Whole House Filtration Calculator

No -- Peak Flow Demand and System Pipe Size are both driven entirely by Number of Bathrooms in this calculator, using a fixed rule of thumb (5 GPM for the first bathroom, plus 3 GPM per additional bathroom). Household Size and the water-quality inputs never move those two outputs.

Just above 3 grains per gallon (GPG), the standard boundary between "moderate" and "hard" water -- the condition is strictly greater than 3, not "3 or more." At exactly 3 GPG the calculator recommends no softener at all; at 3.01 GPG and above, softener sizing and its associated annual salt cost switch on.

Water Hardness, among this calculator's water-quality inputs. It determines both whether a softener is needed and, once needed, how large it has to be -- and softener salt is the one cost in the Annual Maintenance Cost total that scales with hardness.

The calculator reports the actual grains-equivalent capacity your household needs, rounded up to a multiple of the largest standard 64,000-grain unit, rather than capping the number at 64,000 regardless of how much more capacity is actually required -- a large household on very hard water can genuinely need multiple standard units' worth of capacity. Softener Units Required reports how many physical 64,000-grain units that is, and Equipment Cost scales with that count (each additional unit adds another unit's installed cost), matching how Annual Maintenance Cost -- which folds in the softener's salt cost -- already scaled with total capacity. Multiple Softener Units switches to 1 as a plain flag for this unusual, larger-than-typical residential case, so a sudden jump in Equipment Cost doesn't look like an error.

No. Iron Level only determines whether an iron filter is recommended (above 0.3 PPM) and its associated equipment cost -- it has no effect on Peak Flow Demand or System Pipe Size, both of which depend only on the number of bathrooms.