NTP Drift Calculator
Convert clock drift in ppm into error per poll interval, per day and across a holdover window.
Inputs
Drift Per Poll Interval
1.280ms
Maximum Poll Interval For Target
500s
Drift Per Day
1.728s
Drift Over Holdover Window
1.728s
Poll Interval Verdict
Within target — poll interval is fine
Step by step
Values used
Oscillator drift = 20 ppm; NTP poll interval = 64 s; Target accuracy = 10 ms; Holdover window without sync = 24 h
NTP Drift
drift per interval = ppm × interval ÷ 10^6, so drift in ms = ppm × interval in seconds ÷ 1000.
Poll interval for an accuracy target
maximum poll interval = target accuracy ÷ drift rate
Drift Per Poll Interval
= 1.280 ms
Maximum Poll Interval For Target
= 500 s
Drift Per Day
= 1.728 s
Drift Over Holdover Window
= 1.728 s
Poll Interval Verdict
= Within target — poll interval is fine
How it works
A drift figure in ppm is a fraction: 20 ppm means the clock gains or loses 20 microseconds every second. Multiplying by the poll interval gives the error accumulated between corrections, and by the holdover window gives the error if sync is lost entirely. Log correlation, Kerberos authentication and certificate validation all break on clock skew, and a cheap oscillator at 50 ppm drifts more than four seconds a day once upstream sync is gone.
Formulas
NTP Drift
drift per interval = ppm × interval ÷ 10^6, so drift in ms = ppm × interval in seconds ÷ 1000.
- ppm
- Parts per million of frequency error
- interval
- Seconds between NTP polls
- 10^6
- Definition of parts per million
Poll interval for an accuracy target
maximum poll interval = target accuracy ÷ drift rate
- target accuracy
- Largest offset you will tolerate
- drift rate
- ppm expressed as a fraction
Frequently Asked Questions
How is NTP Drift calculated?
drift per interval = ppm × interval ÷ 10^6, so drift in ms = ppm × interval in seconds ÷ 1000. A drift figure in ppm is a fraction: 20 ppm means the clock gains or loses 20 microseconds every second. Multiplying by the poll interval gives the error accumulated between corrections, and by the holdover window gives the error if sync is lost entirely.
Why does NTP Drift matter?
Log correlation, Kerberos authentication and certificate validation all break on clock skew, and a cheap oscillator at 50 ppm drifts more than four seconds a day once upstream sync is gone.
What values do I need to enter?
This calculator takes 4 inputs: Oscillator drift, NTP poll interval, Target accuracy, Holdover window without sync. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
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