
              image: my-image:latest
              command: ["/bin/sh", "-c", "echo Running"]
          restartPolicy: OnFailure

{
  "$schema": "https://openapi.vercel.sh/vercel.json",
  "crons": [
    {
      "path": "/api/cron",
      "schedule": "0 */3 * * *"
    }
  ]
}

# Edit your crontab: crontab -e
0 */3 * * * /path/to/command

# With logging:
0 */3 * * * /path/to/command >> /var/log/cron.log 2>&1

Variations

• 0 */2 * * * On the hour, every 2 hours Twelve runs a day rather than eight, on even hours. Worth it when a three-hour backlog is more than a single run can comfortably work through.
• 0 */6 * * * On the hour, every 6 hours Four runs a day, carving the day into quarters. The step down when three-hourly runs keep finding very little to do.
• 0 0,3,6,9,12,15,18,21 * * * At 12:00 AM, 03:00 AM, 06:00 AM, 09:00 AM, 12:00 PM, 03:00 PM, 06:00 PM and 09:00 PM The same eight hours written out. At this length the step is easier to read, but the list is easier to edit when one hour needs to move.
• 0 */3 * * 1-5 On the hour, every 3 hours, Monday through Friday Three-hourly on weekdays only: 40 runs a week rather than 56. The Friday 21:00 run is then followed by a 51-hour gap.