Circuit Breaker Threshold Calculator

Determine circuit breaker error and latency thresholds to protect downstream services.

About Circuit Breaker Threshold Calculator

A circuit breaker opens once the observed error rate in its evaluation window exceeds a configured threshold, but only after a minimum request volume has been observed to avoid tripping on statistically insignificant samples: opens_when error_rate > threshold% AND volume ≥ min_volume. Once open, the breaker rejects requests immediately (failing fast) until the half-open timeout elapses, at which point it allows a limited number of trial requests through to test whether the downstream dependency has recovered.

Formula

trips_when = error_count >= threshold within rolling_window_sec

error_count: Number of consecutive failures or errors
threshold: Failure count that triggers the circuit breaker
rolling_window_sec: Time window for counting failures (seconds)

Worked example

With the default values below, Circuit Breaker Threshold Calculator returns Circuit Breaker State of Open. Change any input above and every figure here is recalculated in your browser.

Inputs used

Requests in Windowaccepts 1–10,000,000 requests: 200 requests
Failed Requests in Windowaccepts 0–10,000,000 requests: 45 requests
Error Rate Thresholdaccepts 0–100 %: 20 %
Minimum Request Volumeaccepts 0–10,000,000 requests: 20 requests
Half-Open Timeoutaccepts 1–100,000 seconds: 30 seconds

Results

Circuit Breaker State: Open
Observed Error Rate: 22.50%
Margin to Threshold: -2.50 pp
Half-Open Retry After: 0.50 minutes

Step by step

1Observed error rate: failed ÷ total= 22.50%
2Sufficient volume check= yes
3Breaker decision: error rate > threshold?= Open

Assumptions and limits

The result is derived only from the 5 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 is a minimum request volume required before evaluating the threshold?

With very few requests, a single failure can produce a misleadingly high error rate (e.g. 1 failure out of 3 requests = 33%) — requiring a minimum sample size before evaluating prevents the breaker from flapping open on statistical noise during low-traffic periods.

What happens during the half-open state?

The breaker allows a small number of trial requests through to the downstream dependency; if they succeed, the breaker closes and resumes normal traffic, if they fail, it reopens and restarts the half-open timeout, backing off from a struggling dependency.

What's a reasonable error threshold?

20-50% is common for general-purpose circuit breakers, tuned lower for critical dependencies where you want to fail fast sooner, and higher for naturally noisy or already-degraded-tolerant dependencies where occasional errors are expected and acceptable.

How does circuit breaking interact with retries?

Retries should generally be attempted before the circuit breaker's error count is affected by transient failures, or the breaker should count only post-retry failures — otherwise aggressive retrying can itself inflate the observed error rate and trip the breaker prematurely under partial degradation.