Enter your roof length, width, and pitch to get your effective catchment area instantly — in square metres, square feet, and more. The foundation of any rainwater harvesting system.
Enter roof footprint dimensions and pitch to get effective catchment area
Roof Footprint Dimensions
Roof Length
Roof Width
Roof Pitch & Efficiency
Roof Pitch
Runoff Coeff.
0–1
Runoff coefficient guide: Metal roofing ≈ 0.90, Tile / slate ≈ 0.85, Corrugated iron ≈ 0.80, Asphalt shingle ≈ 0.75, Green / living roof ≈ 0.40. Default is 0.85.
⚠ Please enter valid values (greater than zero) for roof length and width. Pitch must be 0–90°.
WaterTankCalculator.com — Roof Catchment Area Results
Your Results
Effective Catchment Area
—
Square Metres (m²)
Gross Footprint
— m²
Effective (ft²)
— ft²
Gross (ft²)
— ft²
Effective (acres)
— ac
Runoff Coeff.
—
Pitch Used
— °
Roof Footprint Visualisation
📖 How To Use
How to Use This Roof Catchment Area Calculator
Measuring your effective catchment area is the essential first step in sizing any rainwater harvesting system. Here's how to do it in under a minute:
Measure your roof footprint — not the slope surface
Walk around the outside of your building and measure its length and width at ground level (or measure the plan area from above). Rainwater falls vertically, so it's the horizontal projected area — not the sloped roof surface — that determines how much rain you collect.
Enter your roof pitch (optional)
For most harvesting calculations, pitch is already accounted for by using the footprint. Entering your pitch lets the calculator show the sloped roof surface separately — useful if you measured along the slope by mistake, or for ordering roofing. Enter degrees (e.g. 30°), percentage (e.g. 58%) or the US pitch ratio (e.g. 7 for 7:12).
Set your runoff coefficient
Not all rain that falls on your roof makes it into your tank — some evaporates from the surface, some is lost at overflowing gutters, and some is absorbed by the material. The default of 0.85 suits most tile and metal roofs. Adjust it for your roofing using the guide shown; rainwater design guides commonly assume 75–90% overall.
Read your effective catchment area
The primary result is your effective catchment area — this is the number to use when calculating how much rainwater you can collect from any given rainfall event.
Pro tip: If your building has multiple roof sections (e.g., a main house plus a garage), calculate each section separately and add the effective areas together. Only include sections whose gutters drain to your collection tank.
📐 The Formula
Roof Catchment Area Formula Explained
The effective catchment area is calculated in two steps — first the gross footprint, then adjusted for runoff efficiency:
Gross Footprint Area = Length × Width Effective Catchment Area = Gross Area × Runoff Coefficient
Metric example: 12 m × 8 m roof, coefficient 0.85
Gross = 12 × 8 = 96 m²
Effective = 96 × 0.85 = 81.6 m²
US example: 40 ft × 30 ft roof, coefficient 0.85
Gross = 1,200 sq ft · Effective = 1,020 sq ft → 1,020 × 0.623 ≈ 635 gallons per inch of rain
The runoff coefficient (also called collection efficiency) accounts for real-world losses between rainfall and your storage tank. It is always between 0 and 1. The higher the number, the more efficiently water is collected.
Runoff Coefficient by Roof Material
Roof Material
Coefficient
Notes
Stainless steel / ZINCALUME®
0.90–0.95
Highest efficiency; minimal absorption
Metal / corrugated iron
0.80–0.90
Slight oxidation losses over time
Glazed tile / slate
0.80–0.90
Smooth surface, low absorption
Concrete tile
0.75–0.85
Slightly porous; wets up before runoff
Asphalt / bitumen shingles
0.70–0.80
Higher evaporation in hot climates
Fibreglass / UPVC
0.85–0.90
Low absorption, smooth finish
Green / living roof
0.30–0.50
Significant retention by plants/soil
Thatched / grass
0.10–0.30
Very low; mostly absorbed and evaporated
Why Horizontal Footprint — Not Slope Area?
Rain falls vertically. No matter how steep your roof is, a 10 m × 8 m building always intercepts a 10 m × 8 m column of falling rain. The slope area is larger, but the additional surface doesn't catch any extra rain — it just means the water travels farther before reaching the gutter. This is why harvesting calculations use the plan (horizontal projected) area rather than the actual roof surface area.
For harvesting volume: always use the footprint × coefficient, not slope area.
💡 Use Cases
When Do You Need to Know Your Roof Catchment Area?
Sizing a Rainwater Harvesting Tank
To size your storage tank, you need to know how much water your roof can deliver in a given period. The formula is: Catchment Area (sq ft) × Rainfall (in) × 0.623 × Coefficient = Volume (US gallons), or in metric Area (m²) × Rainfall (mm) × Coefficient = Volume (litres). Without an accurate catchment area, your tank will be over- or under-sized. Use the Rainwater Harvesting Calculator to go from catchment area to monthly collection volumes.
First Flush Diverter Sizing
First flush diverters work by volume — they capture the first few millimetres of rainfall (which carries the most contamination) before allowing clean water to flow to your tank. The required diverter volume is based directly on your catchment area. A widely used US guideline is one to two gallons for every 100 sq ft of roof (about 0.4–0.8 L per m²), and at least 10 gallons per 1,000 sq ft. Australia's national enHealth guidance suggests discarding the first 20–25 litres from an average roof. Use our First Flush Diverter Size Calculator once you have your effective area.
