Duct component selection

Duct Leakage Check Workbench

Use leakage measurements, airflow, or test pressure to compare leakage percentages and stated limits.

Calculation inputs

This calculator runs in your browser and requires JavaScript.

An example is loaded automatically. Replace it with your project values; the result updates as you edit.

Leakage calculation Choose a documented class/area-allocation method, a multipoint fit or one of the retained supplied-value checks. No code limit is selected automatically.

Measured leakage at test pressure (CFM) Mode: allowance comparison only. Record the actual test pressure and protocol separately.

Externally selected allowance (CFM) Use the applicable specified method/class; this tool does not derive the limit.

Loading example values…

Result

No result yet

Results appear automatically when inputs are valid.

Formula and source basis

Concepts behind this calculation

Assess a duct leakage test against supplied criteria, or fit a pressure–leakage curve from compatible multipoint readings and estimate flow at an explicit target pressure.

Leakage-test flow belongs to a specified specimen and test pressure. It cannot be subtracted from operating system flow unless the leakage has been established at a compatible operating pressure and reference condition.

Leakage class scales allowable flow with pressure and duct surface area. Multipoint fitting instead estimates a coefficient and exponent from measured observations; it does not automatically impose the class exponent.

How to use this calculator

Choose allowance comparison, percentage, compatible delivered-flow subtraction, before/after reduction, class check, section allocation or multipoint fitting.
For class and allocation methods document the test protocol/project criterion and complete tested/system surface-area boundaries; enter pressure magnitude and measured leakage for each specimen.
For multipoint fitting enter the observations as corrected CFM at positive pressure magnitudes in Pa, then enter target duct pressure in Pa. Inspect fitted exponent, residuals and target-range status before using the estimate.

Calculation method and applicability

Class allowance = CL × |p|^0.65 × duct surface area/100. Area allocation = maximum system CFM × supplied target %/100 × section area/system area. Multipoint model: ln Q = ln C + n ln |Δp|, with equal-weight least squares in log space. Target Q = C × |Δp target|^n. Operating-flow arithmetic requires a separately compatible basis.

CL = supplied leakage class (CFM per 100 ft² at 1 in. w.g.); p = test-pressure magnitude (in. w.g.) for the class equation. In the multipoint fit Δp is Pa, Q is corrected leakage CFM, n is the dimensionless fitted exponent and C carries the corresponding CFM/Paⁿ units. Do not mix those two pressure bases.

