What this page is, and what it is not
It is a take-off and a piece of duct arithmetic for a pipe run that has already been laid out by somebody qualified to lay it out. Feed it the lengths, the elbow counts, the pipe size and the flow, and it returns sticks, couplings, equivalent length, velocity and a yearly energy figure for a fan that runs continuously.
It is not a design tool. It does not know where suction points belong, how many a slab needs, what fan curve suits a particular soil, or whether a system works once it is in. It states no measurement threshold of any kind and offers no interpretation of any result. Testing, design, installation and verification all belong with a professional certified in that work; your state radon program keeps the list of who holds that certification locally, and that is the right first call.
Equivalent length, and why the elbows matter more than the pipe
Resistance in a duct comes from friction along the straight sections and from turbulence at every change of direction. The usual way to handle the fittings is to express each one as the number of feet of straight pipe that would cause the same loss, then add the lot up and treat the whole run as one long straight pipe of that length.
On the default run: 25 ft horizontal plus 22 ft vertical is 47 ft of pipe. Four 90 degree elbows at 6 ft each is 24 ft. Two 45s at 3 ft each is 6 ft. Equivalent length 77 ft — the fittings have added 64 percent to the run. That is the number worth taking away: six fittings cost more resistance than the entire vertical section of the stack, which is why a route that avoids two elbows is worth walking twice before the pipe is glued.
| Item | Count | Equivalent feet |
|---|---|---|
| Horizontal pipe | 25 ft | 25.0 |
| Vertical pipe | 22 ft | 22.0 |
| 90 degree elbows | 4 | 24.0 |
| 45 degree elbows | 2 | 6.0 |
| Equivalent length | 77.0 | |
| Lengths of 10 ft pipe, 10% waste | 6 | 51.7 ft needed |
| Couplings | 5 |
The equivalent length per fitting is a field rather than a constant because it genuinely varies. A long-radius sweep and a tight moulded elbow of the same nominal size are not the same fitting, and the figures in general circulation for a 4 in 90 run from about 5 to 8 feet. If the fitting data sheet gives you a number, that number beats the placeholder.
Velocity, and the size the pipe is not
Nominal pipe size is a name, not a measurement. Schedule 40 PVC called 4 inch has an inside diameter of 4.026 in; called 3 inch it is 3.068 in. Cross-sectional area goes as the square of the diameter, so the 3 in pipe has 58 percent of the area of the 4 in one, and at the same flow the air inside it moves 72 percent faster.
At the default 60 CFM through 4.026 in — 12.73 square inches, or 0.0884 square feet — velocity is 679 feet per minute, about 11.3 feet a second. Through 3.068 in pipe the same 60 CFM would be 1,169 fpm. Velocity bears on noise and on whether condensate running back down the stack is being fought by the airflow. It says nothing about performance, and no velocity figure here should be read as a target.
The running cost nobody budgets for
A 60 watt fan running continuously is 1.44 kilowatt hours a day and 525.6 a year. At 17 cents that is 24 cents a day and $87.60 a year, every year, for as long as the system is in service. It is not a large number on its own, but it is the smaller half of the real cost: the fan is also moving air out of the house, and the heating and cooling penalty on the make-up air that replaces it is usually the bigger line. That penalty is not calculated here because it depends on how much of the flow is house air rather than soil gas, and that is a measurement rather than an assumption.
Related arithmetic elsewhere on the site: duct sizing and static pressure handle the same friction question for ventilation ductwork, the caulk calculator covers sealant coverage in more detail, and the vapor barrier calculator handles the membrane take-off if the floor is soil rather than slab.
Questions people ask
Does this calculator design a sub-slab depressurization system?
No, and it should not be used as though it did. It counts pipe and fittings, converts elbows to equivalent feet, divides flow by area to get velocity, and multiplies a fan wattage by a tariff. Everything it needs comes from you. Where suction points go, how many there should be, which fan suits the soil under a particular slab, and whether a completed system does what it was installed to do are all decided by testing and by a professional certified in that work. Your state radon program keeps the list of certified people for your area.
How much equivalent length does a 90 degree elbow add?
It depends on the fitting, which is why it is a field on this form rather than a constant in the code. Figures in general circulation for a long-radius 4 inch 90 sit around 5 to 8 feet of straight pipe, and a tight moulded elbow is worse than a sweep. If the fitting manufacturer publishes a loss coefficient or an equivalent length, use theirs. On the default of 6 feet, four elbows and two 45s add 30 feet to a 47 foot run, which is 64 percent more resistance than the pipe alone.
What is the inside diameter of 4 inch PVC pipe?
Schedule 40 PVC sold as 4 inch has an inside diameter of 4.026 inches, and 3 inch is 3.068 inches. The nominal size is a name rather than a dimension. It matters here because velocity is flow divided by cross-sectional area, and area goes as the square of diameter — the 3 inch pipe has 58 percent of the area of the 4 inch, so at the same 60 CFM the air inside moves from 679 feet per minute to 1,169.
How much does a continuously running fan cost to run?
A 60 watt fan uses 1.44 kilowatt hours a day and 525.6 a year. At 17 cents a kilowatt hour that is 24 cents a day and $87.60 a year. The larger cost is usually invisible: the fan removes air from the house, and heating or cooling the air that replaces it costs more than the fan itself in most climates. That penalty is not calculated here because it depends on what fraction of the flow is house air, which has to be measured.
Can I use this to check whether my existing system is working?
No. Nothing on this page can tell you that, and no arithmetic can. A system is verified by testing the house, by someone certified to do it, using the method that applies. This page contains no measurement, no threshold, no action level and no interpretation of any result, and that is deliberate rather than an oversight. If the question is whether a system is working, the answer comes from a test and from the professional who reads it.