Iron Filter Sizing Calculator: Iron PPM and Backwash

Flow first, chemistry second

An iron filter fails in two different ways. The bed can be too small for the peak, so untreated water channels through. Or the well can be too weak for the backwash, so the bed compacts and pressure drops. Both show up as “the filter did not work.” They are not the same repair.

Cross-section sets both ratings. A 10-inch tank has about 0.55 square feet of bed. At 5 gallons per minute per square foot it serves about 2.7 gpm continuously. At 12 gallons per minute per square foot it wants about 6.5 gpm to backwash. A two-bathroom peak near 9 gpm does not fit that continuous rate in one 10-inch shell. That is why the intermittent allowance exists, and why it is labeled as an allowance you may use only when the sheet agrees. Which iron you are filtering is a separate question, answered in the iron filter for well water guide. This page is the sizing step that guide did not include.

Iron, manganese, and pH

Clear-water ferrous iron is what an air-injection filter is built to oxidize and catch, once pH is high enough for the reaction to finish inside the tank. Red-water ferric iron is already a particle; sediment filtration may be the first stage. Manganese is slower. Above the 0.05 mg/L secondary standard it stains black and gray, and an iron setting on an air filter often leaves it. The manganese in well water guide is the companion, not a duplicate of this calculator.

Pounds of metal per year use a straight conversion: annual gallons times ppm times 8.34, divided by one million, at 80 gallons per person per day. It tells you the load. It does not tell you the year the media dies. Slime from iron bacteria blinds a bed no matter how large it is. That job starts with shock chlorination and oxidation, which the iron bacteria guide covers. Do not size a filter as the first step on a slimy well.

Worked shells

These use the same function as the form. They are examples of the math, not recommended systems.

Where the filter sits

Sediment comes off first so the iron bed is not packed with sand. A neutralizer, if pH is low, comes before air injection. The softener comes after the iron is gone, because iron fouls resin. UV, if bacteria are the reason you are treating, comes last on clear water. The sediment filter sizing calculator, the UV sizing calculator, and the softener sizing calculator cover those stages. The system selector puts them in order.

If the well's sustained flow is the open question, measure it before you order the tank. The well recovery rate calculator turns a short test into specific capacity and a casing-refill rate. It will not invent a backwash the aquifer cannot give you.