Input definitions and units

Leakage calculation: Choose a documented class/area-allocation method, a multipoint fit or one of the retained supplied-value checks. No code limit is selected automatically. Choices: Compare measured test flow with an externally selected allowance; Compute percentage on the same airflow basis; Compute flow less leakage only at compatible operating pressure; Compare before/after tests at compatible conditions; Assess a test against a supplied leakage class; Allocate a system target to tested duct sections; Fit multipoint pressure–leakage data and estimate target flow.
Test protocol and project criterion reference: Identify the actual specified test procedure, class or target, full system/section boundary, pressure direction and CFM correction basis. A reference does not authenticate input data. Applies to Leakage calculation: Assess a test against a supplied leakage class or Allocate a system target to tested duct sections.
Test and area basis: Confirm protocol-corrected CFM, actual duct surface rather than floor area and supplied project criteria. For allocation, sections must be nonoverlapping parts of the same stated complete system. Choices: Not established; Documented test basis and applicable project criterion established. Applies to Leakage calculation: Assess a test against a supplied leakage class or Allocate a system target to tested duct sections.
Specified leakage class (CFM/100 ft² at 1 in. w.g.): Positive project-supplied CL, not an automatically selected standard limit or seal class. Applies to Leakage calculation: Assess a test against a supplied leakage class.
Test-pressure magnitude (in. w.g.): Positive magnitude of the actual test pressure differential; record direction in the protocol reference. Do not enter Pa or a pressure-class designation. Applies to Leakage calculation: Assess a test against a supplied leakage class.
Tested duct surface area (ft²): Actual surface area inside the test boundary, not building/floor area. Use the protocol-defined treatment of fittings and closures. Applies to Leakage calculation: Assess a test against a supplied leakage class.
Measured leakage on the documented test basis (CFM): Already corrected/calibrated as the actual test procedure requires. Zero is valid; instrument resolution still matters. Applies to Leakage calculation: Assess a test against a supplied leakage class.
Maximum design airflow for the complete system (CFM): Use the whole-system maximum design fan flow for this allocation method, not each tested branch flow. Applies to Leakage calculation: Allocate a system target to tested duct sections.
Supplied whole-system leakage target (%): Project-selected percent of maximum system design flow. Enter 3 for 3%; this teaching value is not a nationwide code limit. Applies to Leakage calculation: Allocate a system target to tested duct sections.
Complete system duct surface area (ft²): Surface area on the same complete system boundary as maximum fan flow and percentage. Tested rows can represent only a nonoverlapping subset. Applies to Leakage calculation: Allocate a system target to tested duct sections.
Tested duct sections (ID, area ft², test magnitude in. w.g., measured CFM): One nonoverlapping section per row, no header. Unique integer ID; positive actual surface area and test-pressure magnitude; nonnegative already protocol-corrected test CFM. Different test pressures remain separate assessments, not simultaneous operating flows. At most 100 sections. Applies to Leakage calculation: Allocate a system target to tested duct sections.
Multipoint test boundary and procedure reference: Identify specimen/section, pressure direction, pressure monitoring and uniformity, baseline and instrument/airflow corrections, and why the target represents this same boundary. Do not mix pressure directions, before/after sealing or different sections. Applies to Leakage calculation: Fit multipoint pressure–leakage data and estimate target flow.
Multipoint measurement basis: Readings must share one unchanged boundary, one pressure direction and compatible corrections. No density, baseline or pressure-distribution correction is performed by this fit. Choices: Not established; Same specimen/direction and compatible corrected measurement basis established. Applies to Leakage calculation: Fit multipoint pressure–leakage data and estimate target flow.
Multipoint leakage observations (pressure magnitude Pa, corrected leakage CFM): One observation per row, no header. Enter 3–100 positive finite pressure/flow pairs at at least three distinct pressures. Repeated pressure observations retain equal weight. Use baseline-adjusted differential pressure magnitudes in Pa, not signed readings, in. w.g., absolute pressure or tester fan-pressure signal. Zero/negative airflow cannot enter a log fit. Applies to Leakage calculation: Fit multipoint pressure–leakage data and estimate target flow.
Target duct pressure magnitude (Pa): Explicit reference or representative pressure across the same test specimen. Outside the measured pressure range, the result is flagged EXTRAPOLATED; no universal reference or fan TESP is assumed. Applies to Leakage calculation: Fit multipoint pressure–leakage data and estimate target flow.
Measured leakage at test pressure (CFM): Mode: allowance comparison only. Record the actual test pressure and protocol separately. Applies to Leakage calculation: Compare measured test flow with an externally selected allowance.
Externally selected allowance (CFM): Use the applicable specified method/class; this tool does not derive the limit. Applies to Leakage calculation: Compare measured test flow with an externally selected allowance.
Measured leakage flow (CFM): Use only when the leakage flow and total system flow share a compatible reference pressure. Applies to Leakage calculation: Compute percentage on the same airflow basis.
System airflow at same basis (CFM): Do not pair CFM25 with operating airflow as if they were the same condition. Applies to Leakage calculation: Compute percentage on the same airflow basis.
System airflow at compatible operating condition (CFM): Only subtract operating leakage on a compatible airflow/pressure basis; do not subtract CFM25. Applies to Leakage calculation: Compute flow less leakage only at compatible operating pressure.
Leakage at same operating condition (CFM): Must represent leakage at the system operating condition, not a separate test pressure. Applies to Leakage calculation: Compute flow less leakage only at compatible operating pressure.
Before-sealing test leakage (CFM): Compare tests using the same method, pressure and system boundary. Applies to Leakage calculation: Compare before/after tests at compatible conditions.
After-sealing test leakage (CFM): Use the same instrument method and test pressure as the before test. Applies to Leakage calculation: Compare before/after tests at compatible conditions.

How to interpret the result

Retain pressure, specimen area and measurement basis with every comparison. A reduction is meaningful only between compatible tests, and extrapolated multipoint estimates need the stated model limitations.

Worked examples

Compare measured test flow with an externally selected allowance

Inputs: Measured 80 CFM; separately selected allowance 60 CFM.

Calculation: Compare the supplied test result with the supplied limit.

Result: 80 CFM, above the entered 60 CFM allowance.

Compute percentage on the same airflow basis

Inputs: 80 CFM leakage and 1,000 CFM system flow at the same basis.

Calculation: 80/1,000×100.

Result: 8.0% of the stated system flow.

Compute flow less leakage only at compatible operating pressure

Inputs: 1,000 CFM operating flow and 80 CFM leakage on the same basis.

Calculation: Subtract compatible operating leakage.

Result: 920 CFM delivered under this simplified balance.

Compare before/after tests at compatible conditions

Inputs: Before 100 CFM and after 40 CFM using the same test conditions.

Calculation: Subtract 40 from 100 and divide reduction by the before value.

Result: 60 CFM or 60% reduction.

Assess a test against a supplied leakage class

Inputs: Constructed test: supplied class 4, 1,000 ft² duct surface area, 3 in. w.g. test-pressure magnitude and 80 CFM measured on the documented protocol basis.

Calculation: The pressure factor is 3^0.65, about 2.042343632. Allowance = 4 × 2.042343632 × 1,000 / 100. Measured class = 80 × 100 / (1,000 × 3^0.65).

Result: Allowable leakage is 81.693745 CFM. The 80 CFM measurement is within the entered criterion by 1.693745 CFM. This is not operating leakage or proof of a code limit.

