Calculator
Enter space size, target ACH, and scrubber airflow to estimate count.
Formula used
The calculator estimates the airflow needed to reach a target number of air changes per hour. Then it divides by the effective airflow of each scrubber (after derating), and rounds up.
- Required CFM = (Volume × ACH) ÷ 60
- Adjusted CFM = Required CFM × (1 + Safety%) × (1 + Leakage%) (leakage optional)
- Effective unit CFM = Rated airflow × Derate%
- Scrubbers needed = ceil(Adjusted CFM ÷ Effective unit CFM)
- ACH achieved = (Total effective CFM × 60) ÷ Volume
- Time to reduction ≈ (-ln(remaining) × 60) ÷ ACH achieved
How to use this calculator
- Select your unit system and input method.
- Enter the space dimensions or a measured volume.
- Set the ACH target required by your scope or standard.
- Enter the scrubber airflow rating and choose a realistic derate.
- Add a safety factor and optional leakage allowance if needed.
- Press Calculate and download a CSV or PDF summary for records.
Example data table
| Scenario | Space | Target ACH | Scrubber rating | Derate | Safety | Estimated count |
|---|---|---|---|---|---|---|
| Small room | 12 ft × 10 ft × 8 ft | 6 | 500 CFM | 85% | 10% | 2 |
| Medium work zone | 20 ft × 15 ft × 9 ft | 8 | 2000 CFM | 80% | 10% | 2 |
| Large bay | 50 ft × 30 ft × 12 ft | 6 | 3000 CFM | 75% | 15% | 5 |
| Two identical rooms | 2 × (18 ft × 14 ft × 8 ft) | 6 | 1000 CFM | 80% | 10% | 3 |
| Metric example | 10 m × 6 m × 3 m | 6 | 3400 m3/h | 80% | 10% | 4 |
Examples are illustrative; actual requirements depend on containment performance and project scope.
ACH to airflow conversion
Air changes per hour (ACH) convert directly into required airflow. The calculator uses CFM = (Volume × ACH) ÷ 60, so a 2,400 ft3 zone at 6 ACH needs 240 CFM before margins. Doubling ACH doubles airflow demand. For multi-room repeats, the tool multiplies volume by the room count, keeping the same ACH target across identical spaces. For quick checks, confirm units and ceiling height.
Volume inputs and unit handling
Room volume drives everything, so measuring accurately matters. Dimensions mode computes L × W × H, while Volume mode lets you enter a surveyed value for irregular areas. A 20 ft × 15 ft × 9 ft space equals 2,700 ft3; at 8 ACH it needs 360 CFM baseline. Switching to metric accepts meters and cubic meters and converts internally. Include mezzanines only when air is connected.
Derating scrubber performance
Scrubber ratings rarely match field performance, so the derate input adjusts capacity for loaded filters, bends, and duct runs. For example, a 2,000 CFM unit derated to 80% provides 1,600 effective CFM. Two such units yield 3,200 effective CFM. The count is rounded up to avoid under-ventilation when conditions change during the shift. Use manufacturer curves when available.
Safety and leakage allowances
Safety and leakage allowances add practical resilience. A 10% safety factor increases required airflow by 1.10, and a 5% leakage allowance multiplies again to 1.155 overall. These margins help when doors open, seams flex, or make-up air pathways vary. If you have measured negative pressure behavior, tune leakage to match site observations. When unsure, start conservative, then verify with pressure tests.
Cleanup time estimation
The results estimate cleanup time using a mixing model. Once ACH achieved is known, time to a remaining fraction uses t = (−ln(remaining) × 60) ÷ ACH. At 10 ACH achieved, reaching 99% reduction (1% remaining) takes about 27.6 minutes. Use this as a guide, not a compliance certificate. Schedule filter changes to maintain the achieved ACH daily.
FAQs
1) How do I choose a target ACH?
Use your project specification or containment plan first. When none is stated, many teams start around 4–12 ACH, then verify performance with field checks and adjust scrubber count as needed.
2) What derate percentage should I enter?
If you run long ducting, multiple bends, or loaded filters, derate more. A practical starting point is 70–90%. Use measured airflow or manufacturer curves whenever you can.
3) Should I enable leakage allowance?
Enable it when enclosures are temporary, doors open often, or you expect make-up air variability. Start with 3–8%, then refine after observing pressure stability and smoke movement.
4) Can I calculate multiple rooms at once?
Yes. Enter the number of identical rooms and the tool multiplies the volume accordingly. If rooms differ, calculate each room separately and sum the scrubber totals.
5) Does the calculator support metric inputs?
Yes. Select Metric to enter meters and cubic meters. Scrubber airflow can be entered as m3/h or CFM, and all results are normalized internally for consistent calculations.
6) Are the reduction-time estimates guaranteed?
No. They assume well-mixed air and steady effective airflow. Use them for planning and sequencing. Real cleanup time can change with obstructions, leakage, and filter loading.