Rainwater Harvesting Calculator

Rainwater Harvesting Calculator – What It Is, Why It’s Important, and How to Use It in House Construction (India)

What is a Rainwater Harvesting Calculator?

A Rainwater Harvesting Calculator is a simple tool used to estimate how much rainwater you can collect from your roof or other catchment areas during the rainy season.

It calculates the total water you can harvest in litres per year, based on the roof size, amount of rainfall, and roof type (which affects how efficiently water is collected).

 Basic Formula:

Rainwater Collected (litres) = Roof Area (sq.m) × Rainfall (mm) × Runoff Coefficient × 0.95

This tool is very helpful in planning a proper rainwater harvesting system at home or in buildings.

 Why is Rainwater Harvesting Important in House Construction in India?

India faces frequent water shortages, especially during summer and in cities with growing populations. At the same time, monsoons bring heavy rainfall which usually goes to waste.

 Benefits of Rainwater Harvesting in India:

Saves Water: Reduces dependency on borewells or municipal water.

Reduces Water Bills: Especially in urban homes and apartments.

Recharges Groundwater: Helps improve water levels in wells and borewells.

Prevents Waterlogging: Reduces surface runoff and soil erosion.

Required by Law: Many cities like Delhi, Bangalore, Chennai, etc., mandate rainwater harvesting for new buildings.

According to the Central Ground Water Board (CGWB), rainwater harvesting is one of the best ways to combat declining groundwater levels in India.

How to Make a Rainwater Harvesting System in a House (Step-by-Step)

Here’s how you can set up a simple and effective rainwater harvesting system for your home:

 Step 1: Measure the Catchment Area (Roof)

The roof is your primary rain collection surface. Measure the length and width of the roof and calculate the area in square meters.

For example: A 10m × 10m roof = 100 sq.m

 Step 2: Know the Annual Rainfall

Find the average annual rainfall in your city. You can get this from local authorities or weather websites. In India, average rainfall ranges between 400 mm (Rajasthan) to 2000+ mm (North-East India).

Example: Delhi receives around 800 mm annually.

 Step 3: Install Gutters and Downpipes

Fix gutters around the edge of your roof to collect rainwater and direct it through downpipes to a storage tank or recharge pit.

• Use mesh screens to filter leaves and debris.
• Make sure the pipes are sloped for easy flow.

 Step 4: First Flush System (Very Important)

Install a first flush diverter to discard the first few litres of dirty rainwater (contains dust, bird droppings, etc.) from the roof.

This keeps your storage tank or recharge pit clean.

 Step 5: Choose Your Storage Method

You can store the water in 2 main ways:

 Storage Tank

• A clean, covered tank to store filtered rainwater for direct use (gardening, washing, flushing toilets, etc.).
• Add a filter chamber before the tank (sand, charcoal, gravel).

 Recharge Pit

• A pit or borewell to recharge groundwater.
• Use layers of gravel, coarse sand, and charcoal for natural filtration.
• Cover with a concrete or metal lid.

Step 6: Maintenance

• Clean roof and gutters regularly (especially before monsoon).
• Empty the first flush chamber after every rainfall.
• Check for cracks or leaks in the system.

 How the Rainwater Harvesting Calculator Helps

Using a rainwater harvesting calculator, you can:

• Estimate your water collection capacity
• Decide tank size or recharge pit depth
• Plan your plumbing design
• Compare cost vs benefit of setting up a system

Example Using the Calculator:

• Roof Area = 120 sq.m
• Rainfall = 850 mm/year
• Roof Type = RCC (Runoff Coefficient = 0.85)

Rainwater Collected = 120 × 850 × 0.85 × 0.95 = 81,945 litres/year

That’s enough water to run a 4-person household for 3–4 months!

Materials You May Need:

• PVC or GI Gutters and Downpipes
• Mesh filter
• First flush valve
• Storage tank or HDPE drum
• Sand, charcoal, gravel (for filters)
• Cement and bricks (for recharge pit)

Government Support & Rules

• In many Indian states, rainwater harvesting is mandatory for new homes above a certain plot size.
• You may also get subsidies or technical support from municipal corporations or Jal Shakti Ministry.

 Final

Rainwater harvesting is not just an eco-friendly choice — it’s becoming a necessity in Indian house construction. By installing a simple system and using a rainwater harvesting calculator, you can save water, reduce costs, and help recharge groundwater.

Whether you are building a new house or renovating an old one, investing in rainwater harvesting is a smart, long-term solution — for your home and for the planet.