Planning Gutter and Downpipe Capacity
Gutters and downpipes must be sized to handle peak rainfall intensity without overflowing. Larger catchment areas require wider gutters and more downspouts. Gutter tables — in the US, those in the International Plumbing Code — use the roof area each gutter drains and the local design rainfall intensity.
Annual Savings and ROI Calculations
If you're evaluating the financial case for rainwater harvesting, your catchment area combined with local annual rainfall data gives you the maximum harvestable volume per year — the starting point for any ROI calculation.
Irrigation and Agricultural Planning
Farmers and market gardeners use roof catchment area to size farm dams and tanks that collect rain from barn roofs and shed roofs. Combined with crop water need data, catchment area determines whether an on-site supply is feasible or supplemental mains water is needed.
🧮 Worked Example
Worked example: a house and garage on one tank
A house roof measures 50 ft × 28 ft at ground level with a 6:12 pitch, and a detached garage roof 24 ft × 22 ft; both are asphalt shingle (about 0.75) and drain to the same tank.
House footprint: 50 × 28 = 1,400 sq ft. The 6:12 pitch (26.6°) makes the sloped surface about 1,565 sq ft — but only the 1,400 sq ft footprint counts for rain.
Per inch of rain: 1,446 × 0.623 ≈ 900 gallons; per 10 mm, about 1,340 litres.
❓ FAQ
Frequently Asked Questions
How do I calculate roof catchment area for rainwater harvesting?
Measure the horizontal length and width of your roof at ground level (the building footprint, not the sloped surface), then multiply them together. This gives your gross area in sq ft or m². Multiply by your runoff coefficient (typically 0.75–0.90) to get the effective catchment area. Each inch of rain on 1 sq ft of effective area gives 0.623 US gallons; each mm on 1 m² gives 1 litre.
Does roof pitch affect catchment area?
No — for rainwater harvesting purposes, pitch does not affect how much rain you collect. Because rain falls vertically, the horizontal projected (plan) area is what matters, not the slope surface. A steeper roof has a larger physical surface, but intercepts exactly the same rainfall as a flat roof of the same footprint. Pitch matters for gutter design and flow rates, but not for collection volume.
What is a good runoff coefficient for my roof?
Most hard roofing materials (metal, tile, slate) have runoff coefficients of 0.80–0.90. Asphalt shingles run 0.70–0.80. Green or living roofs drop to 0.30–0.50 because plants and soil absorb a large portion. The default of 0.85 is a reasonable estimate for typical tile or metal roofs; rainwater design guides commonly assume 75–90% overall. Use a lower figure for rough or porous roofs, many small showers (each one wets the roof before anything runs off), or gutters that overflow in heavy rain.
How much water can I harvest per mm of rainfall?
The formula is: Litres = Catchment Area (m²) × Rainfall (mm) × Runoff Coefficient. For example, a 100 m² effective catchment area and 10 mm of rain yields 100 × 10 × 1 = 1,000 litres. With a coefficient of 0.85, that's 850 litres. One mm of rainfall over 1 m² of effective catchment area delivers 1 litre. In US units, 1 inch over 1 sq ft gives 0.623 gallons — so 1,000 sq ft and 1 inch of rain gives about 623 gallons before losses.
What if my roof has multiple sections draining to different gutters?
Calculate each section separately using its own length × width, then add only the sections that drain to your collection tank. If your main house roof and garage both drain to the same tank, add their effective areas. If the garage drains to a separate outlet, exclude it. This is especially important for L-shaped or multi-gabled roofs where only part of the roof feeds your harvesting system.
How large a tank do I need for my roof catchment area?
Tank size depends on your catchment area, local rainfall patterns, and your water demand. It depends on your demand and dry spells as much as on the roof. Design guides advise sizing storage for the longest expected dry spell. As a feel for the numbers: a 1,000 sq ft catchment with 2 in of rain a month delivers about 1,250 gallons a month before losses (100 m² with 50 mm gives about 5,000 litres). Use the Rainwater Harvesting Calculator for a full analysis.
Can I use roof catchment area to size my gutters?
Yes — gutter capacity is directly related to catchment area and local peak rainfall intensity. Most gutter sizing tables ask for the catchment area in m² and the design storm intensity in mm/hr. A 100 m² (1,076 sq ft) roof in a 100 mm/hr (4 in/hr) storm sheds about 2.8 L/s (44 gal/min). How much roof one gutter can serve depends on its size and profile, its slope and the number of downspouts — use your local plumbing code's tables (in the US, the International Plumbing Code) or the gutter maker's data.
✅ Sources & Review
Sources and review
The catchment method is standard geometry; the efficiency and first-flush figures on this page come from these references:
United States — Texas Water Development Board, The Texas Manual on Rainwater Harvesting (3rd ed.), one of the most widely used US design guides: about 0.62 gallons per sq ft per inch; 75–90% efficiency; first flush of 1–2 gallons per 100 sq ft.
Australia — enHealth, Guidance on Use of Rainwater Tanks (2010): first-flush diversion of the first 20–25 litres for an average roof, tank inspection for sediment every 2–3 years, and drinking-water safety.
Europe and UK — BS EN 16941-1:2018, On-site non-potable water systems — Systems for the use of rainwater: design, sizing and installation of rainwater systems.
International Code Council — International Plumbing Code, Chapter 11 (Storm Drainage): gutter and leader sizing by roof area and rainfall rate.
Unit conversions from NIST SP 811: 1 m² = 10.764 sq ft; 1 acre = 4,046.9 m²; 1 ft³ = 7.48 US gallons.
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
