How much rainwater can I collect from a 1000 sq ft roof in Sri Lanka?

A 1,000 ft² roof is about 92.9 m². In Colombo (≈2,400 mm/year) with a tiled roof (runoff 0.80) and 85% system efficiency, that yields 92.9 × 2,400 × 0.80 × 0.85 ≈ 151,600 litres a year, or roughly 415 litres a day averaged over the year. Drier districts collect proportionally less.

Is rainwater harvesting mandatory in Sri Lanka?

It is national policy (National Policy & Strategies on Rainwater Harvesting, 2005) and, under Urban Development Authority Gazette Extraordinary No. 1585/23 of 2009 and later amendments, rainwater harvesting systems are required for specified developments — larger plots, high-rise, and certain commercial buildings — as a building-approval condition. Ordinary small houses are encouraged rather than compelled, so check with your local authority.

What is the runoff coefficient for a tiled, asbestos, or metal roof?

This tool uses Lanka Rainwater Harvesting Forum / TWDB guidance: clay tile 0.80, asbestos or metal sheet 0.85, concrete flat slab 0.75, and green/other roofs 0.60. Smooth impervious sheets shed the most water; textured, planted, or thatched surfaces absorb more, so they collect less.

How big should my rainwater storage tank be?

Size the tank to bridge your longest dry spell: daily household demand × dry-spell days. A 4-person home using 90 L/person/day over a 15-day dry spell needs 4 × 90 × 15 = 5,400 L, which rounds up to a standard 10,000 L tank. Bigger dry-zone buffers need proportionally more storage.

How much of my monthly water bill can rainwater harvesting save?

It depends on how much of your demand rain can cover. If the tool shows 50% of demand met, you could roughly halve the metered water you draw from NWSDB in an average year — though the saving is seasonal, largest in the monsoon months and smallest in dry spells when the tank runs low.

Does this use this year's actual rainfall?

No. Version 1 uses long-run annual rainfall normals (Department of Meteorology, 1991–2020) so the estimate is deterministic and works offline. A wetter or drier year will collect more or less. You can override the district figure with "Enter rainfall manually" if you have your own station data.

Is roof area the flat footprint or the sloped surface?

Use the plan (footprint) area — the flat area the roof covers when viewed from above. Rain falls vertically, so the horizontal catchment is what determines the volume collected, regardless of how steep the roof is. Measure the building outline, including eaves overhang.

Can I drink harvested rainwater?

This tool sizes supply and storage, not treatment. Roof rainwater is generally fine for gardening, washing, and flushing without treatment. For drinking, it needs first-flush diversion, filtering, and disinfection (boiling or an approved filter). Water quality and potability are outside this calculator's scope.

Sri Lanka · Utility

Sri Lanka Rainwater Harvesting Calculator

Estimate how many litres your roof can harvest each year from its area, roof type, and your district's rainfall — then see what share of your household's water that covers and what storage-tank size to buy. No signup, sources cited below.

By Induwara Ashinsana— Executive Director, Ryzera TechnologiesUpdated Jul 15, 2026

Rainwater harvest estimateroof catchment

LRWHF method · verified two ways

Roof catchment area

The roof's plan (footprint) area, not the sloped surface.

Roof type

Traditional Sri Lankan tiled roof — small losses in the tile gaps.

District (annual rainfall)

Colombo · Dept. of Meteorology 1991–2020 normal

System efficiency (%)

First-flush + filter losses. 85% is typical.

Try

Household demand

People

Persons in the home.

Litres / person / day

Piped homes ≈ 90.

Dry-spell days

Days the tank should bridge.

Harvestable / year

195,840 L

196 m³ (kL)

Average per day

537 L

annual ÷ 365

Share of demand met

149.04%

storage-limited

Suggested tank

10,000 L

needs 5,400 L buffer

Rainwater alone can meet a full year of demand — storage, not supply, is the limiting factor. Size the tank for the dry spells.

Assumptions used

Roof type / runoff Cr: Clay tile · 0.80
Annual rainfall: 2,400 mmColombo · Dept. of Meteorology 1991–2020 normal
System efficiency: 85%
Effective capture depth: 1,632 mmrainfall × Cr × efficiency

Sri Lanka's rain is seasonal (Yala & Maha monsoons), so real collection is uneven across the year — storage bridges the dry months.

