Conversion Formula
Pa to inWG: inWG = Pa ÷ 249.089
Quick check: 1 inWG ≈ 249 Pa | 0.5 inWG ≈ 125 Pa
inWG to Pa Quick Reference Table
| inWG | Pa | inWG | Pa |
|---|---|---|---|
| 0.05 | 12.5 | 0.6 | 149.5 |
| 0.1 | 24.9 | 0.7 | 174.4 |
| 0.15 | 37.4 | 0.8 | 199.3 |
| 0.2 | 49.8 | 0.9 | 224.2 |
| 0.25 | 62.3 | 1.0 | 249.1 |
| 0.3 | 74.7 | 1.5 | 373.6 |
| 0.4 | 99.6 | 2.0 | 498.2 |
| 0.5 | 124.5 | 3.0 | 747.3 |
Furnace and Air Handler Static Pressure Reference
| Equipment / Condition | inWG | Pa | Notes |
|---|---|---|---|
| Residential furnace TESP rating | 0.5 | 124.5 | Typical maximum rated static |
| Target actual TESP (well-designed) | 0.3 - 0.5 | 75 - 125 | Good airflow with margin |
| Light commercial AHU rating | 0.5 - 1.5 | 125 - 375 | Higher-capacity fan |
| Commercial VAV main fan | 1.0 - 3.0 | 250 - 750 | Full system including VAV boxes |
Filter Pressure Drop and Building Airtightness Reference
| Item | inWG | Pa | Notes |
|---|---|---|---|
| Clean 1-inch fiberglass filter | 0.03 - 0.08 | 7 - 20 | Low resistance, low filtration |
| Clean pleated MERV 8-11 filter | 0.08 - 0.15 | 20 - 37 | Common residential upgrade filter |
| Clean high-MERV (13+) filter | 0.15 - 0.30 | 37 - 75 | Better filtration, higher resistance |
| Heavily loaded / dirty filter | 0.30 - 0.50+ | 75 - 125+ | Replace before this point |
| Blower door test reference pressure | 0.2 | 50 | Standard test condition (ACH50) |
| Typical wind/stack pressure on envelope | 0.004 - 0.02 | 1 - 5 | Natural infiltration driving force |
Use the Pressure Converter for the full range of pressure units including psi, bar, and kPa, or the Refrigerant Unit Converter for refrigerant-specific pressure work.
How to Use the inH₂O to Pa Converter
Type an inWG value or a Pascal value -- the other field updates instantly. Use the quick presets for common HVAC reference points: 0.5 inWG furnace rating, 0.2 inWG or 50 Pa blower door test standard, and clean filter pressure drop.
The result panel identifies what typical HVAC condition your static pressure value matches: filter pressure drop, duct static, or building airtightness test pressure. This helps quickly confirm a converted value falls in the expected range.
The quick reference table covers 0.05 to 3.0 inWG in one glance. The furnace/AHU table shows typical fan rating ranges by equipment type. The filter and blower door table covers filter pressure drop stages and the standard 50 Pa blower door test condition.
For other pressure units -- psi, bar, atm, mmHg, or the gauge/absolute distinction -- use the Pressure Converter. For refrigerant-specific pressure work and P-T saturation data, use the Refrigerant Unit Converter.
inWG and Pascals in Canadian HVAC Duct Work
Inches of water column, commonly abbreviated inWG or in w.g., is the dominant Imperial unit for duct static pressure in North American HVAC. Fan curves, furnace and air handler ratings, and filter pressure drop specifications are almost universally shown in inWG. Canadian engineering documents, commissioning reports, and provincial code references use Pascals -- so conversion between the two is a routine part of Canadian HVAC work.
The Conversion: 249.089
The exact factor is 1 inWG = 249.089 Pa. For quick field estimates, rounding to 250 Pa per inWG is close enough for most sanity checks. Key reference points worth memorizing: 0.5 inWG = 124.5 Pa (the standard residential furnace TESP rating), and 0.2 inWG = 49.8 Pa (essentially the 50 Pa standard blower door test pressure). This exact factor derives from the inch-of-water and Pascal definitions set out in the NIST Guide to the SI, Appendix B.
Total External Static Pressure (TESP)
TESP is the sum of all pressure drops in the duct system external to the furnace or air handler cabinet -- supply ductwork, return ductwork, filter, and any coils in the airflow path. Most residential furnaces are rated for a maximum TESP of 0.5 inWG (124.5 Pa). Exceeding this rating reduces delivered airflow below design values, which reduces heating and cooling capacity, increases equipment operating temperatures, and can shorten equipment life. Measuring actual installed TESP with a manometer and comparing it against the equipment's rated maximum is a standard commissioning step, and the Duct Sizing Calculator is where that supply and return duct pressure drop gets worked out during design.
Filter Pressure Drop Over Time
Filter pressure drop increases progressively as dust accumulates. A clean standard filter might add only 0.05 to 0.1 inWG (12 to 25 Pa), but a heavily loaded filter can add 0.3 to 0.5 inWG (75 to 125 Pa) or more. Since the entire system TESP budget is often just 0.5 inWG, a dirty filter alone can consume the whole available static pressure allowance, starving the rest of the system of airflow. Regular filter replacement intervals should account for this progressive pressure increase, not just a fixed calendar schedule. Filter MERV ratings themselves come from lab testing under ASHRAE Standard 52.2, which measures how much a filter's resistance rises as it loads with dust.
Blower Door Testing and Building Airtightness
Blower door tests measure how much air a fan must move to create a standardized pressure difference across the building envelope -- typically 50 Pascals (0.2 inWG). The resulting airflow, divided by the building volume, gives air changes per hour at 50 Pa (ACH50), a standard metric the Blower Door Calculator works out directly for comparing building airtightness across different homes and different test days. This standardization matters because natural pressure differences from wind and stack effect vary constantly, making a fixed test pressure necessary for consistent, comparable results.
Frequently Asked Questions
Multiply inWG by 249.089 to get Pascals. So 0.5 inWG = 124.5 Pa, and 1 inWG = 249.1 Pa. To reverse, divide Pascals by 249.089. For quick field estimates, 250 Pa per inWG is close enough for most sanity checks. This conversion is essential for comparing North American fan and furnace ratings (inWG) against Canadian engineering documents and commissioning reports (Pa).
Most residential furnaces are rated for a maximum total external static pressure (TESP) of 0.5 inWG (124.5 Pa). This is the combined pressure drop of the supply duct, return duct, filter, and any coils external to the furnace cabinet. Exceeding this rating reduces airflow and equipment performance. Well-designed systems typically operate below the rated maximum to leave margin for filter loading and future changes.
The standard test pressure is 50 Pascals (0.2 inWG). This standardized pressure is used to measure how much air must be moved by the blower door fan to maintain that pressure difference between inside and outside, which is then used to calculate ACH50 -- a comparable measure of building airtightness. Standardizing on 50 Pa allows results to be compared consistently across different homes regardless of actual weather conditions during testing.
A clean standard filter typically adds 0.05 to 0.15 inWG (12 to 37 Pa). As it loads with dust, pressure drop climbs, often reaching 0.3 to 0.5 inWG (75 to 125 Pa) or more when significantly dirty. Since most systems are only rated for 0.5 inWG total, a heavily loaded filter can consume the entire available static pressure budget on its own, starving the rest of the system of airflow. Regular replacement based on visible loading, not just calendar time, helps avoid this.