Rainwater Harvesting Calculator

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See how much rainwater your roof can collect in a year and what size storage tank or recharge pit you need.

Inputs

Roof area (sq ft)

Annual rainfall (mm)

Roof type

People in the house

How to use this calculator: Rainwater Harvesting Calculator

See how much rainwater your roof can collect in a year and what size storage tank or recharge pit you need. The example below is calculated by this page's real engine from the displayed inputs.

1Confirm that the Rainwater Harvesting Calculator matches the quantity or design check you need.
2Enter Roof area, Annual rainfall, and People in the house using the units printed beside each field.
3Select the applicable Roof type options; these choices change the calculation method or factors.
4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

Input	Example value	Why it matters
Roof area	1200 sq ft	Measured or known roof area used by the calculation engine.
Annual rainfall	900 mm	Measured or known annual rainfall used by the calculation engine.
Roof type	Concrete / terrace (0.85)	Select the option that matches the real installation or scenario.
People in the house	4	Measured or known people in the house used by the calculation engine.

Formula inputs & variables for Rainwater Harvesting Calculator

These are the named quantities used by this calculator. When the source formula does not define a mathematical symbol, OneCalcApp keeps the real input label instead of inventing one.

Variable / input	Unit	Meaning in this calculation
Roof area	sq ft	Measured or known roof area used by the calculation engine.
Annual rainfall	mm	Measured or known annual rainfall used by the calculation engine.
Roof type	—	Select the option that matches the real installation or scenario.
People in the house	—	Measured or known people in the house used by the calculation engine.

How the Rainwater Harvesting Calculator works

The Rainwater Harvesting Calculator uses Roof area, Annual rainfall, Roof type, and People in the house to calculate Rainwater harvested per year, Share of family demand, Suggested storage tank, Roof area, and Runoff coefficient used. Its engine applies Harvest = roof area × rainfall × runoff coefficient; the worked values below come from that same live calculation rather than a separately typed example.

With Roof area 1200 sq ft, Annual rainfall 900 mm, Roof type Concrete / terrace (0.85), and People in the house 4, the main worked-example result is Rainwater harvested per year = 85,285 litres.

How each Rainwater Harvesting Calculator input is used

Roof area

The Rainwater Harvesting Calculator worked example uses Roof area = 1200 sq ft. This value is passed directly into the calculation, with an allowed minimum 10.

Annual rainfall

The Rainwater Harvesting Calculator worked example uses Annual rainfall = 900 mm. This value is passed directly into the calculation, with an allowed minimum 10.

Roof type

The Rainwater Harvesting Calculator worked example selects “Concrete / terrace (0.85)”. Available choices include Concrete / terrace (0.85), Tiled roof (0.75), and Metal sheet (0.9). This selection may change the method or factor used by the engine, so choose the option that matches the real case.

People in the house

The Rainwater Harvesting Calculator worked example uses People in the house = 4. This value is passed directly into the calculation, with an allowed minimum 1.

Rainwater Harvesting Calculator formulas and result interpretation

Formula 1: relationship used

In the Rainwater Harvesting Calculator, Harvest = roof area × rainfall × runoff coefficient. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 2: relationship used

In the Rainwater Harvesting Calculator, 1 m² × 1 mm of rain = 1 litre. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 3: relationship used

In the Rainwater Harvesting Calculator, Storage ≈ harvest from the largest rain spell (about 20 days of rainfall). The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Rainwater harvested per year

For the displayed Rainwater Harvesting Calculator worked example, Rainwater harvested per year is 85,285 litres. 85.3 m³ Verify Roof area, Annual rainfall, and Roof type and their units before relying on this output.

Share of family demand

For the displayed Rainwater Harvesting Calculator worked example, Share of family demand is 43%. 158 days of supply Verify Roof area, Annual rainfall, and Roof type and their units before relying on this output.

Suggested storage tank

For the displayed Rainwater Harvesting Calculator worked example, Suggested storage tank is 5,000 litres. Verify Roof area, Annual rainfall, and Roof type and their units before relying on this output.

Roof area

For the displayed Rainwater Harvesting Calculator worked example, Roof area is 111.5 m². Verify Roof area, Annual rainfall, and Roof type and their units before relying on this output.

Runoff coefficient used

For the displayed Rainwater Harvesting Calculator worked example, Runoff coefficient used is 0.85. Verify Roof area, Annual rainfall, and Roof type and their units before relying on this output.

Rainwater Harvesting Calculator accuracy, checks and limitations

Rainwater Harvesting Calculator units check: confirm Roof area (sq ft), Annual rainfall (mm), Roof type, and People in the house before calculating.
Rainwater Harvesting Calculator result check: compare Rainwater harvested per year, Share of family demand, Suggested storage tank, Roof area, and Runoff coefficient used with the substituted formula steps and the displayed rounding precision.
Rainwater Harvesting Calculator: Use this result for preliminary design and cross-checking. Confirm the applicable code edition, manufacturer data, site conditions and qualified-engineer approval before final design or installation.

