How is peak demand flow estimated from fixture units?

The tool uses a simplified Hunter-curve fit for residential loads, where peak GPM rises with the square root of the water supply fixture unit count. It enforces a four GPM minimum so even small homes get a realistic peak figure for sizing filters.

What grain capacity softener do I need?

Daily softening load equals daily gallons times hardness in grains per gallon. Multiplying by the days you want between regenerations gives the grain capacity. Round up to the next standard size such as 24,000, 32,000, or 48,000 grains.

How do I convert mg/L hardness to grains per gallon?

Divide hardness in milligrams per liter (or ppm as calcium carbonate) by 17.1 to get grains per gallon. For example, 257 mg/L is about 15 grains per gallon, a common moderately hard supply.

Why does regeneration frequency matter for sizing?

Regenerating too often wastes salt and water, while too rarely risks hard water breakthrough. Sizing for a three to seven day cycle balances salt efficiency and reserve. Fewer regenerations need a larger resin bed and higher grain capacity.

What permeate quality should I expect from an RO membrane?

Thin-film composite RO membranes reject roughly 95 to 99 percent of dissolved solids. At 97 percent rejection a 500 mg/L feed yields about 15 mg/L permeate. The tool shows the expected permeate TDS for your chosen rejection rate.

Water Treatment Equipment Flow Rate Calculator

Sizing residential water treatment means matching equipment to two different demands at once: the peak flow that filters and RO systems must pass without pressure loss, and the daily contaminant load that sets softener capacity and regeneration frequency. This calculator works out both from a handful of household figures.

How it works

Peak flow comes from the fixture-unit count, while softener capacity comes from daily usage and water hardness:

peak GPM        ≈ √(fixture units) × 1.9 + 1   (min 4 GPM)
daily gallons    = occupants × gallons per person per day
daily load       = daily gallons × hardness (grains/gal)
grain capacity   = daily load × days between regenerations
RO permeate TDS  = feed TDS × (1 − rejection)

The fixture-unit term is a smooth fit to the Hunter curve for small loads. The softening side uses the fact that one grain per gallon equals 17.1 mg/L of hardness as calcium carbonate, so daily grains times the regeneration interval gives the resin bed capacity you need.

Example and tips

A four-person home using 75 gallons per person per day at 15 grains per gallon hardness generates about 4,500 grains of softening load daily, so a four-day cycle needs roughly an 18,000 grain capacity, met by a standard 24,000 grain softener. At 28 fixture units the peak flow is around 11 GPM, which sets the minimum filter and RO booster sizing. Choose softener and filter ratings at the next standard step above the calculated figures, and confirm the RO permeate TDS meets your target before specifying the membrane.