How to Calculate Relative Percent Difference for Radon Tests

On this page (8 sections)

1. What is the relative percent difference formula for radon tests?
2. Why are duplicate radon measurements compared?
3. What does a high or low RPD tell you?
4. What can make duplicate radon results differ?
5. What if one measurement is zero or very small?
6. Does RPD tell you whether a home needs follow-up?
7. Which home radon test kits support this process?
8. What should you do after calculating relative percent difference?

Relative percent difference for radon tests is the absolute gap between two measurements divided by their average, multiplied by 100. It helps compare duplicate measurements as a quality-control check, but the acceptable limit depends on the test method and its instructions.

For example, results of 3.0 and 3.6 pCi/L differ by 0.6 pCi/L and average 3.3 pCi/L. Their relative percent difference is about 18.2%.

What is the relative percent difference formula for radon tests?

Use RPD = |A − B| ÷ ((A + B) ÷ 2) × 100%. Here, A and B are two comparable radon measurements, and the vertical bars mean to use the positive difference.

This formula expresses the difference as a share of the average, so it works for comparing the pair without choosing one result as the baseline. It is commonly shortened to RPD.

Use results with the same units, such as pCi/L, and the same reporting basis. If the measurements come from different periods or methods, check whether the test or lab protocol says they can be compared.

How do you calculate radon testing relative percent difference by hand?

First subtract the lower measurement from the higher one, then divide that difference by the average of the two readings. Multiply the quotient by 100 to express it as a percent.

For readings of 3.0 and 3.6 pCi/L:

1. Find the absolute difference: |3.0 − 3.6| = 0.6.
2. Find the average: (3.0 + 3.6) ÷ 2 = 3.3.
3. Divide: 0.6 ÷ 3.3 = 0.1818.
4. Convert to a percent: 0.1818 × 100 = 18.2% RPD.

Keep a few decimal places during the calculation, then round the final percentage as your instructions or report require. Rounding the readings too early can change the result.

How do you calculate RPD in a spreadsheet?

In a spreadsheet, enter the two measurements in cells A1 and B1, then use =ABS(A1-B1)/AVERAGE(A1,B1)*100. The result is a number such as 18.2; label it as percent or add a percent sign in the output.

If you format the cell as a percentage, use =ABS(A1-B1)/AVERAGE(A1,B1) instead. Spreadsheet percentage formatting multiplies the displayed value by 100, so using both methods at once would show a value 100 times too high.

Why are duplicate radon measurements compared?

Duplicate measurements help show whether two devices or measurements taken under comparable conditions produce similar results. A large gap can prompt a review of placement, exposure time, handling, or the protocol used.

A duplicate check is a quality-control step, not a replacement for following test instructions. It also doesn't mean a result is accurate just because two readings are close.

For home screening, the test method and placement rules matter. Review what can affect a radon test before interpreting a difference between measurements.

What does a high or low RPD tell you?

A lower RPD means the two values are closer relative to their average; a higher RPD means they differ more. Neither value alone says whether the radon level is safe or whether the test should be accepted.

For instance, 3.0 and 3.6 pCi/L produce about 18.2% RPD. But the same absolute gap of 0.6 between 0.5 and 1.1 pCi/L produces 75% RPD because the average is smaller.

Always compare the calculated value with the applicable quality-control threshold in the test instructions, lab report, or protocol. Don't apply a cutoff from a different test type without confirming it fits.

Is there one acceptable RPD limit for all radon tests?

No single cutoff applies to every testing method and situation. Duplicate charcoal devices, continuous monitors, and other methods can have different quality-control procedures, so use the instructions tied to the measurements.

If the report does not explain its threshold, ask the lab or testing professional how to interpret the duplicate comparison. Don't decide a test failed or passed based only on a general number found elsewhere.

What can make duplicate radon results differ?

Differences can come from real variation in radon levels, testing conditions, device handling, or measurement uncertainty. The RPD calculation describes the gap; it does not explain its cause.

Check whether both devices were placed and exposed as directed, and whether they were handled and returned as instructed. Confirm that the readings use matching units and cover comparable periods.

A result can also differ if one value is rounded more than the other or reported near a detection limit. The guide to how to check radon levels and what that means explains basic measurement context.

What if one measurement is zero or very small?

If both readings are zero, the usual RPD formula has a zero average in the denominator and cannot produce a meaningful result. If one is zero and the other is nonzero, the calculation is mathematically possible, but confirm what “zero” means in that report.

A displayed zero may represent rounding or a value below the method's detection limit rather than an exact absence of radon. Check the report notes before calculating, and ask the lab if you are unsure.

Very small averages can also make RPD look large even when the absolute difference is small. Keep both the raw values and the percentage in view.

Does RPD tell you whether a home needs follow-up?

No, RPD only compares two measurements; use the reported radon concentration and the relevant public health guidance to decide on follow-up. For context, see normal radon levels in a home.

A close pair can still show a raised concentration, while a wide pair needs interpretation under the testing protocol.

Read the result units and any notes about short-term screening or longer-term measurement. Our radon levels chart can help put a reported concentration in context, and how to read radon test results covers the report itself.

For background on methods, see what radon testing involves. If you are preparing to test, how to test for radon at home covers setup basics.

Which home radon test kits support this process?

A home kit can collect a sample for lab analysis, but the kit's instructions determine how and when to expose it. These options offer different short-term collection approaches; check the instructions for the specific steps and return process.

The First Alert Radon Gas Test Kit includes test materials and lab fees, with New Jersey state fees excluded. It is recommended for use only within the United States.

The PRO-LAB Radon Test Kit for Home is a short-term detector with a 48-hour collection period and lab analysis included. Its instructions describe sealing and returning the sample for analysis.

The Leaping Lynx Radon Test Kit for Home uses an activated charcoal detector for a 48-hour test and includes lab analysis; return postage is required. Follow its placement and submission instructions so the sample is handled as directed.

What should you do after calculating relative percent difference?

Use the RPD as a comparison aid, then check the applicable test or lab rule before deciding whether duplicate results are acceptable. Keep the original readings, units, test dates, and calculation together so the comparison can be reviewed.

If results differ more than the protocol allows, contact the lab or testing professional for the next step. If you are deciding what the radon concentration means, interpret the measured level separately from the RPD.

Frequently asked questions

What is a good relative percent difference for radon tests?

There isn't one universal cutoff for every radon test or lab. Use the acceptance rule from the test instructions, lab report, or applicable quality-control protocol.

Relative percent difference shows how far two measurements differ in relation to their average, but it doesn't by itself confirm that either result is accurate.

Can I use relative percent difference for different radon units?

Use two results in the same units and on the same reporting basis. Convert units first if one result is in pCi/L and the other is in Bq/m³, and make sure both represent comparable testing periods and conditions.

Otherwise, the calculated percentage may be misleading.

What if one radon result is zero?

If both results are zero, the formula's denominator is zero, so relative percent difference is undefined. If only one is zero, calculate using the usual formula, but first check whether the instrument or lab reports zero, a detection limit, or a rounded value.

Does a low relative percent difference prove a radon test is accurate?

No. A low relative percent difference means two results are close to each other, but both could still be affected by the same placement, timing, or measurement issue.

Follow the test instructions and use the relevant lab or quality-control guidance to judge whether the pair is acceptable.