Radon Mitigation System Sizing Calculator

Calculate

Floor area of the foundation zone the mitigation system is designed to treat

Select the leakage class that best describes the slab and foundation condition

Select based on expected ability of suction to extend beneath the slab from a single suction point

Percentage margin added to the preliminary sizing result — leave blank for no safety factor

Overview

The Radon Mitigation System Sizing Calculator estimates the airflow required for a radon mitigation system, typically an active soil depressurization (ASD) or sub-slab depressurization setup.

This calculator uses a fixed preliminary sizing model based on treatment area, foundation leakage condition, sub-slab communication quality, and a design safety factor. The result is a required mitigation airflow, which can then be used as a practical starting point for fan selection, vent pipe review, and suction point planning.

ASTM E2121 describes field diagnostic procedures (sub-slab communication testing, pressure field extension measurement) for radon mitigation. This calculator provides a preliminary area-based airflow estimate using illustrative leakage coefficients. It does not implement ASTM diagnostic procedures and does not replace field diagnostics, communication testing, pressure measurements, or post-installation verification.

How to Use This Calculator

  1. Select Imperial or Metric units — to match your project data.

  2. Enter the treatment area — in ft² (Imperial) or m² (Metric). This is the area of the foundation that the mitigation system is designed to treat.

  3. Choose the foundation leakage class — Tight, Typical, or Leaky, based on the visible condition of the slab and penetrations.

  4. Choose the sub-slab communication condition — Good, Fair, or Poor, based on how easily suction can be expected to extend beneath the slab.

  5. Enter the design safety factor (%) — to add margin to the preliminary sizing result. Leave blank for no safety factor.

  6. Click Calculate — review the required mitigation airflow and the result category.

  7. Use the resulting airflow as a preliminary basis for mitigation fan selection, vent piping review, and suction point strategy.

This is a preliminary sizing tool. Final fan selection also depends on pressure loss, fan curve, pipe routing, and actual field diagnostics. Post-installation verification is always required.

Inputs & Outputs

Inputs

  • •Treatment Area (m² / ft²)
  • •Foundation Leakage Class — Options: Tight — minimal cracks and penetrations, Typical — moderate cracks and typical penetrations, Leaky — visible cracks, joints, or heavy penetrations
  • •Sub-Slab Communication Condition — Options: Good — suction expected to extend well, Fair — moderate sub-slab communication, Poor — limited or uncertain sub-slab communication
  • •Design Safety Factor (%)

Outputs

  • •Required Mitigation Airflow (m³/h / CFM)
  • •Sizing Category

Formula

Calculator Formula

Imperial:

CFM_required = A × L × C × S

Metric:

Q_required = A × L × C × S

Where:

  • A = treatment area (ft² / m²)
  • L = base leakage airflow intensity (CFM/ft² / m³/h·m²)
  • C = sub-slab communication multiplier (dimensionless)
  • S = safety factor multiplier (dimensionless)

Imperial Formula Detail

1) Treatment Area

A = treatment area, ft²

2) Leakage Airflow Intensity

Use the selected leakage class:

Leakage Class L (CFM/ft²)
Tight 0.05
Typical 0.10
Leaky 0.20

These leakage coefficients are practical preliminary sizing assumptions intended to represent tight, typical, and leaky slab/foundation conditions. They are not universal code constants or ASTM-prescribed values.

3) Sub-Slab Communication Multiplier

Communication Condition C
Good 1.00
Fair 1.25
Poor 1.50

4) Safety Factor Multiplier

S = 1 + (SF / 100)

Where SF = design safety factor in %

5) Final Imperial Result

CFM_required = A × L × C × S

Metric Formula Detail

1) Treatment Area

A = treatment area, m²

2) Leakage Airflow Intensity

Use the selected leakage class:

Leakage Class L (m³/h·m²)
Tight 0.91
Typical 1.83
Leaky 3.66

These are the metric equivalents of the fixed Imperial airflow-intensity values (1 CFM/ft² = 18.288 m³/h·m²).

3) Sub-Slab Communication Multiplier

Same as Imperial — Good: 1.00, Fair: 1.25, Poor: 1.50

4) Safety Factor Multiplier

S = 1 + (SF / 100)

5) Final Metric Result

Q_required = A × L × C × S

Where Q_required = required mitigation airflow, m³/h

Formula Meaning

This calculator assumes that mitigation airflow increases when the treatment area is larger, the slab or foundation is leakier, sub-slab communication is weaker, or more design margin is needed.

This calculator does not predict indoor radon concentration directly and should not be interpreted as a direct health-risk model.

