Indoor Radon Mitigation Effectiveness Calculator

Introduction to indoor radon mitigation estimates

When a radon fan, vent pipe, or depressurization system is planned, the important question is not merely whether a reading will fall. It is how far the indoor concentration may fall after the system and the room’s air exchange have had time to work. This calculator turns a starting radon reading, an air-change estimate, a mitigation percentage, and elapsed hours into a projected post-mitigation concentration.

The result is useful for comparing assumptions in one specific space: for example, a basement before and after a fan installation, or a cautious contractor estimate against a stronger-performance estimate. It is deliberately a simple model, so every assumption remains visible rather than being hidden behind a vague “better” or “worse” label.

Radon is measured here in picocuries per liter, written pCi/L. Keep the baseline reading and the scenario tied to the same room whenever possible. A basement result, a crawlspace result, and a first-floor result may describe different conditions and should not be casually substituted for one another.

What this radon mitigation calculator compares

This indoor radon mitigation calculator estimates what concentration might remain after an expected removal step and ongoing ventilation decay. It can help you see whether a proposed reduction percentage has a large practical effect, whether additional hours materially change the projection, and how sensitive the forecast is to the air changes per hour, or ACH, assumption.

The calculation is best treated as a scenario comparison. Enter one credible set of values, note the projected pCi/L, then change just one input for a second scenario. That approach makes it much easier to identify whether the baseline reading, expected mitigation performance, or ventilation assumption caused the difference.

How to use the indoor radon mitigation calculator

Start with a measured radon result from the room you want to evaluate. Enter that number as the initial concentration, then enter an ACH estimate for that same space. Add the percentage reduction you expect from mitigation and the number of hours since the system began influencing the room. Select Estimate Post-Mitigation Level to update the result panel.

1. Use the baseline Initial radon concentration in pCi/L.
2. Enter Air changes per hour as an hourly rate, not as a daily total.
3. Enter the expected Mitigation reduction as a percentage from 0 to 100.
4. Enter the Hours after mitigation for the modeled period.
5. Compare the projected pCi/L with your own target, test history, and follow-up plan.

The prefilled values are a worked scenario, not a recommendation. Replace them before using the result to discuss a particular property or mitigation system.

Radon inputs that should match the modeled room

The initial radon concentration is the starting reading for the space under review. It provides the amount of radon the model has available to remove. A reading from a different floor or a different testing period can be useful context, but it is not automatically interchangeable with the room being modeled.

ACH describes how quickly indoor air is replaced each hour. In this page’s formula, a larger ACH causes the remaining modeled concentration to decline more quickly over time. Mitigation reduction is separate from ACH: it represents the fraction removed by the mitigation step before the time-based decay is applied. For example, 80% is entered as 80 in the form and becomes 0.80 in the formula.

Hours after mitigation is the length of the modeled period. If an input is uncertain, run more than one case instead of treating a single estimate as precise. A conservative case might use a smaller reduction percentage or lower ACH, while an optimistic case can use a better-supported system-performance assumption. Comparing that range is generally more honest than relying on one guessed number.

Radon decay formulas used for the projection

The model applies mitigation first and air exchange second. The initial concentration is multiplied by the fraction that remains after mitigation, then the remaining amount is reduced exponentially as ACH and elapsed hours increase.

The calculator’s core estimate is:

In this expression, C₀ is the initial pCi/L reading, m is the mitigation fraction, a is ACH, and h is elapsed hours. The exponential term is why changing ACH or time can have a larger effect over a longer period than over a short period.

After calculating C_f, the page adds a small 0.2 pCi/L offset and converts the adjusted concentration into its displayed comparison score:

The percentage shown beside the concentration is therefore a logistic comparison score centered on 4 pCi/L. It rises as the modeled level moves above that point and falls as it moves below it. It is not a medical probability, a guarantee of system performance, or a replacement for a measured radon result.

Worked example with the calculator’s starter radon values

Suppose a room starts at 10 pCi/L, the mitigation system is expected to reduce that amount by 80%, the ACH estimate is 0.50, and the system has been running for 24 hours. The mitigation portion leaves 10 × (1 − 0.80), or 2 pCi/L, before the ventilation decay term is applied. With an exponent of −0.50 × 24, the modeled remaining amount becomes very small; the calculator then adds its 0.2 pCi/L offset and displays approximately 0.20 pCi/L.

This example also shows why the result needs sensible assumptions. A real home may not sustain one steady air-exchange rate or one fixed mitigation percentage for an entire day. Use the example to understand the calculation, then substitute the measurements and performance range that fit the property you are reviewing.

Radon scenario sensitivity after a mitigation change

The starting concentration and mitigation percentage usually have the most immediate influence on this model because they determine how much radon remains before time-based decay begins. A higher baseline gives the model more radon to remove, while a higher mitigation percentage leaves less remaining concentration at the start of the decay period.

ACH and elapsed time work together. Raising ACH or waiting longer makes the exponential factor smaller, which lowers the projection under the model’s steady-condition assumption. To understand sensitivity, keep three values fixed and adjust the fourth. That simple discipline prevents a comparison from mixing several changes together and obscuring the real driver of the difference.

How to interpret a post-mitigation radon result

The projected pCi/L is a planning estimate, not a certification that the indoor level has reached a particular target. Check whether the result is plausible for the room, whether the inputs are measured or merely assumed, and whether the number moves in the expected direction when a major input changes. Those checks are often more valuable than treating a precisely rounded output as proof.

For a real health, property, or contractor decision, pair this calculator with an appropriate follow-up radon measurement and applicable local guidance. The page is most useful when it helps you ask sharper questions about fan performance, source control, ventilation, and testing conditions.

Radon model limitations and practical assumptions

This indoor radon estimate assumes a smooth, steady response. Actual homes can behave differently when weather changes, stack effect shifts, windows open, a fan cycles, soil moisture changes, or pressure pathways through the foundation vary. The formula also does not model new radon entering the room during the elapsed period as a separate source term.

Use pCi/L, ACH, percent, and hours exactly as labeled. The displayed concentration and score are rounded for readability, so minor differences from a hand calculation are normal. Most importantly, a modeled result should not replace a post-mitigation test. Its role is to make the assumptions explicit, help compare alternatives, and show why both source reduction and air movement matter.