📖 How To Use
How to Use This First Flush Diverter Size Calculator
Sizing a first flush diverter correctly takes less than a minute with the right inputs:
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Enter your roof catchment area
Measure the plan (horizontal) area of your roof that drains into a single downpipe system. Use the area unit selector to switch between m², ft², or yd².
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Set the runoff coefficient
This accounts for how much rainfall actually runs off versus evaporates or absorbs. Metal roofs: 0.90–0.95. Tiled roofs: 0.75–0.90. Green roofs: 0.15–0.50. The default 0.9 suits most metal and tile roofs.
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Choose your flush rate
The standard is 1 litre per m² of roof area (or 0.02 gal/ft²). Some authorities require up to 2 L/m² for heavily trafficked or pollution-prone areas. Check your local rainwater harvesting guidelines.
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Set pipe diameter and number of downpipes
Enter the internal diameter of your first flush chamber pipe (typically 90–150 mm / 4–6 in). If you have multiple downpipes draining the same roof, enter the count — the volume is divided evenly across them.
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Read your results
The calculator outputs total flush volume, volume per downpipe, required pipe length, and bore area. Use the pipe length figure to cut and install your first flush standpipe.
Tip: Always round the pipe length up to the nearest 50 mm (2 in) when cutting. A slightly oversized chamber catches a little more contaminated first flush — it never hurts. Undersizing means polluted water reaches your storage tank.
📐 The Formula
First Flush Diverter Size Formula Explained
The calculation involves two steps — determining the required flush volume, then working out the pipe length to hold that volume:
Step 1 — Total Flush Volume (L)
V_flush = Roof Area (m²) × Runoff Coefficient × Flush Rate (L/m²)
Step 2 — Volume per Downpipe (L)
V_per_pipe = V_flush ÷ Number of Downpipes
Step 3 — Required Pipe Length (m)
L_pipe = V_per_pipe ÷ (π × (D/2)² × 1000)
where D = internal pipe diameter in metres
The pipe length formula simply calculates how tall a cylinder of the chosen diameter must be to hold the required flush volume. The factor of 1000 converts litres to cubic metres.
Worked Example
Roof area: 80 m², runoff coefficient: 0.9, flush rate: 1 L/m², pipe diameter: 100 mm, 1 downpipe.
V_flush = 80 × 0.9 × 1 = 72 litres
V_per_pipe = 72 ÷ 1 = 72 L
Pipe bore area = π × (0.05)² = 0.007854 m²
L_pipe = 0.072 ÷ 0.007854 ≈ 9.17 m
Flush Rate Reference by Location & Roof Type
| Context | Flush Rate | Reason |
|---|
| Clean rural metal roof | 0.5–1 L/m² | Low dust and pollution load |
| Standard suburban tile | 1–1.5 L/m² | Bird droppings, dust, leaves |
| Urban area / near roads | 1.5–2 L/m² | Higher particulate and oil deposits |
| Heavy tree canopy | 2 L/m² | Leaf tannins, insects, organic matter |
| Industrial / coastal | 2–2.5 L/m² | Airborne chemical deposits |
Common Pipe Diameters & Their Bore Areas
| Diameter | Bore Area (cm²) | Volume per metre (L) |
|---|
| 75 mm (3 in) | 44.2 cm² | 4.42 L/m |
| 90 mm (3.5 in) | 63.6 cm² | 6.36 L/m |
| 100 mm (4 in) | 78.5 cm² | 7.85 L/m |
| 110 mm (4.3 in) | 95.0 cm² | 9.50 L/m |
| 150 mm (6 in) | 176.7 cm² | 17.67 L/m |
| 200 mm (8 in) | 314.2 cm² | 31.42 L/m |
🌧️ When to Use
When You Need a First Flush Diverter
A first flush diverter is essential whenever you intend to store or use rainwater for any purpose beyond simple garden irrigation of non-edible plants. The first millimetres of rain wash accumulated dust, bird droppings, organic matter, and atmospheric pollutants off your roof — and into your storage tank if no diverter is fitted.
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Household Potable Supply
Required before any treatment system (filters, UV) if water will be used for drinking, cooking, or bathing.
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Edible Garden Irrigation
WHO guidelines recommend first flush diversion for produce that will be consumed raw to reduce pathogen risk.
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Appliance Longevity
Tannins and particulates from first flush damage washing machines, dishwashers, and heat exchangers over time.
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New Construction
Concrete and construction dust on a new roof can leach into storage for months without a diverter fitted from day one.
You may be able to skip a first flush diverter for greywater toilet flushing or industrial cooling water where contamination thresholds are higher — but it is still best practice to install one and protect downstream equipment.
❓ FAQ
Frequently Asked Questions
What is a first flush diverter and how does it work?
A first flush diverter is a pipe-based device fitted to a downpipe that captures the initial, most contaminated portion of each rainfall event before it reaches the storage tank. When rain begins, the standpipe fills with dirty first-flush water. Once the standpipe is full, a floating ball seals the inlet and clean water diverts to the tank. The standpipe slowly drains between rain events via a small bleed hole (typically 3–5 mm), resetting it for the next storm.
How do I calculate first flush diverter size for my roof?
Multiply your roof catchment area (m²) by your runoff coefficient (typically 0.9) and your design flush rate (commonly 1 L/m²). This gives the total first flush volume in litres. Then divide by the number of downpipes to get volume per chamber. Finally, divide by the cross-sectional area of your chosen pipe (in m²) to get the required pipe length. This calculator automates all three steps.
What flush rate should I use — 1 L/m² or 2 L/m²?
The standard recommendation for most residential roofs is 1 litre per m² of catchment area. Increase to 1.5–2 L/m² if your roof is near busy roads, under heavy tree canopy, in an urban environment with high airborne particulates, or if the roof hasn't been cleaned in over six months. Some Australian state guidelines and Texas AgriLife guidance specify 1 L/m² as the baseline minimum.
What pipe diameter should I choose for my first flush diverter?
The most common sizes are 100 mm (4 in) and 150 mm (6 in) PVC stormwater pipe, which are widely available and inexpensive. A larger diameter means a shorter, more compact chamber — but if the diameter is too large relative to the downpipe, the ball float may not seal properly. Match your first flush chamber diameter to your downpipe diameter, or one size larger at most. Use this calculator to compare the resulting pipe lengths across different diameter options.
How long does it take for the first flush diverter to drain and reset?
Drain time depends on the bleed hole size and the volume in the chamber. A 4 mm bleed hole in a 100 mm diameter, 9 m long chamber (≈70 L) typically drains in 8–24 hours depending on temperature and water pressure head. This is long enough to ensure the chamber is empty before the next significant storm but fast enough to not trap water for days (which would encourage mosquitoes or algae). Size the bleed hole at 3–5 mm for most installations.
Do I need a first flush diverter if I already have a roof filter?
Yes — a first flush diverter and a roof washer filter serve different functions and should be used together for potable or near-potable use. The diverter removes the initial high-contaminant water before it enters your system at all. A filter then removes fine sediment and debris from the cleaner water that follows. Relying on a filter alone to handle first-flush contamination will clog your filter rapidly and may not remove dissolved pollutants or pathogens.
Can I use this calculator for a commercial or large-scale rainwater system?
Yes. Enter the total roof area served by a single downpipe (or the total area divided by the number of downpipes if they're evenly distributed). For commercial buildings, consider using a 1.5–2 L/m² flush rate, and use the number of downpipes field to split the volume correctly across all chamber installations. For very large roofs (>2,000 m²), consult a civil engineer — you may need custom fabricated chambers rather than standard PVC pipe sections.