Allocate a system target to tested duct sections

Inputs: ASHRAE 2025 Table 5 geometry and target: 5,000 CFM maximum system flow, a supplied 3% target and 3,117 ft² of system duct surface. Five sections have areas of 500, 667, 750, 200 and 1,000 ft², tested at 2, 1, 1.5, 1 and 1 in. w.g. Measurements of 22, 30, 35, 9 and 45 CFM are constructed, not published test data.

Calculation: The whole-system budget is 150 CFM. Multiply by each section area divided by 3,117 ft². Maximum section class = allowable CFM × 100 / (section area × test pressure^0.65).

Result: Allowances are 24.061598, 32.098171, 36.092397, 9.624639 and 48.123195 CFM; all five constructed measurements pass separately. The source prints 24, 32, 36, 10 and 48 CFM, with classes 3.1, 4.8, 3.7, 4.8 and 4.8. Both reproduce at printed precision. Intermediate rounding does not govern acceptance.

Fit multipoint pressure–leakage data and estimate target flow

Inputs: Constructed series: (4 Pa, 20 CFM), (16, 40), (36, 60), (64, 80); target 25 Pa. One unchanged specimen, direction and corrected measurement basis.

Calculation: Fit ln(CFM) against ln(Pa) with equal observation weights. These points give n = 0.5 and C = 10 CFM/Pa^0.5. At 25 Pa, 10 × √25 = 50 CFM.

Result: 50 CFM at 25 Pa, within the measured 4–64 Pa range; log-space R² = 1 and zero residuals within rounding. These constructed data are not a field test or proof of test acceptance.

Assumptions and limits

The multipoint mode is an explicitly derived equal-weight log-linear model, not an implementation or certification of the full ASTM E779 procedure. At least three distinct pressures and a natural-log pressure span of 0.000001 are implementation requirements; the latter is a numerical stability guard, not a protocol limit. Repeated observations retain weight. R² and flow residuals do not establish measurement uncertainty or an acceptance threshold.
Target-pressure estimates can be extrapolations and are labeled accordingly. The law may change outside the test range or when seals, dampers, pressure direction or boundary conditions change. Pressure nonuniformity and baseline/instrument/airflow corrections must be addressed externally. A fan TESP or one test cannot automatically establish whole-system operating leakage.
CFM at a duct pressurization test point (such as CFM25) is not operating leakage and must not simply be subtracted from system CFM.
Named I-P class and area-allocation equations calculate a supplied criterion; no residential floor-area limit, seal-class mapping, standard default class or nationwide maximum leakage percentage is selected. Protocol/instrument/density corrections and actual test certification remain external.
Before/after tests require comparable pressure, boundary and measurement methods.
Allocation uses actual nonoverlapping duct surface areas and the complete system maximum fan flow. A passing subset does not accept untested sections; spare allowance in one section does not excuse another section exceeding its allocation. Mixed-pressure test sums are bookkeeping, not operating system leakage.
Published source examples and independent constructed measurements are distinguished. Source table values are rounded for print; unrounded arithmetic governs this comparison. Actual project criterion, current code adoption and test basis must be supplied.

FAQs

Can a leakage percentage replace leakage class?

Percentage uses a system-flow denominator; class uses tested duct surface area and pressure. They need different evidence and cannot be exchanged by changing a label.

Does a good multipoint fit validate the test?

A small residual only describes agreement with the fitted model. Specimen isolation, corrected instrument flow, stable pressure and unchanged leakage direction still need independent support.

Sources

PNNL: Central exhaust and ventilation duct leakage testingDescription tab: power-law pressure conversion and empirically determined multipoint exponent; baseline and pressure-distribution precautions. This guide concerns central ventilation systems. Its program criteria and default 0.65 are not automatically applied to the fitted model.
NIST/SEMATECH: Least SquaresStraight-line least-squares slope/intercept formulas applied here to natural-log pressure and natural-log leakage with equal observation weights. This transformation and residual presentation are explicit local derivations, not an ASTM-compliance claim.
ASHRAE 2025 Fundamentals: Duct DesignDuct leakage I-P Equations 37–39 and Table 5: test-pressure magnitude, class per 100 ft², maximum system fan flow and section surface-area allocation. Printed allowance and class labels were independently reproduced; no default code target is selected.
ASHRAE 90.1-2016 I-P official errata, June 15, 2023Confirms the I-P exponent 0.65, surface-area normalization and pressure-difference definition. This is edition-specific context; the tool does not impose class 4 or a local-code requirement.
SMACNA Air Duct Leakage AppOfficial task-discovery page describes class and multiple-section calculation. Its current public page was inspected; no app installation or paid handbook access was performed.
PNNL: Duct Leakage TestsGovernment definitions distinguish test-pressure flow from operating system airflow.

Related calculators

Available Static Pressure and Friction Budget Subtract component losses from available pressure and use route length to find the friction budget. Duct Traverse Airflow Calculator Combine traverse readings with duct area and air properties to estimate average velocity and airflow.

Find a tool.