Rapid Sand Filter Area Calculator

Work out rapid sand filter area instantly with clear inputs, formula shown and shareable results.

About Rapid Sand Filter Area Calculator

Total filter area is design flow divided by the filtration rate over 24 hours. That area is split into units small enough to backwash conveniently, usually under 40-50 square metres each, with at least four units so that one can be washed without pushing the others past their maximum rate.

Formulas

Filter area

Total area (m2) = flow (m3/day) / (24 x filtration rate (m/h))

Unit count

Filters = ceil(total area / maximum area per filter) + standby

Worked example

With the default values below, Rapid Sand Filter Area Calculator returns Total Filter Area Required of 52.08. Change any input above and every figure here is recalculated in your browser.

Inputs used

Design Flowminimum 0 m3/day: 10,000 m3/day
Filtration Rateminimum 0 m/h: 8 m/h
Standby Filtersminimum 0: 1
Maximum Area Per Filterminimum 0 m2: 40 m2

Results

Total Filter Area Required: 52.08
Number Of Filters: 3
Area Per Filter: 26.042
Rate With One Filter Out Of Service: 8.0000

Step by step

1Values used
2Filter area
3Unit count
4Total Filter Area Required= 52.08
5Number Of Filters= 3
6Area Per Filter= 26.042
7Rate With One Filter Out Of Service= 8.0000

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why at least four filters?

So that removing one for backwash raises the rate on the others by only a third. With two filters, washing one doubles the rate on the other, which is unacceptable.

What is filter-to-waste?

Discarding the first few minutes of filtrate after a backwash, when turbidity spikes as the bed re-ripens. It is an important pathogen barrier and is required by some regulators.