Enter supply pressure and the positive magnitude of return pressure in inches water column. Enter the applicable manufacturer external-static limit separately; 0.5 is an example rather than a universal equipment rating. Measurements must use the correct test locations and included/excluded components for that equipment’s definition.
The calculation does not specify where to drill or authorize an adjustment.
A percentage of an entered limit is not an airflow measurement. Blower behavior depends on equipment controls and its performance table; constant-speed and controlled blowers can respond differently. Filter and coil location, test ports, dampers and operating speed affect interpretation.
Use the actual equipment instructions and qualified measurement practice.
Total external static pressure (in. wg) from supply-side static
| Supply-side static | 0.35 |
|---|---|
| Entered manufacturer external-static limit | 0.5 |
| Return-side static | 0.15 |
| Total external static pressure (in. wg) | 0.5 |
| Percent of the entered external-static limit | 100.0% |
Common error: Reading only one side. The total is the sum of the supply and return readings, and a problem on either side raises it.
Static Pressure: formula
Static Pressure: changing Supply-side static
| Supply-side static | Total external static pressure (in. wg) | Percent of the entered external-static limit |
|---|---|---|
| 0.28 | 0.43 | 86.0% |
| 0.32 | 0.47 | 94.0% |
| 0.35 (example) | 0.5 | 100.0% |
| 0.39 | 0.54 | 108.0% |
| 0.42 | 0.57 | 114.0% |
Static Pressure: compare input ranges
| Input | Tested from | To | Total external static pressure (in. wg): low input | Total external static pressure (in. wg): high input | Swing |
|---|---|---|---|---|---|
| Supply-side static | 0.32 | 0.39 | 0.47 | 0.54 | 0.07 (14%) |
| Return-side static | 0.14 | 0.17 | 0.49 | 0.52 | 0.03 (6.0%) |
| Entered manufacturer external-static limit | 0.45 | 0.55 | 0.5 | 0.5 | 0 |
Static Pressure: individual input comparisons
| Input | Value | Total external static pressure (in. wg) | Percent of the entered external-static limit |
|---|---|---|---|
| Supply-side static | 0.28 | 0.43 | 86.0% |
| 0.32 | 0.47 | 94.0% | |
| 0.35 | 0.5 | 100.0% | |
| 0.39 | 0.54 | 108.0% | |
| 0.42 | 0.57 | 114.0% | |
| Entered manufacturer external-static limit | 0.4 | 0.5 | 125.0% |
| 0.45 | 0.5 | 111.1% | |
| 0.5 | 0.5 | 100.0% | |
| 0.55 | 0.5 | 90.9% | |
| 0.6 | 0.5 | 83.3% | |
| Return-side static | 0.12 | 0.47 | 94.0% |
| 0.14 | 0.49 | 98.0% | |
| 0.15 | 0.5 | 100.0% | |
| 0.17 | 0.52 | 104.0% | |
| 0.18 | 0.53 | 106.0% |
Total external static pressure (in. wg): Supply-side static against Return-side static
| Supply-side static / Return-side static | 0.12 | 0.15 | 0.18 |
|---|---|---|---|
| 0.28 | 0.4 | 0.43 | 0.46 |
| 0.32 | 0.44 | 0.47 | 0.5 |
| 0.35 | 0.47 | 0.5 | 0.53 |
| 0.39 | 0.51 | 0.54 | 0.57 |
| 0.42 | 0.54 | 0.57 | 0.6 |
Static Pressure: values and definitions
| Quantity and definition | Value used |
|---|---|
| Supply-side static — Static pressure reading on the supply side of the air handler, in inches of water column. | 0.35 |
| Entered manufacturer external-static limit — Use the applicable equipment rating and measurement boundary, in inches water column. | 0.5 |
| Return-side static — Static pressure reading on the return side of the air handler, in inches of water column (entered as a positive magnitude). | 0.15 |
| Total external static pressure (in. wg) | 0.5 |
| Percent of the entered external-static limit | 100.0% |
Supply-side static: half and double
| Supply-side static | Total external static pressure (in. wg) |
|---|---|
| 0.175 | 0.325 |
| 0.35 | 0.5 |
| 0.7 | 0.85 |
Static pressure is the resistance the fan pushes against, and it is a health check for an air system. Adding a supply-side reading of 0.35 to a return-side reading of 0.15 inch of water gives a total external static pressure of 0.5, right at a typical rated maximum of 0.5. Restricted filters, undersized ducts or a dirty coil raise it, and a system running at its limit moves less air and works harder.
Compare the total with the rated external static pressure on the equipment's label, not with a general figure, since ratings differ between units.
Size ducts on the duct sizing calculator, airflow on the CFM page, and equipment on the HVAC tonnage calculator. The SEER upgrade page covers efficiency, and the pressure converter changes units. Compare readings before and after changing a filter to see the effect.
Can I enter the return reading with a minus sign?
This version expects its positive magnitude. A reading of −0.15 in. wg enters as 0.15, which is added to supply pressure.
Does a low percentage prove adequate room airflow?
No. The tool has no airflow or room-balance measurement. Compare the correct blower table and actual delivered airflow separately.