Error Rate Calculator
Calculate error rate and success rate from request counts, and see the equivalent reliability in “nines”.
Formula last reviewed 4 August 2026 · How we verify our calculators
Error rate
- Success rate
- 99.5%
- Failed requests
- 250
- Roughly equivalent to
- ≈ 99.500% (between table bands)
Updates live as you type
Frequently asked questions
Error rate = failed requests ÷ total requests × 100. 250 failures out of 50,000 requests is a 0.5% error rate, or 99.5% success.
It is shorthand for an availability percentage: “three nines” is 99.9%, “four nines” is 99.99%. Each extra nine cuts allowed failure roughly tenfold.
A request-success rate is the most common availability SLI. Measured over your SLO window and compared to the target, this same number becomes your SLI.
Typically HTTP 5xx (and sometimes specific 4xx) count as failures, while 2xx/3xx count as successes. Define the exact set for your service so the rate is meaningful.
Your error budget is the total failures allowed by your SLO; the error rate tells you how quickly you’re spending it. A sustained error rate above your budget pace will exhaust it early.
99.5% isn't "basically three nines" — it's stated as what it is
50,000 total requests, 250 failed: error rate is 250 ÷ 50,000 × 100 = 0.5%, success rate the complement, 99.5%. Placed on the nines scale, 99.5% sits between "two nines" (99%) and "three nines" (99.9%) rather than matching either — and the calculator states that plainly instead of rounding to the nearer band and misrepresenting actual reliability. Halve the failures to 125 on the same 50,000 requests, and the error rate drops to 0.25% — success rate 99.75%, now genuinely closer to three-nines territory.
The "nines" context matters because reliability is usually discussed in those terms. The calculator maps your success rate onto the standard table — 99% (two nines), 99.9% (three nines), 99.99% (four nines), and so on — so you can immediately place your result on the scale teams actually talk in. When your number sits between table bands, it says so rather than rounding misleadingly.
Be deliberate about what counts as a failure: the convention is that HTTP 2xx and 3xx are successful while 5xx (and sometimes a chosen subset of 4xx) are failures. The rate is only meaningful once that definition is fixed for your service.
Error rate is the real-time companion to the error budget: the budget is the *total* failure your SLO allows, while the error rate is the *pace* at which you're using it. A steady error rate higher than your budget's pace means you'll run out early — which is exactly what the burn-rate calculator quantifies.