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Cloud Firewall Rule Calculator

Measure cloud firewall policy hygiene: quota headroom, shadowed and duplicate rules, any-any exposure and effective rule count per policy.

About Cloud Firewall Rule Calculator

Firewall policies are evaluated in priority order, so a rule that nothing has matched in ninety days is either dead or a latent hole, and a rule matching any source and any destination makes everything beneath it unreachable. The score penalises dead rules proportionally, any-any rules absolutely, and quota pressure only once you pass 80% — the point at which a routine change starts failing. Nobody deletes firewall rules, so policies grow until a change fails at 3am on the quota limit, and by then the any-any rule somebody added 'temporarily' during a migration has been the effective policy for two years.

Formulas

Cloud Firewall Rule

Hygiene = 100 − 2 × dead-rule percentage − 4 × any-any rule count − quota overshoot above 80%, clamped to 0–100. Dead rules = shadowed + duplicate.

deadRatio
Shadowed plus duplicate rules ÷ all rules × 100
anyAnyRules
Rules matching any source and any destination
quotaPenalty
Points lost once rule count passes 80% of the policy limit

Effective rule count

Effective rules = total rules − shadowed − duplicate: the count that actually decides whether a packet passes.

effectiveRules
Rules that can still match traffic
totalRules
Rules configured in the policy

Worked example

With the default values below, Cloud Firewall Rule Calculator returns Policy Hygiene Score of 45.8. Change any input above and every figure here is recalculated in your browser.

Inputs used

Rules in the policyaccepts 1–20,000 rules
320 rules
Rules per policy limitaccepts 1–20,000 rules
1,000 rules
Allow rulesaccepts 0–20,000 rules
240 rules
Deny rulesaccepts 0–20,000 rules
80 rules
Rules with any source and any destinationaccepts 0–20,000 rules
7 rules
Rules never matched in 90 daysaccepts 0–20,000 rules
19 rules
Duplicate or fully overlapping rulesaccepts 0–20,000 rules
23 rules
Free priority slots between rulesaccepts 0–20,000 slots
120 slots
Rule evaluations per dayaccepts 0–100,000,000,000 evaluations
42,000,000 evaluations

Results

Policy Hygiene Score
45.8 / 100
Rule Quota Used
32.0%
Rules Remaining
680 rules
Dead Rules
42 rules
Dead Rule Ratio
13.1%
Effective Rules
278 rules
Allow Rule Share
75.0%
Evaluations per Second
486.1 eval/s
Verdict
Remove the any-any rules first — every rule below them is decoration

Step by step

  1. 1Values used
  2. 2Cloud Firewall Rule
  3. 3Effective rule count
  4. 4Policy Hygiene Score= 45.8 / 100
  5. 5Rule Quota Used= 32.0
  6. 6Rules Remaining= 680 rules
  7. 7Dead Rules= 42 rules
  8. 8Dead Rule Ratio= 13.1
  9. 9Effective Rules= 278 rules

Assumptions and limits

  • The result is derived only from the 9 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 1 decimal place, 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

How is Cloud Firewall Rule calculated?

Hygiene = 100 − 2 × dead-rule percentage − 4 × any-any rule count − quota overshoot above 80%, clamped to 0–100. Dead rules = shadowed + duplicate. Firewall policies are evaluated in priority order, so a rule that nothing has matched in ninety days is either dead or a latent hole, and a rule matching any source and any destination makes everything beneath it unreachable. The score penalises dead rules proportionally, any-any rules absolutely, and quota pressure only once you pass 80% — the point at which a routine change starts failing.

Why does Cloud Firewall Rule matter?

Nobody deletes firewall rules, so policies grow until a change fails at 3am on the quota limit, and by then the any-any rule somebody added 'temporarily' during a migration has been the effective policy for two years.

What values do I need to enter?

This calculator takes 9 inputs: Rules in the policy, Rules per policy limit, Allow rules, Deny rules, Rules with any source and any destination, Rules never matched in 90 days, Duplicate or fully overlapping rules, Free priority slots between rules, Rule evaluations per day. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

What makes a rule shadowed?

A higher-priority rule already covers its entire match space, so it can never fire. Shadowed rules are dangerous during clean-up: deleting the higher-priority rule silently activates the shadowed one, and if that one is broader you have just opened a hole while doing hygiene work.

Why do priority slots matter?

Azure NSG priorities run 100 to 4096 and evaluation is strictly ordered, so inserting a deny above an existing allow needs a free number between them. Policies numbered 100, 101, 102 have to be fully renumbered to insert anything, which is a change-window-sized problem rather than a five-minute one.

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