HEPA Filter Sizing Calculator

Use the form to estimate airflow, filter face area, filter quantity, and optional pressure drop.

Example Data Table

Examples are illustrative; confirm with project specs and filter submittals.

Formula Used

• Room volume: V = L x W x H (ft3 or m3)
• Airflow: Q = (V x ACH) / 60 for CFM (imperial); Q = V x ACH for m3/h (metric)
• Safety factor: Qdesign = Q x (1 + SF/100)
• Required face area: Areq = Qdesign / v (ft2), where v is face velocity (fpm)
• Filters required: N = ceil(Areq / Afilter)
• Actual face velocity: vactual = Qdesign / (N x Afilter)
• Optional pressure drop: dP about dPrated x (Q/Qrated)^2 per filter

How to Use This Calculator

1. Select your unit system and enter room dimensions.
2. Set the ACH required by your ventilation plan.
3. Add a safety factor for duct losses and site conditions.
4. Choose a filter face size and target face velocity.
5. Optional: enter rated flow and rated pressure drop from the submittal.
6. Click Calculate, then export results as CSV or PDF.

For final design, verify fan capability, housing leakage class, and local code requirements.

HEPA Sizing Goals on Active Jobsites

HEPA filtration in construction is usually sized to control airborne dust migration, support negative pressure containment, and protect adjacent occupied areas. This calculator converts your room size and target air changes into a design airflow, then translates that airflow into filter face area and filter quantity. The result helps align equipment selections with project requirements and reduces guesswork during submittals.

ACH to Airflow: Translating Containment Plans

Air changes per hour (ACH) is a planning metric, while fans and filters are specified in flow rate. The calculator uses the room volume and ACH to estimate required airflow. A safety factor adds margin for leakage, duct losses, and loading. Use the same dimensions applied in your containment drawings so airflow targets match field conditions.

Face Velocity and Filter Loading Considerations

Filter face velocity affects pressure drop, noise, and how quickly media loads. Lower velocities generally improve performance and extend service life, but require more face area. By selecting a standard or custom filter size, the calculator computes required face area and rounds up to whole filters. The “actual face velocity” output verifies the final configuration.

Pressure Drop, Fans, and System Losses

If you enter rated flow and rated resistance from a filter submittal, the calculator estimates initial pressure drop using quadratic scaling. Treat this as a screening value. Final fan selection should include housing losses, flexible duct friction, fittings, prefilters, and anticipated loading. Confirm that the fan curve can deliver the design flow at the expected total static pressure.

Example Data for Quick Verification

Example: a 30 ft x 20 ft x 10 ft space at 8 ACH produces about 800 CFM base flow. With 10% safety, design flow is about 880 CFM. Using 24x24 in filters at 250 fpm, required face area is about 3.52 ft2, so one filter typically meets the target. For a 12 m x 6 m x 3 m space at 10 ACH with 15% safety, design flow is about 1,250 CFM, often needing two 24x24 in filters at similar velocity.

FAQs

1) What ACH should I use for a containment area?

Use the ACH specified in your project plan, ICRA, or containment protocol. Many temporary containment setups target 6–12 ACH, but requirements vary with hazard level, room leakage, and adjacent occupancy.

2) Why add a safety factor?

Safety factor accounts for real losses such as duct friction, imperfect seals, filter loading, and equipment tolerances. It helps avoid undersizing when conditions change between design and field installation.

3) What is a good target face velocity?

Lower face velocity typically reduces pressure drop and bypass risk. Common design targets are often in the 200–500 fpm range, depending on the housing, filter type, and allowed resistance.

4) Does the filter count include prefilters?

No. The count is based on HEPA face area. If your system uses prefilters, they affect pressure drop and loading rate. Include them in fan static pressure checks and maintenance planning.

5) How is pressure drop estimated?

When enabled, the calculator scales rated resistance using a quadratic relationship with flow per filter. This estimates initial clean resistance only. Loading, ductwork, and fittings can significantly increase total static pressure.

6) Why does the calculator round up to whole filters?

Filters are installed as discrete units, and partial filters are not practical. Rounding up ensures the total face area meets or exceeds the required area, keeping face velocity at or below the target.

7) Can I size for metric projects?

Yes. Select Metric units and enter dimensions in meters. The calculator converts to a consistent sizing basis and reports both CFM and m3/h so you can coordinate equipment schedules and submittals.