Radon Fan Sizing Calculator
Choosing a radon fan is the most consequential decision in a mitigation system, and it is the one most often made by guesswork. This calculator does the arithmetic that can be done at a desk: the airflow your slab is likely to demand, and the friction your pipe run will cost you. It also tells you what it cannot work out, because a fan is sized against the soil, and no form can measure that.
This single answer decides the fan class more than anything else on this form.
The area the system has to reach: length x width of the basement or slab.
Suction reaches across the slab.
Most residential systems use 4 inch PVC.
Total straight pipe, from the suction point to the discharge above the roof.
Every elbow adds resistance: a 90° costs roughly twice what a 45° does.
Recommended fan class
Mid-range
Look for a fan that delivers about 90 CFM against at least 0.38 in. w.c. of piping loss, plus whatever the soil under your slab adds.
- Required airflow90 CFM
- Total equivalent pipe length71 ft
- Piping friction loss0.38 in. w.c.
- Sub-slab resistanceNeeds a manometer
The one number no calculator can give you. A fan has to overcome the piping loss above and the resistance of the soil itself, and that second figure only shows up on a manometer, held against a test hole in the actual slab. Mixed fill sits between the two extremes and suits a general-purpose fan. A site review measures it.
This estimates a starting point, not a finished design.
The Three Numbers Behind Every Radon Fan
A radon fan has to move a certain amount of air, and it has to pull against a certain amount of resistance. That resistance comes from two places, which is why sizing trips people up. The first is the pipe: every foot of PVC and every elbow costs the fan some of its capacity, and that cost can be calculated exactly. The second is the ground itself: how hard the soil under your slab resists giving up its air. The calculator above works out the airflow and the pipe friction. It does not pretend to know the third number, and neither should anyone else, because the resistance of the soil under one specific house is not something you can look up.
What Is Under the Slab Decides the Fan
If your slab sits on clean gravel, or has a drain-tile loop, there is a lot of air down there and it moves freely. A system like that needs a fan built for airflow rather than suction. If your slab sits on sand, silt, clay or packed dirt, the opposite is true: there is little air to be had, and the fan has to pull hard to get it. That is a fundamentally different machine, and it is why two houses of the same size on the same street can need entirely different fans. Fitting a high-airflow fan to tight soil is the classic mistake: the fan is barely working, the radon stays high, and nobody can see why.
Pipe Diameter Matters More Than People Expect
Friction rises steeply as pipe gets narrower, and the effect is much larger than it looks. Take a system moving about 56 cubic feet of air per minute through roughly 40 feet of pipe and a handful of elbows. In 4-inch PVC, friction costs about 0.18 inches of water column. In 3-inch pipe, the same system costs about 0.69, nearly four times as much, all of it wasted before the fan has pulled a single molecule of radon out of the ground. Most residential systems use 4-inch pipe for exactly this reason, and the calculator will tell you when going up a size would hand a meaningful chunk of capacity back to the fan.
When One Suction Point Is Not Enough
The airflow a slab demands scales with its area and with how freely the fill passes air. Push both up (a large basement over clean gravel) and the required airflow can climb past what any residential radon fan is built to move. That is not a sign you need a bigger fan, because the fan you want does not exist. It is a sign the system needs more than one suction point, so each fan is asked to cover a share of the slab rather than all of it. The calculator flags this when the numbers reach it, rather than quietly printing a specification nothing on the market can meet.
How a Mitigator Actually Sizes a Fan
The honest answer is that fan sizing is a measurement, not a lookup. A mitigator drills a small test hole in the slab, applies suction, and reads the vacuum on a manometer. That reading is then matched against the fan curve, the chart every manufacturer publishes showing how much air a given fan moves at a given pressure. Two fans that look interchangeable on a shelf can differ by a factor of two once you know the actual pressure your system runs at. This is the part the calculator above cannot do for you, and it is the part that decides whether the system works. It is also, in practice, why radon systems get designed on site rather than over the phone.
Where These Numbers Come From
The airflow figure uses a published sizing method: the slab area, multiplied by how freely the fill passes air, adjusted for how far suction actually travels beneath the slab, with a safety margin. The friction figure uses the standard engineering equation for airflow in round pipe, along with the accepted allowances for what each elbow costs, the same arithmetic used to size any duct. Both are general methods rather than Peerless Environmental figures. What they cannot account for is your particular soil, which is measured, not estimated.
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Common Questions
What size radon fan do I need?
It depends far less on the size of your house than on what sits under the slab. Gravel or drain tile means plenty of air moving freely, which calls for a high-airflow fan. Sand, clay or packed dirt means very little air and high resistance, which calls for a high-suction fan. The calculator above estimates the airflow and pipe friction for your layout and returns the fan class to shop against.
What is static pressure in a radon system?
Static pressure is the resistance the fan has to pull against, measured in inches of water column. It has two parts: the friction of the pipe and elbows, which can be calculated, and the resistance of the soil under the slab, which has to be measured with a manometer. A fan is chosen by finding the point on its fan curve where it still delivers the airflow you need at the pressure your system actually runs at.
Can I size a radon fan without a manometer?
Not completely, and anyone who tells you otherwise is guessing. You can work out the airflow your slab is likely to demand and the friction your pipe run will cost, which is what this calculator does. But the resistance of the soil itself is specific to your house, and the only way to learn it is to apply suction to a test hole and read the result. That measurement is what separates a system that works from one that runs for years without fixing the problem.
Request a mitigation reviewIs a 3-inch or 4-inch pipe better for radon mitigation?
Four inch, in almost every residential case. Friction rises sharply as pipe narrows: the same system that costs about 0.18 inches of water column in 4-inch pipe can cost around 0.69 in 3-inch, nearly four times the loss, spent before the fan does any useful work. The narrower pipe also makes the fan work harder and run louder for the same result.
Why would a radon system need more than one suction point?
Because a single point can only draw air from so far. A large slab, or one over open gravel, can demand more airflow than any residential radon fan is capable of moving. Splitting the system across two or more suction points covers the area without asking a fan to do something no fan can do. Poor communication under the slab, where suction does not travel far from the pit, is the other common reason.
Does a bigger radon fan always lower radon more?
No, and an oversized fan can make things worse. A fan too powerful for tight soil pulls conditioned air out of the house, drives up heating and cooling bills, and adds noise without lowering radon any further. The goal is the smallest fan that holds radon below the action level, which is also the cheapest one to run.
Radon fan operating cost calculator →