Iron Filter Sizing Guide: What Size Do You Need?

I size an iron filter from how many people use the water, your iron and manganese, and how much water your well can give the filter to clean itself. Every size we sell handles the same 30 ppm of iron and 15 ppm of manganese, so more iron does not mean the biggest tank: your test moves you between the 1.5 and the 2.5 cubic foot, and it tells me whether your pH needs correcting first. Here is how I size one when someone sends me their test.

Caller "So you have to back flush it less or what?"

Aidan "No, it just has higher capacity and it accepts higher flow rates. If you're doing a large irrigation field, you'd use a 3.5. Most of the time for residential we use the 2.5, and occasionally for smaller homes and less water usage, we use the 1.5 for lower levels of iron."

A caller in Ohio asked me that in July while deciding between two of our sizes. This page is the long version of my answer.

How iron or manganese changes the size

Iron and manganese decide where you fall between the 1.5 and the 2.5, not whether you need the biggest tank. In 32 years in this business, these are the four things that matter, in the order I check them: your iron level, how many people live in the house, how many bathrooms get used at the same time, and how much water your well can give the filter to backwash.

Iron is measured in parts per million (ppm) on a water test. The higher the iron, the harder the media bed works between backwash cycles, and the more capacity you want in the tank. The media bed is the Katalox Light filter media inside the tank. With 1 or 2 bathrooms, 2 people in the house and about 3 to 5 ppm of iron, the 1.5 cubic foot is pretty much all you need. A family of four with three bathrooms, high water use and 5 to 10 ppm of iron or higher gets the 2.5 from me every time.

Manganese counts about double. Watch Water, the company that makes Katalox Light, rates it to hold about 85,000 mg of iron per cubic foot of media but only about 42,500 mg of manganese. So a ppm of manganese uses up the media about twice as fast as a ppm of iron. That is my arithmetic from their sheet, not a rule they print. It rarely changes the size: manganese shows up in about 6 in 10 of the well tests sent to us that have iron (53 of 91), but it is usually a fraction of a ppm, and the highest since April was about 6.

Manganese also changes the pH you need. Katalox Light removes iron best near pH 8, and as I tell people, "If you have any manganese, you have to get it to 8.5." The filter raises the pH some on its own. When your pH is low, an acid neutralizer goes in too, and I set its place from your test.

Every size we sell handles up to 30 ppm of iron, 15 ppm of manganese and 10 ppm of hydrogen sulfide (the rotten egg smell), so the 3.5 is not the high-iron size. At about 30 ppm of iron, I put two 2.5 tanks in a row, because on high-usage days one tank can let iron through. More on the two metals is in iron and manganese in well water, and on the kinds of iron in types of iron in well water.

Which size fits how many people

Count the people who live in the house, then check that size against your well. A big house with 2 people in it still uses the water of 2 people.

Bathrooms matter only when people use them at the same time. Six people with three bathrooms can all be in the shower getting ready for work or school, and that pushes you to the 2.5. A lot of bathrooms with only a few people does not: I have the 2.5 in houses with seven full bathrooms and ten people.

If you are between the 1.5 and the 2.5 and your well can backwash the 2.5, take the 2.5. It holds more media, so it goes longer between media changes: a little better than 6 to 8 years. What the whole job costs over the years, media included, is in the iron filter cost guide.

In the video I say 99% of customers buy the 2.5. That overstates it. The 2.5 is the size we ship most, and the 1.5 is next.

Whether your well can backwash the tank

A bigger tank needs more water from your well to clean itself, and a well that can't give it will foul the media. Every so often the valve runs a backwash: it sends water up through the tank, lifts the Katalox Light and rinses the trapped iron down the drain. Why it does that, and where the water goes, is in why does an iron filter backwash.

Watch Water says Katalox Light needs 10 to 12 GPM of backwash for every square foot across the tank, so the wider the tank, the more water it takes. Their sheet lists about 6 GPM for a 10 inch wide tank, 10.5 for a 13 inch tank and 16 for a 14 inch tank. In the video I put the 3.5 at 15 GPM. Most wells I see give about 8 to 10 GPM, which backwashes the 2.5 and keeps the tank clean. That is the main reason the 2.5 is my usual pick.

A weaker well can still run a bigger tank if the backwash runs longer, and I set that for you. A caller in Connecticut with 3 people had a pump making about 7 GPM. I told him I'd probably go with the 2.0, and that the 2.5 would work too if the backwash ran 25 minutes instead of 14. What I would not do is put the 3.5 on a 5 GPM well. On that little flow you could get fouling of the media bed, and I don't think you want to take that tank outside, dump it, clean it and start all over again. How many gallons a minute the house itself uses at its busiest is in whole house water filter flow rate sizing.

How to find your well's flow rate

1. Check the well record. The county or the health department usually has the flow rate from when the well was drilled.
2. Or time a bucket. Run a garden hose from an outside spigot, or a short hose off the well tank, into a 5 gallon bucket and time how long it takes to fill. Let the water run until the pump comes on first, so you time the well and not the water stored in the pressure tank.
3. Work out the gallons per minute. Divide 300 by the seconds it took. A caller in July filled his bucket in 15 seconds, which is 20 GPM: plenty for any size we sell.

Have your test and your well's flow?

Text Aidan your water test numbers

Text a photo of your water test, how many people live in the house and your well's flow rate. Aidan will tell you which size fits and whether your pH calls for a neutralizer too.

Text Aidan443-277-2204 Call800-460-5810Aidan

8 AM to 5 PM Eastern, seven days a week. A real person, not a call center.

Sizes I recommended for real homes

These are real calls from this year.

What goes wrong with the wrong size

A tank that is too small lets iron through, and a tank too big for your well fouls the media. Too small, and it's going to be overwhelmed by iron, and you're going to get bleed through: iron slips past the media into the house, and the orange stains come back even though you have a filter.

Too big for the well, and the backwash can't lift and rinse the media, so the trapped iron stays in the tank and the media fouls. The fix is the one I described above: take the tank outside, dump it, clean it and fill it with new media. If you are between two sizes and your well can backwash the bigger one, go bigger. How the air and the media take the iron out is in how does an iron filter work, and where it goes in line with your other equipment is in the correct order for well water treatment systems. The whole job, from test to install, is in the complete iron filter guide.

What buyers wrote after installing one

These are verified purchases of our iron filters, newest first.

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