Capture Efficiency by Particle Size
Full Results Table
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How to Use This Calculator
Choose a preset filter size matching common portable purifier and HVAC HEPA dimensions, or enter the exact face area from your filter's specification sheet.
Enter the airflow the system will push through the filter. The rated face velocity is the rated airflow divided by the face area on the manufacturer's data sheet. A standard-capacity 24 x 24 in HEPA is typically rated near 1.27 m/s (250 fpm), and high-capacity models near 2.5 m/s (500 fpm).
Room volume calculates the equivalent air changes per hour. Select the environment dust loading and whether a pre-filter is installed. Both significantly affect estimated HEPA service life.
The results show whether your actual face velocity is within the rated range for full 99.97% efficiency, the capture efficiency across particle sizes, estimated pressure drop, and projected filter replacement interval.
HEPA Filtration: Standards, Performance, and Sizing
HEPA is one of the most misused terms in air filtration marketing, but the underlying technology is genuinely effective when properly specified and sized. Understanding the formal standard, the face velocity relationship, and realistic service life expectations helps separate marketing claims from actual performance for wildfire smoke protection, allergy relief, and healthcare applications.
The HEPA Standard
As EPA explains, HEPA is officially defined by the US Department of Energy as a filter that removes at least 99.97% of particles 0.3 microns in diameter. The 0.3 micron size corresponds to the worst case, the most penetrating particle size (MPPS), and particles that are larger or smaller are trapped with even higher efficiency. Individual filters have their own MPPS, which is why EN 1822 tests each filter at its own most penetrating size and grades it at 99.95% for H13 and 99.995% for H14. Certified HEPA filters are individually tested and labelled with their measured efficiency, and the grades most commonly treated as equivalent to or better than US HEPA are H13 and H14.
Face Velocity and Capture Efficiency
HEPA filters are certified at a rated airflow, and the face velocity is that airflow divided by the face area. Manufacturer data shows a standard-capacity 24 x 24 x 11.5 in module rated near 1,000 CFM (about 1.27 m/s, or 250 fpm) at roughly 1.0 in w.g. (250 Pa), and high-capacity versions near 2,000 CFM (about 2.5 m/s) at about 1.5 in w.g. Inside the pleated media the air moves far more slowly, at 2 to 5 cm/s, and that media velocity is what efficiency depends on. Above the rated flow the filter is outside its certified condition, pressure drop rises, and measured efficiency typically falls slightly as media velocity increases. Below the rated flow the filter is more lightly loaded but you are paying for more filter than the airflow needs. The air filtration calculator covers multi-stage systems combining HEPA with pre-filters.
Pressure Drop and Energy
HEPA media has inherently higher pressure drop than coarser MERV-rated filters because the dense fibre matrix needed for sub-micron capture also restricts airflow more. Standard-capacity 24 x 24 modules show about 1.0 in w.g. (250 Pa) at rated flow in one manufacturer's data, high-capacity modules about 1.5 in w.g., and another manufacturer describes typical HEPA at 1.4 to 1.8 in w.g., so roughly 250 to 450 Pa in all. This calculator uses 250 Pa at rated velocity and scales it with velocity to the power 1.4, a planning assumption. That is well above what a 50 mm MERV 13 filter adds, so HEPA-equipped systems need a fan sized for the extra resistance or accept reduced airflow. The static pressure calculator adds up total system resistance so you can check that the fan can carry the HEPA stage. Portable HEPA purifiers manage this with airflow targets matched to fan capability, and whole-building HEPA bypass systems need dedicated booster fans rather than relying on the main air handler.
Service Life and Loading
HEPA filters load with particles over time, which tends to improve capture of the smallest particles because the dust cake helps trap what follows, but it raises pressure drop. Replacement is normally triggered by pressure drop, and a commonly used trigger is about twice the clean value, though manufacturers set their own final resistance. This calculator's planning values start at 36 months for low loading, 18 for medium, 9 for high, and 4 for very high, all without a pre-filter, and a MERV 8 to 13 pre-filter multiplies life by 1.8 because it captures the larger particles that would otherwise load the HEPA media fastest. A face velocity above 1.2 times rated cuts the estimate by 30% and above 1.0 times rated by 10%. Treat these as estimates and let measured pressure drop decide.
