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Calculation Machine

Duct Leakage Calculator

CFM25 leakage against code limits.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
Leakage rate
6 CFM25/100 sq ft
Code limit
4
Verdict
Fail — seal and retest
Allowed total
80 CFM25
Excess
40 CFM25

Quick answer

Duct leakage is reported as CFM at 25 Pa per 100 square feet of conditioned floor area.

The formula

leakage rate = CFM25 / (area / 100)

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Duct blaster result120 CFM25
  • Conditioned area2000 sq ft
  • Code limit4 CFM25/100 sq ft

Result

  • Leakage rate6 CFM25/100 sq ft
  • Code limit4
  • VerdictFail — seal and retest
  • Allowed total80 CFM25
  • Excess40 CFM25

How to use the Duct Leakage Calculator

  1. 1Pressurise the duct system to 25 Pa with a duct blaster and record the flow.
  2. 2Enter conditioned floor area served by that system.
  3. 3Seal boots, plenums and joints with mastic, then retest.

Frequently asked questions

Total or to outside?

Total leakage is easier to test; leakage to outside is the number that actually wastes energy.

Typical old system?

Untested existing ducts often leak 15-30% of system airflow.

About this calculator