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WAF Rule Impact Calculator

Work out waf rule impact instantly with clear inputs, formula shown and shareable results.

About WAF Rule Impact Calculator

A WAF rule is judged on both sides of its confusion matrix at production scale. At 40 million requests a day, blocking 0.8 percent with a 4 percent false-positive rate silently rejects over 12,000 legitimate requests daily — which is why new rules should run in count-only mode first. The latency term matters too, since it applies to every request, not just the blocked ones.

Formula

WAF rule impact

blocked = requests x block rate; false blocks = blocked x false positive rate; aggregate latency = requests x per-request latency

Worked example

With the default values below, WAF Rule Impact Calculator returns Legitimate requests blocked per day of 12,800. Change any input above and every figure here is recalculated in your browser.

Inputs used

Requests per dayminimum 1
40,000,000
Requests blocked by the ruleaccepts 0–100 %
0.8 %
Share of blocks that are legitimateaccepts 0–100 %
4 %
Added latency per requestminimum 0 ms
1.2 ms

Results

Legitimate requests blocked per day
12,800
Total requests blocked per day
320,000
Genuine attacks blocked per day
307,200
Aggregate latency added per day
13.33 h

Step by step

  1. 1Values used
  2. 2WAF rule impact
  3. 3Legitimate requests blocked per day= 12,800
  4. 4Total requests blocked per day= 320,000
  5. 5Genuine attacks blocked per day= 307,200
  6. 6Aggregate latency added per day= 13.33 h

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

Why deploy rules in count-only mode first?

It reveals what would have been blocked without breaking anything. Managed rule sets frequently match legitimate traffic in ways that are impossible to predict from the rule text.

What is the usual cause of WAF false positives?

Legitimate payloads containing SQL or script-like fragments — rich text editors, code snippets, search queries and base64 blobs are the classic offenders.

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