Interpretation Thresholds

Imperial (CFM)

Range Category
Less than 50 CFM LOW
50 to 99 CFM NORMAL
100 to 199 CFM HIGH
200 CFM and above VERY HIGH

Metric (m³/h)

Range Category
Less than 85 m³/h LOW
85 to 169 m³/h NORMAL
170 to 339 m³/h HIGH
340 m³/h and above VERY HIGH

Calculator Variables

Variable Meaning Units
treatmentArea Foundation area to be treated ft² / m²
leakageClass Base leakage airflow intensity CFM/ft² (Imperial)
communicationCondition Sub-slab communication multiplier dimensionless
safetyFactor Design safety factor %
S Safety factor multiplier = 1 + SF/100 dimensionless
requiredVentilation Required mitigation airflow CFM / m³/h

What is Radon Mitigation System Sizing?

Radon mitigation system sizing is the process of estimating how much airflow a mitigation fan and piping system may need to create and maintain effective sub-slab depressurization or sub-membrane depressurization. In practice, the required system size depends on slab leakage, foundation type, sub-slab communication, suction point effectiveness, vent piping losses, and fan capability.

For early design work, airflow sizing helps determine whether the mitigation system looks relatively small, typical, or more demanding before detailed field refinement. This calculator uses a fixed area-based preliminary sizing model: required airflow equals treatment area multiplied by a leakage intensity coefficient, sub-slab communication multiplier, and safety factor. The result gives a practical first-pass airflow estimate for ASD fan selection and vent pipe review.

This calculator follows the general principle behind active soil depressurization sizing — that mitigation airflow demand increases with larger treatment area, leakier slabs, weaker sub-slab communication, and greater design margin requirements. ASTM E2121 describes field diagnostic procedures (sub-slab communication testing, pressure field extension measurement) rather than prescribing CFM/ft² airflow coefficients. This calculator provides a preliminary area-based estimate that supports early planning alongside those diagnostics.

How Radon Mitigation Airflow Sizing Works

The fundamental sizing calculation is: required airflow equals treatment area multiplied by a base leakage airflow intensity, a sub-slab communication multiplier, and a safety factor multiplier. The leakage intensity represents how much airflow per unit of slab area is needed to control soil gas under the stated slab condition — tight slabs require less airflow than leaky ones.

The sub-slab communication multiplier adjusts the sizing result for how easily suction is expected to extend beneath the slab. Good communication means suction can reach further with less airflow effort; poor communication means the system must work harder, effectively increasing the airflow demand. A safety factor multiplier then adds margin to the result to account for uncertainty in the field condition estimates.

The result is a preliminary airflow in CFM (Imperial) or m³/h (Metric) that can be used to shortlist mitigation fans, review vent pipe sizing, and assess whether the system appears low, normal, high, or very high in demand before proceeding to detailed fan selection and installation planning.

Result Categories Explained

This calculator classifies the required mitigation airflow into one of four categories — LOW, NORMAL, HIGH, or VERY HIGH — based on the calculated airflow in CFM (Imperial) or m³/h (Metric). A LOW result indicates a relatively limited airflow demand, typically associated with smaller or tighter slab conditions. A NORMAL result falls within the typical range for many residential ASD applications. A HIGH result suggests a more demanding mitigation requirement, possibly requiring more careful fan and pipe selection. A VERY HIGH result indicates a heavy-duty requirement that may need multi-point suction layout, careful pressure-loss review, or a more detailed site assessment before final design.

Key Facts

  • Radon mitigation system sizing is commonly linked to active soil depressurization (ASD) or sub-slab depressurization.
  • Required airflow generally increases with larger treatment area.
  • Leakier slabs and foundations usually require higher airflow to maintain effective depressurization.
  • Poor sub-slab communication increases mitigation system demand and may require additional suction points.
  • Safety factor increases the final required airflow to provide design margin on uncertain installations.
  • Airflow is only part of the system sizing picture — real performance also depends on fan pressure capability, pipe losses, and suction point effectiveness.
  • Typical radon mitigation fans operate in the 20–150 CFM range for residential applications, but demanding conditions may require larger or multiple fans.

Applications

  • Preliminary ASD fan sizing.
  • Early-stage sub-slab depressurization planning.
  • Comparing tight vs leaky slab conditions.
  • Estimating the effect of good vs poor sub-slab communication.
  • Reviewing system demand before selecting vent pipe size.
  • Building a first-pass mitigation design basis before field refinement.
  • Sanity-checking field estimates against a fixed preliminary sizing model.

