Flow rate at a glance
- Advertised
- New filter, clean water
- Real
- Drops as it clogs
- Restored by
- Backflushing
Why flow rate decides whether you use a filter
A filter's flow rate decides whether you use it. A slow filter turns every water stop into a chore, and the practical outcome is that people drink less and refill less often, which is a real problem on a hot day. It matters more than most specs on the box.
The same words do a second job on a hydration reservoir, where flow is set by the drink tube's bore and the bite valve's opening rather than by an element. Nothing clogs there, so a slow reservoir is slow by design and the fix is a wider tube, not a backflush.
Why the advertised figure is a best case
The number on the packaging is a best case: a new element, clean water, ideal conditions. Field flow is always lower and falls further as the pores load up with sediment. Silty water clogs a filter fast, and glacial melt with rock flour in it can halve the flow in an afternoon.
What you can actually do about it
Most of what you can do about it is upstream. Let cloudy water settle, or pre-filter it through a bandana or the coffee-filter trick, before it reaches the element. Collect from the clearest part of the source rather than the bank. And backflush regularly rather than waiting until the flow is unbearable, since a badly clogged element is much harder to recover.
What this means when you're choosing
- Whether the quoted rate is gravity-fed or squeezed, which are very different
- How easy the filter is to backflush in the field
- Group size, since a slow filter is fine solo and painful for four
The common mistake
Reading the advertised figure as what you will get
It is measured on a brand-new element with clear water. Real flow starts lower and falls from there. Judge a filter on how fast it is after a few days and how easy it is to backflush, not on the number on the box.
Common questions
Why has my water filter got so slow?
Sediment loading the pores, almost always. Backflush it thoroughly, and start pre-filtering cloudy water through a cloth before it reaches the element.
Does a slower filter clean water better?
No. Flow rate and filtration are set by different things: pore size determines what's removed, and flow depends on surface area and how clogged it is.