Sources: Lanka Rainwater Harvesting Forum (runoff coefficients); Department of Meteorology, Sri Lanka (rainfall normals, 1991–2020). Figures are long-run averages, not this year's actual rainfall. Full citations below.

How it works

The calculator uses the standard rainwater-harvesting yield formula recommended by the Lanka Rainwater Harvesting Forum and mirrored in international manuals such as the Texas Water Development Board's. It rests on one hydrology identity: one millimetre of rain falling on one square metre of catchment delivers exactly one litre of water. So the volume a roof can collect in a year is:

V = A × R × Cr × Es

A — catchment area (m²).Enter the roof's plan footprint, not the sloped surface — rain falls vertically, so the horizontal area is what counts. Square feet are converted at ×0.092903.
R — annual rainfall (mm). Each district loads a long-run rainfall normal from the Department of Meteorology (1991–2020). You can type your own station figure instead.
Cr — runoff coefficient. The fraction of rain a roof actually sheds to the gutters, by material:
- Clay tile: 0.80
- Asbestos / metal sheet: 0.85
- Concrete flat slab: 0.75
- Green / other: 0.60
Es — system efficiency. First-flush diversion and filter losses, default 0.85. Rain from the first minutes of a storm is diverted to wash dust off the roof, and filters lose a little more.

Dividing the annual volume by 365 gives an average daily yield. If you enter a household size, the tool compares the harvest against annual demand (people × per-capita litres × 365) to show the share met. When that share tops 100%, supply is not the constraint — storage is, because Sri Lanka's rain is concentrated in the Yala and Maha monsoons. The suggested tank size (demand × dry-spell days, rounded up to a standard 1,000 / 2,000 / 5,000 / 10,000 L tank) is what bridges the dry months. Every figure the tool uses is shown in the “Assumptions used” panel so the result is auditable, and the yield is verified two ways — the volume identity and an effective-depth cross-check that must agree to the litre.

Worked examples

Colombo, tiled roof (wet zone)

A = 120 m², clay tile (Cr 0.80), R = 2,400 mm, Es = 0.85, 4 people @ 90 L/day, 15 dry days

Annual volume: 120 × 2,400 × 0.80 × 0.85 = 195,840 L/year (≈ 195.8 m³)
Per day: 195,840 ÷ 365 ≈ 536 L/day
Demand: 4 × 90 × 365 = 131,400 L/year
Share met: 195,840 ÷ 131,400 = 149% (exceeds demand → storage is the limiter)
Tank: 4 × 90 × 15 = 5,400 L → rounds up to a 10,000 L tank

Anuradhapura, metal-sheet roof (dry zone)

A = 90 m², metal sheet (Cr 0.85), R = 1,350 mm, Es = 0.85, 5 people @ 90 L/day, 20 dry days

Annual volume: 90 × 1,350 × 0.85 × 0.85 ≈ 87,784 L/year (≈ 87.8 m³)
Per day: 87,784 ÷ 365 ≈ 240 L/day
Demand: 5 × 90 × 365 = 164,250 L/year
Share met: 87,784 ÷ 164,250 = 53% (rain covers about half — a strong dry-zone supplement)
Tank: 5 × 90 × 20 = 9,000 L → rounds up to a 10,000 L tank

Small ft²-entered roof (unit conversion edge case)

A = 1,000 ft² → 92.9 m², clay tile (Cr 0.80), Colombo R = 2,400 mm, Es = 0.85

Convert area: 1,000 × 0.092903 = 92.903 m²
Annual volume: 92.903 × 2,400 × 0.80 × 0.85 ≈ 151,600 L/year
Per day: 151,600 ÷ 365 ≈ 415 L/day
A 1,000 ft² roof in Colombo collects roughly 150,000 L a year before use.

Frequently asked questions

Sources & references

Runoff coefficients and the yield formula were last cross-checked against the Lanka Rainwater Harvesting Forum and TWDB guidance, and district rainfall against Department of Meteorology normals, on 2026-07-15. District figures are long-run averages rounded to the nearest 50 mm and are user-overridable.

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Found a bug, edge case, or want to suggest an improvement?

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