Enter design airflow, filter area, and static data. See velocity, losses, and replacement guidance fast. Export results as files for site teams and clients.
This tool assumes a typical system curve where static pressure is proportional to the square of airflow: ΔP ∝ Q². Total static is calculated as Ptotal = Pother + Pfilter.
Current airflow is estimated by Qcurrent = Qdesign × √(Pdesign,total / Pcurrent,total). Face velocity is V = Q / A.
This is a practical estimate for field use. For precise results, use fan curves and measured system data.
| Scenario | Design airflow | Filter area | Other static | Clean drop | Current drop | Estimated current airflow | Loss |
|---|---|---|---|---|---|---|---|
| Typical temporary ventilation | 1200 CFM | 8 ft² | 0.60 in.w.g | 0.30 in.w.g | 0.90 in.w.g | ~925 CFM | ~275 CFM |
| Higher resistance duct run | 1500 CFM | 10 ft² | 1.00 in.w.g | 0.35 in.w.g | 1.20 in.w.g | ~1165 CFM | ~335 CFM |
Example results are rounded and may differ from your exact inputs and units.
HEPA filters remove fine particles, but they add resistance to the air path. The clean filter pressure drop is the baseline at design flow, while the current pressure drop rises as dust loading increases. In construction ventilation, temporary ducting, bends, and diffusers already consume static pressure, so a loaded filter can quickly push a fan toward a lower operating point.
Most site air systems behave like a quadratic resistance network where total static pressure increases with the square of airflow. By treating “other system static” as constant and updating only the filter drop, the calculator estimates the new airflow using a square-root relationship. This provides a practical field estimate when full fan curves or commissioning data are unavailable onsite.
Face velocity is the airflow divided by effective filter face area. High velocity can raise pressure drop, create uneven loading, and shorten service life. Keeping velocity within typical design ranges improves filtration efficiency stability and reduces energy use. The calculator reports velocity so you can judge whether filter area is adequate for the target airflow.
Airflow loss is presented as both absolute flow reduction and percentage. Use percentage loss to compare different zones, and absolute loss to decide whether makeup air, negative pressure targets, or drying requirements will be compromised. If current drop approaches the final recommended drop, plan replacement to avoid sudden airflow collapse and noise issues.
Regular logging of filter pressure drop supports predictable maintenance and safer indoor air quality. Exported CSV and PDF outputs help attach calculations to daily reports, commissioning packets, or client handover files. Pair the calculated airflow with measured readings when possible, and record the filter model, installation date, and sampling location for traceability. Where strict containment is required, confirm airflow with anemometers or balancing hoods, then update the inputs. Consistent records support warranty claims and demonstrate compliance with project specifications later.
It covers all resistance except the filter, such as duct length, fittings, diffusers, dampers, and equipment. Use commissioning data when available, or a reasonable estimate for temporary site ductwork.
Many duct systems follow a quadratic relationship where static pressure rises with airflow squared. When total static increases, the new operating flow can be estimated using the square‑root ratio of baseline to current static.
Yes. Select Pa, then enter all pressure values in Pascals. The calculator converts internally and reports totals back in your chosen unit.
Not always, but higher velocity usually increases pressure drop and can shorten service life. Use the velocity output to judge whether filter area should be increased for your target airflow.
Replace when the measured drop reaches the manufacturer’s final recommended value or your project limit. If you are near that threshold, plan replacement to avoid abrupt airflow loss.
Measure airflow with a balancing hood, anemometer traverse, or fan flow station, then compare with the estimate. If differences are large, refine “other static” or use a fan curve based method.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.