SLI Measurement Window Calculator

Work out sli measurement window instantly with clear inputs, formula shown and shareable results.

About SLI Measurement Window Calculator

An SLI window must contain enough failure events for the measured ratio to mean anything. At 150 requests per second and a 0.1 percent target failure rate, only 0.15 failures occur per second, so a five-minute window sees about 45 — workable, but halve the traffic and the same window becomes noise. Low-traffic services need long windows or count-based rather than ratio-based SLIs.

Formula

Window sizing

expected events = rate x failure rate x window seconds; minimum window = events needed / (rate x failure rate)

Worked example

With the default values below, SLI Measurement Window Calculator returns Minimum usable measurement window of 2.22. Change any input above and every figure here is recalculated in your browser.

Inputs used

Request rateminimum 0.001 req/s: 150 req/s
Failure rate the SLI must resolveaccepts 0–100 %: 0.1 %
Failure events needed for confidenceminimum 1: 20
Proposed alert windowminimum 0.1 min: 5 min

Results

Minimum usable measurement window: 2.22 min
Expected failure events in the proposed window: 45.00
Requests observed in the window: 45,000
Assessment: Window is long enough to measure this failure rate reliably

Step by step

1Values used
2Window sizing
3Minimum usable measurement window= 2.22 min
4Expected failure events in the proposed window= 45.00
5Requests observed in the window= 45,000
6Assessment= Window is long enough to measure this failure rate reliably

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 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

What about services with very low traffic?

Use longer windows, aggregate across similar endpoints, or switch to synthetic probes that generate a known, steady request rate.

Why do multi-window alerts exist?

A short window catches fast burns quickly while a long window suppresses noise. Combining both with different burn-rate thresholds gives fast detection without constant false pages.