Interactive Calculator · 2026
Clarifier Size Calculator
Size a secondary clarifier from your flow and surface overflow rate (SOR). Enter the design flow and the overflow rate, and get the required tank diameter — the number every clarifier layout starts from.
Required surface area falls as the surface overflow rate rises
Design inputs
Average daily flow reaching the clarifier.
Typical secondary clarifiers use ≈3–4 m.
The design overflow rate — secondary clarifiers usually sit around 16–32 m³/m²·day.
How surface overflow rate drives the size
Required surface area across the practical SOR band — your selected rate is highlighted.
Surface area is set almost entirely by the overflow rate: a higher SOR packs more flow onto each square metre, so the tank shrinks — but push it too far and settling suffers.
At 200 m³/day and 24 m³/m²·day you need ≈8.3 m² — a 3.3 m diameter circular clarifier.
How this is calculated
- Surface area = flow ÷ SOR = 200 ÷ 24 = 8.3 m².
- Diameter = √(4 × area ÷ π) = √(4 × 8.3 ÷ π) = 3.3 m.
- Tank volume = area × side water depth = 8.3 × 3 = 25 m³.
Indicative sizing for concept planning. A detailed design must also check solids loading, weir loading and peak flow — confirm with a qualified consultant before procurement.
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