HEPA in Canadian Wildfire Smoke Response
Wildfire smoke particles are predominantly in the fine range, well within HEPA's strong capture zone. Health Canada's wildfire smoke guidance names fine particles (PM2.5) as the main health risk and recommends keeping windows and doors closed, using the best filter your ventilation system can handle, and using a certified portable air cleaner that filters fine particles. The key sizing factor for a portable unit is matching CADR (clean air delivery rate, which already accounts for the filter's actual capture at rated airflow) to room volume and target air changes per hour, which the air purifier sizing calculator handles. This HEPA filter calculator focuses on the filter itself: face velocity, pressure drop, and service life for a given filter and airflow combination.
Healthcare and Critical Applications
HVAC in Canadian healthcare facilities falls under CSA Z317.2, and the filtration required for each type of space is set by that standard and its related documents, not by this calculator. Critical spaces that use HEPA terminals typically require certified, leak-tested installations with on-site scan testing per IEST-RP-CC034, well beyond the consumer-grade HEPA found in portable purifiers. The distinction matters: a portable HEPA purifier rated 99.97% at 0.3 microns under laboratory chamber conditions is a different deployment context than a hospital HEPA terminal filter that must maintain that efficiency with zero bypass leakage around the frame. The hospital HVAC calculator covers the wider ventilation picture for these spaces.
Frequently Asked Questions
True HEPA filters meet the formal definition of capturing at least 99.97% of particles 0.3 microns in diameter, tested and certified to standards such as IEST-RP-CC001 or, in Europe, EN 1822 grades H13 and H14. HEPA-type, HEPA-like, and 99% HEPA are marketing terms with no formal definition, and a filter labelled that way has not been shown to meet the 99.97% at 0.3 micron standard. For health-critical applications like wildfire smoke protection, allergy relief, or healthcare settings, only filters labelled True HEPA or with documented test certification to the formal standard should be relied upon.
HEPA filters are certified at a rated airflow, and the face velocity is that airflow divided by the face area. A standard-capacity 24 x 24 x 11.5 in HEPA module is typically rated near 1,000 CFM, about 1.27 m/s (250 fpm), at roughly 1.0 in w.g. (250 Pa), and high-capacity modules near 2,000 CFM, about 2.5 m/s, at about 1.5 in w.g. Inside the pleated media the velocity is only 2 to 5 cm/s, which is what the efficiency depends on. Operating above the rated flow raises pressure drop and puts the filter outside its certified condition, and measured efficiency typically falls slightly as velocity rises. Operating well below it means paying for more filter than the airflow needs. Use the rated value from the data sheet as the reference in this calculator.
HEPA filter replacement is normally based on pressure drop rather than a fixed calendar schedule, since particle loading varies significantly by environment. A commonly used trigger is about twice the clean-filter pressure drop, though manufacturers set their own final resistance. This calculator's planning values range from about 4 months for very high loading to 36 months for low loading without a pre-filter, and a pre-filter multiplies the estimate by 1.8. Homes in wildfire smoke regions or with pets sit toward the shorter end. Commercial and healthcare HEPA installations typically use a pre-filter ahead of the HEPA stage specifically to extend HEPA service life, since the pre-filter captures larger particles before they load the more expensive HEPA media.
No. HEPA filters work by mechanical particle capture through interception, impaction, and diffusion, with no capacity to adsorb gas-phase contaminants like VOCs, formaldehyde, ozone, or odours. Activated carbon or other sorbent media must be added separately. Many air purifiers combine a HEPA stage for particles with a carbon stage for gases. See the VOC calculator and formaldehyde calculator for gas-phase contaminant control.
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