If your home runs on a rainwater cistern, the usual sizing shortcuts do not work well. A city or well home draws from a supply that arrives on demand. A cistern home draws from a fixed reserve that refills only when it rains, and the treatment equipment sits between that reserve and your taps. The question is not only how much water the house uses, but how fast it is drawn and how much is left in storage during the dry weeks.
Start with the draw you actually make
Write down every fixture that runs in your home: kitchen tap, bathroom sinks, shower, toilet fill, washer, dishwasher, and any outdoor hose or irrigation line. For each one, note how many people use it on a typical day and roughly how long each use lasts. If you can, keep a simple count for a week. A rough daily total is enough to start. Then note the busiest hour, such as a morning when two showers and a load of laundry overlap, because peak draw is what tests the equipment.
Match the peak, not only the average
Treatment equipment has a rated flow, and that rating only helps if your peak demand stays under it. Ask each provider for the rated flow at the pressure your pump actually delivers, and compare it with the simultaneous-use figure you wrote down. If the house can run three taps at once, the unit has to handle three taps at once without the flow dropping to a trickle. Check whether the rating is for continuous flow or for short bursts, since a cistern pump often cycles on and off.
Check the storage and the pump together
The cistern feeds a pump, and the pump feeds the treatment. Ask how the pump is sized against your peak draw and whether it has a pressure tank that smooths the flow. Ask where the treatment sits in the line, before or after the pressure tank, and why. Placement changes the pressure and flow the treatment sees, so a provider who cannot explain the order is not ready to size the job. Also ask what happens when the cistern runs low, because a pump that runs dry can be damaged regardless of the treatment.
Plan for the dry stretch
Rain-fed supply has low periods. Work out how many days of your normal use the cistern can cover without new rain, using the cistern's storage volume and your daily total. Treat the number as a warning. If the reserve is short, the treatment sizing matters less than a supply plan. Ask a provider to walk you through how the system behaves when the reserve is low, and whether any part of the treatment needs a minimum flow to work as intended.
What to bring to the conversation
Bring the fixture list, your peak-use estimate, the pump model and its rated output if you know them, the cistern size, and a simple sketch of where the cistern, pump, and house supply line meet. Ask for the sizing in writing, with the rated flow, the flow under your peak conditions, and the pressure assumed. Compare providers on those points rather than on the headline capacity of the unit. To see how we review providers, read our methodology page at https://missouriwaterexperts.com/methodology.html. To find providers who serve your area, start with our directory at https://missouriwaterexperts.com/index.html.