Example Calculation

Imperial Example

Inputs:

  • Treatment Area = 1,200 ft²
  • Leakage Class = Typical
  • Communication Condition = Fair
  • Safety Factor = 15%

Step 1 — Select base leakage intensity:

Typical slab: L = 0.10 CFM/ft²

Step 2 — Select communication multiplier:

Fair communication: C = 1.25

Step 3 — Calculate safety factor:

S = 1 + 15/100 = 1.15

Step 4 — Calculate required airflow:

CFM_required = 1200 × 0.10 × 1.25 × 1.15 = 172.5 CFM

Rounded result:

Required Mitigation Airflow = 173 CFM

Metric Example

Inputs:

  • Treatment Area = 111.5 m²
  • Leakage Class = Typical
  • Communication Condition = Fair
  • Safety Factor = 15%

Step 1 — Select base leakage intensity:

Typical slab: L = 1.83 m³/h·m²

Step 2 — Select communication multiplier:

Fair communication: C = 1.25

Step 3 — Calculate safety factor:

S = 1.15

Step 4 — Calculate required airflow:

Q_required = 111.5 × 1.83 × 1.25 × 1.15 = 293.3 m³/h

Rounded result:

Required Mitigation Airflow = 293 m³/h

Standards & References

  • AARST/ANSI — Radon Mitigation Standards — RMS-LB for low-rise buildings and SGM-SF for soil-gas mitigation; the primary professional standards for radon mitigation design and installation.
  • ASTM E2121 — Standard Practice for Installing Radon Mitigation Systems in Existing Low-Rise Residential Buildings. Describes field diagnostic procedures including sub-slab communication testing and pressure field extension measurement.
  • Project-specific contractor practice, fan data, and installation constraints — should always govern final design.

Limitations

  • This is a preliminary sizing calculator, not a full mitigation design tool.
  • It uses a fixed area-based decision model chosen for calculator consistency.
  • It does not calculate actual pressure field extension, sub-slab vacuum, or post-mitigation radon concentration.
  • It does not account for vent pipe friction loss, fitting losses, fan curve interaction, or the effect of using multiple suction points.
  • It does not replace communication testing, site diagnostics, pressure measurements, post-installation verification, or qualified mitigation contractor design.
  • The calculator is not a direct compliance tool for ASTM, EPA, state, provincial, or local radon regulations.

Common Mistakes to Avoid

  • Using the building floor area instead of the actual treatment area.
  • Selecting Tight leakage when the slab is visibly cracked or highly penetrated.
  • Assuming good sub-slab communication without any diagnostic basis.
  • Using too small a safety factor on uncertain projects.
  • Interpreting airflow as a direct prediction of indoor radon level.
  • Ignoring fan pressure capability and vent pipe losses.
  • Treating the calculator as a final contractor design instead of a first-pass sizing tool.
  • Forgetting that poor slab communication may require more than one suction point even if airflow seems reasonable.

Frequently Asked Questions

What does this calculator estimate?
It estimates the required mitigation airflow for a radon mitigation system, usually an ASD or sub-slab depressurization system.
Does this calculator predict indoor radon concentration?
No. It estimates airflow demand for system sizing, not post-mitigation indoor radon levels.
Why do slab leakage and communication matter?
Because mitigation performance depends on how easily suction can move beneath the slab and how much soil gas leakage the system must control.
What is active soil depressurization?
Active soil depressurization is a mitigation method that uses a fan to create negative pressure beneath the slab or membrane so soil gas is captured before entering the building.
Is this calculator enough to choose the final fan?
No. Final fan selection also depends on pressure loss, fan curve, pipe routing, and actual field diagnostics.
Can one suction point always handle the whole slab?
Not always. Weak sub-slab communication or a large treatment area may require additional suction points or layout changes.
What should I do if the calculated airflow is very high?
First, recheck the selected leakage class and sub-slab communication assumption. A very high result may indicate that the slab is modeled as too leaky, communication is too weak, or the treatment area is too large for a simple single-point system. In some cases, multiple suction points or a different mitigation layout may be more appropriate.
Do I still need post-mitigation testing?
Yes. Final system performance should always be verified after installation.

Frequently Used Together

Engineers often use these calculators in combination for complete project workflows:

Blower Door Test Cfm50

Fan Power Calculator

Cfm Calculator

Related Calculators

Explore similar calculators that might be useful for your project:

Ventilation Rate Calculator

Air Changes Per Hour Calculator

Building Envelope Tightness

Static Pressure Calculator

Indoor Air Quality Co2 Calculator

Duct Velocity Calculator

Free HVAC Quick Reference. Formulas & Checks.

Airflow, loads, refrigerant & duct checks — one printable page for the job site.

  • Key formulas for airflow, load, refrigerant charge & duct sizing
  • Quick sanity checks for the most common HVAC design errors
  • Printable one-pager for field use and design review

No spam. Unsubscribe any time.