NTP Drift Calculator

Convert clock drift in ppm into error per poll interval, per day and across a holdover window.

About NTP Drift Calculator

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

Worked example

With the default values below, NTP Drift Calculator returns Drift Per Poll Interval of 1.280. Change any input above and every figure here is recalculated in your browser.

Inputs used

Oscillator driftaccepts 0.01–500 ppm: 20 ppm
NTP poll intervalminimum 1 s: 64 s
Target accuracyminimum 0.001 ms: 10 ms
Holdover window without syncminimum 0.1 h: 24 h

Results

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

Step by step

1Values used
2NTP Drift
3Poll interval for an accuracy target
4Drift Per Poll Interval= 1.280 ms
5Maximum Poll Interval For Target= 500 s
6Drift Per Day= 1.728 s
7Drift Over Holdover Window= 1.728 s
8Poll Interval Verdict= Within target — poll interval is fine

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

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.