Formula, derivation and worked example

Every square metre of roof collects one litre for each millimetre of rain, minus losses from splash, evaporation and the first flush. A concrete terrace keeps about 85% of it. Compare the annual harvest with your family's demand to see how many months of water the roof can supply.

Harvest = roof area × rainfall × runoff coefficient

1 m² × 1 mm of rain = 1 litre

Storage ≈ harvest from the largest rain spell (about 20 days of rainfall)

Substitution steps

1. Roof area
sq ft × 0.0929
= 111.48 m²
2. Harvest
area × rainfall × 0.85
= 85,285 litres
3. Family demand
people × 135 × 365
= 197,100 litres

Computed example results

Rainwater harvested per year: 85,285 litres; 85.3 m³
Share of family demand: 43%; 158 days of supply
Suggested storage tank: 5,000 litres
Roof area: 111.5 m²
Runoff coefficient used: 0.85

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Assumptions and calculation scope

Roof area is assumed to be entered in sq ft; converting from another unit before entry avoids changing the numerical meaning of the calculation.
Roof type is assumed to match the real case. Choosing a different available option can change the method, factor or interpretation used by this calculator.
Rainwater Harvesting Calculator applies the displayed Harvest = roof area × rainfall × runoff coefficient relationship to the entered construction inputs. Factors that are not exposed as inputs or stated assumptions are outside this calculator's calculation scope.

Common mistakes when using Rainwater Harvesting Calculator

Do not mix units for Roof area (sq ft), Annual rainfall (mm). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
Do not leave Roof type on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
Do not replace the displayed Harvest = roof area × rainfall × runoff coefficient relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
Do not treat Rainwater harvested per year = 85,285 litres from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the Rainwater Harvesting Calculator is useful

Rainwater Harvesting Calculator is designed for cases where Roof area, Annual rainfall, Roof type, People in the house are known and you need Rainwater harvested per year, Share of family demand, Suggested storage tank. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is Harvest = roof area × rainfall × runoff coefficient. If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Roof area and Annual rainfall: what changes the answer

The worked example uses Roof area = 1200 sq ft, Annual rainfall = 900 mm, Roof type = Concrete / terrace (0.85), People in the house = 4. With those values, Rainwater harvested per year is 85,285 litres. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Roof area (sq ft): Measured or known roof area used by the calculation engine. Annual rainfall (mm): Measured or known annual rainfall used by the calculation engine. Roof type: Select the option that matches the real installation or scenario. Available choices include Concrete / terrace (0.85), Tiled roof (0.75), Metal sheet (0.9). People in the house: Measured or known people in the house used by the calculation engine.

How to sanity-check a Rainwater Harvesting Calculator result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Rainwater harvested per year, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with Water Tank Size Calculator

Water Tank Size Calculator is directly connected from Rainwater Harvesting Calculator as a source-defined continuation or comparison.

Open Water Tank Size Calculator

Formula

Harvest = roof area × rainfall × runoff coefficient
1 m² × 1 mm of rain = 1 litre
Storage ≈ harvest from the largest rain spell (about 20 days of rainfall)

Worked example

11,200 sq ft = 111.5 m² of roof, 900 mm annual rainfall.
2Harvest = 111.5 × 900 × 0.85 = 85,300 litres a year.
3A family of 4 uses about 197,000 litres a year, so the roof covers 43% of the demand.

Tips

Fit a first-flush diverter — the first 2 mm of rain carries all the roof dust.
If you cannot store it, recharge it: a 1 m × 1 m × 2 m pit filled with gravel recharges the borewell.

Frequently asked questions

How much rainwater can a 1000 sq ft roof collect?

About 71,000 litres a year in a place with 900 mm rainfall — enough for 130 days of a family of four.

Is rainwater safe to drink?

Only after filtration and disinfection. Most homes use it for washing, flushing and gardening, or recharge it into the ground.

What tank size do I need?

Storing the harvest from one heavy spell is enough — typically 5,000–10,000 litres for a house.

What inputs does the Rainwater Harvesting Calculator use?

It uses Roof area, Annual rainfall, Roof type, and People in the house. Follow the unit printed for each field and choose any selectable option to match the real scenario.

What does the Rainwater Harvesting Calculator calculate?

It calculates Rainwater harvested per year, Share of family demand, Suggested storage tank, Roof area, and Runoff coefficient used. With the displayed default inputs, Rainwater harvested per year is 85,285 litres.

Which formula does the Rainwater Harvesting Calculator use?

The primary relationship is Harvest = roof area × rainfall × runoff coefficient. The page also shows substituted values and calculation steps so the result can be checked independently.

How can I verify a Rainwater Harvesting Calculator result?

First verify the units for Roof area, Annual rainfall, and Roof type. Then compare the substituted formula steps with Rainwater harvested per year and its displayed precision.

What are the limitations of the Rainwater Harvesting Calculator?

Rainwater Harvesting Calculator: Use this result for preliminary design and cross-checking. Confirm the applicable code edition, manufacturer data, site conditions and qualified-engineer approval before final design or installation.

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