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TOTP Time Drift Calculator

Work out totp time drift instantly with clear inputs, formula shown and shareable results.

About TOTP Time Drift Calculator

TOTP derives the code from floor(unix time / step), so any clock difference larger than the step produces a different code. Servers therefore accept a window of adjacent steps, typically one either side, which tolerates 30 seconds of drift in each direction. Every extra step of tolerance triples or expands the number of simultaneously valid codes, weakening the factor proportionally.

Formula

TOTP validation window

counter = floor(unix time / step); tolerated drift = window x step; codes valid at once = 2 x window + 1

Worked example

With the default values below, TOTP Time Drift Calculator returns Drift measured in time steps of 1.50. Change any input above and every figure here is recalculated in your browser.

Inputs used

Client clock driftaccepts -600–600 s
45 s
TOTP time stepaccepts 10–300 s
30 s
Acceptance window either sideaccepts 0–10 steps
1 steps

Results

Drift measured in time steps
1.50 steps
Drift the server tolerates
30 s
Codes valid at any instant
3
Validation outcome
Code will be rejected - client clock must be synchronised

Step by step

  1. 1Values used
  2. 2TOTP validation window
  3. 3Drift measured in time steps= 1.50 steps
  4. 4Drift the server tolerates= 30 s
  5. 5Codes valid at any instant= 3
  6. 6Validation outcome= Code will be rejected - client clock must be synchronised

Assumptions and limits

  • The result is derived only from the 3 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 2 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

Why not use a large acceptance window?

Each additional step multiplies the number of valid codes, increasing both guessing probability and the replay window. One step either side is the usual compromise.

How is drift handled properly?

Run NTP on servers and store a per-user drift correction learned from successful authentications, rather than widening the window for everyone.

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TOTP Time Drift Calculator | Calcrivo