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Rainwater Harvesting Tank Size Calculator: Demand-Supply Balanced Sizing, Not a Naive Total

Sizing a rainwater tank to your full annual harvest produces an impractically huge, expensive tank that spends most of the year sitting mostly empty. This water storage tank calculator works the way reservoir engineers actually size storage, using a sequent-peak (Rippl mass-curve) balance between your monthly rainfall supply and steady daily demand, finding the tank size just large enough to carry you through the single longest dry-season deficit run, not a generic multiple of monthly rainfall. As a complete roof catchment calculator, it starts from your roof area and material, RCC, tiled, metal/GI or green roof, each with its own runoff coefficient, always editable. Switch to Monthly Pattern mode with a regional preset like the Indian monsoon and the tool shows you month by month exactly which months run a deficit and by how much. It also estimates the first-flush diverter volume to discard the initial dirty roof-wash and shows how much harvested water would overflow your chosen capacity and need a recharge pit instead of going to waste.

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How to Use

Step 1: Set up your roof catchment

• Enter your Roof Area in square metres, the plan (footprint) area of your catchment surface.
• Select your Roof Material: RCC Flat Roof (~0.85), Tiled/Sloped (~0.75), Metal/GI Sheet (~0.90) or Green Roof (~0.30). This pre-fills the runoff coefficient for that surface.
• Check the Runoff Coeff. field that fills in automatically and adjust it directly if you have a more specific value for your exact roof condition, since real coefficients vary with slope, surface wear and maintenance.

Step 2: Choose how you want to enter rainfall

• Select Annual Total if you only have one overall figure and enter your Annual Rainfall in mm.
• Select Monthly Pattern instead for a far more realistic tank size, since two locations with the same annual total but very different seasonal spread need very differently sized tanks.
• In Monthly Pattern mode, optionally pick a Load pattern preset: Indian Monsoon (SW), Fairly even or Winter-dominant, to auto-fill a representative starting shape, then adjust any individual month’s rainfall figure directly to match your actual local data.

Step 3: Set your demand and first-flush

• Enter your Water Demand in litres per day, your household’s steady daily consumption that the tank needs to supply.
• Enter your First-Flush value in mm per m², the depth of initial roof-wash water you want diverted away before it reaches storage, since the first rain after a dry spell washes dust, bird droppings and debris off the roof. Roughly 0.5 to 1 mm is a common range.

Step 4: Calculate and read your result

• Tap Calculate Tank Size. In Annual Total mode, the result shows your total annual harvest potential, the first-flush volume to discard, and a recommended tank size.
• In Monthly Pattern mode, the result additionally shows a full month-by-month table, each month’s rainfall, harvested supply, demand and net surplus or deficit, the number of dry-deficit months and the demand-supply balanced tank size calculated using the sequent-peak method across your specific seasonal pattern, along with the overflow volume beyond that capacity that should feed a recharge pit rather than being lost.

Step 5: Export

• Use Print / PDF for a clean printable copy or Copy to paste the figures into your project report.

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Key Features

• Roof material presets for RCC, Tiled/Sloped, Metal/GI and Green Roof, each with its own runoff coefficient, always editable
• Two rainfall input modes, a quick Annual Total or a full Monthly Pattern with regional presets, Indian Monsoon, Fairly even or Winter-dominant
• Sequent-peak (Rippl mass-curve) balanced tank sizing, the same principle used to size reservoirs, finding the smallest tank that bridges the longest actual dry-season deficit
• Month-by-month deficit table, showing exactly which months fall short and by how much, not just an annual summary
• First-flush diverter volume, keeping the initial dirty roof-wash out of your stored water
• Overflow estimate beyond tank capacity, pointing you toward a recharge pit instead of wasted runoff
• Clear flag if annual demand exceeds annual harvest entirely, since no tank size can bridge that gap
• Export as PDF or copy the result.

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Formula / Logic Used

Harvest Potential

$$Harvest\ (L) = Roof\ Area\ (m^2) \times Rainfall\ (mm) \times Runoff\ Coefficient$$

This works because 1 mm of rain falling on 1 m² of catchment delivers exactly 1 litre of water before losses.

First-Flush Volume

$$First\text{-}Flush\ (L) = Roof\ Area\ (m^2) \times First\text{-}Flush\ Depth\ (mm)$$

Sequent-Peak (Rippl Mass-Curve) Balanced Tank Size

For each month, cumulative net supply minus demand is tracked and the required tank size equals the largest cumulative deficit that occurs anywhere across the sequence, the single deepest drawdown between a supply surplus and the point it’s fully used up :

cumulative[i] = cumulative[i-1] + (monthly_supply[i] - monthly_demand[i])
required_tank_size = max(0, -min(cumulative[i]) across all months)

This finds the storage volume needed to carry demand through the worst dry run in your specific rainfall pattern, rather than assuming a fixed multiple of average monthly rainfall that may not match your actual seasonal spread.

Overflow

$$Overflow = \max(0, \ Total\ Harvest - Tank\ Capacity - Total\ Demand\ Met)$$

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Who Should Use This Tool

Homeowners and site engineers planning a rainwater harvesting system who want a tank sized to actually match their local rainfall pattern rather than an oversized, expensive guess. Also useful for diploma and B.tech civil and environmental engineering students learning runoff coefficients and mass-curve reservoir sizing methods for their coursework.

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Frequently Asked Questions (FAQs)

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Related Tools

• Water Treatment Dosing Calculator – for treating harvested water before use
• Carbon Footprint Calculator – another environmental planning tool for the same site
• Solid Waste Generation Estimator – plan waste management alongside water harvesting