TryBuildCalcRainwater Runoff Calculator(Roof Catchment, Rainfall & Tank Storage)
Estimate rainwater collection and tank storage.
🕒 Last updated: July 29, 2026
Inputs
Section 1: Catchment & Rainfall
ℹ️Use roof plan area, terrace area, or paved collection area.
ℹ️Typical annual rainfall can range from 250 to 3,000 mm. Dry regions may be lower; very wet regions can exceed this.
Section 2: Collection Efficiency
ℹ️Surface type sets a default runoff coefficient. Choose custom to enter your own value.
ℹ️Typical roof values range from 0.75 to 0.95. Lower values collect less water.
ℹ️Allows for filter, gutter, conveyance, and practical collection losses.
ℹ️Optional rainfall depth diverted before storage. Enter 0 if not used.
Section 3: Storage & Demand
ℹ️Optional. Used to estimate overflow and usable stored water.
ℹ️Optional. Used for demand coverage and days of supply.
ℹ️Used to recommend a practical tank size from daily demand.
💧 You can harvest up to 94,860 litres of rainwater per year.
Effective capture is 67.8% after runoff, first flush, and system efficiency losses.
This can cover 86.6% of your annual water demand.
Inputs Used
Catchment Surface: Tile roof
Catchment Area: 200 m²
Annual Rainfall: 700 mm
Runoff Coefficient: 0.8
System Efficiency: 85%
First Flush Depth: 2 mm
Other Conversions
Cubic Meters: 94.86 m³
US Gallons: 25,059.4 US gal
Imperial Gallons: 20,866.3 imp gal
Cubic Feet: 3,349.9 ft3
Current Tank & Demand Planning
Entered Tank Capacity: 8,000 L
Max Stored at One Time: 8,000 L
Limited by the entered tank capacity.
Annual Overflow With This Tank: ⚠️ 86,860 L will overflow without additional storage
Compares total annual harvest with the entered tank size. Actual overflow depends on rainfall timing and water use.
Annual Demand: 1,09,500 L
Potential Demand Coverage: 86.6%
Based on total annual harvest, before detailed seasonal timing.
Backup Water Still Needed: 14,640 L
Supply From Full Entered Tank: 26.7 days
At 300 L/day demand.
Recommended Backup Tank: 2,500 L
Sized for 7 dry days plus 10% safety. Calculated need: 2,310 L. This is not sized to store all annual rainfall.
Loss Breakdown
Gross rainfall volume is 1,40,000 L. Runoff capture is 1,12,000 L, with 400 L reserved as first flush loss.
Harvestable water is calculated from catchment area, rainfall, runoff coefficient, first flush loss, and system efficiency.
Approximate results for planning only. Verify with a professional.
Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Rainwater Harvesting Calculator.
Rainwater runoff estimate
This page focuses specifically on the runoff step for a tile roof — 200 m² at 700mm annual rainfall with a 0.82 tile-roof coefficient, giving 114,800 liters of runoff before first flush and efficiency losses, and about 97,240 liters net after those losses.
Tile roofs sit between metal and concrete in runoff efficiency — smoother and less porous than raw concrete, but with more surface texture and joints than a metal sheet roof, giving a middle-ground 0.82 coefficient.
- Tile roof selected.
- System efficiency: 85%.
- Catchment area: 200 m².
How the rainwater harvesting calculation works
The calculator converts the catchment area to square metres and rainfall to millimetres. Since 1 mm of rain over 1 m² equals 1 litre, the water quantity is simple to estimate.
Step 1 - Calculate gross rainfall volume
Litres = m² × mm
This is the theoretical water falling on the roof or terrace before any surface, gutter, filter, or first flush losses.
Step 2 - Apply runoff coefficient
The runoff coefficient adjusts for the catchment surface. Metal roofs collect more water, while rough or porous surfaces collect less.
Step 3 - Subtract first flush loss
Water After First Flush = Runoff Captured Water - First Flush Loss
First flush is the initial dirty roof water diverted away from the storage tank. If you enter 0, the calculator skips this loss.
Step 4 - Apply system efficiency
System efficiency accounts for practical losses in gutters, downpipes, filters, leakage, splash, and maintenance conditions.
Step 5 - Check storage, overflow, and demand
Period Demand = Daily Demand × Days in Period
Demand Coverage = Net Harvestable Water / Period Demand × 100
Backup Tank Need = Daily Demand × Dry Days × Safety Factor
Storage planning compares net harvest with the entered tank capacity and daily demand. The recommended backup tank is sized for dry days, not for storing all annual or monthly rainfall. If you already know the required litres, the Water Tank Capacity Calculator can convert that storage requirement into rectangular or cylindrical tank dimensions.
Real-World Rainwater Harvesting Example
This example uses the active inputs above and follows the same steps from the formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Rainfall Period | Annual | Determines whether results are shown per year or per month |
| Catchment Area | 200 m2 | Multiplied by rainfall to get gross rainfall volume |
| Annual Rainfall | 700 mm | Multiplied by catchment area to get gross rainfall volume |
| Catchment Surface | Tile roof (coefficient 0.8) | Reduces gross rainfall to account for surface losses |
| System Efficiency | 85% | Reduces captured water for filter, gutter, and conveyance losses |
| First Flush Depth | 2 mm | Diverts the initial dirty runoff before it reaches storage |
| Existing Tank Capacity | 8000 L | Used to estimate overflow and maximum stored water |
| Daily Water Demand | 300 L/day | Used to estimate demand coverage and days of supply |
| Backup Dry Days | 7 days | Used to size a practical backup tank |
| Safety Factor | 10% | Adds a buffer to the recommended backup tank size |
| Primary Output Unit | liters | Sets the unit used for capacity figures in the result card |
Step 1 — Gross Rainfall Volume
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Gross rainfall | Catchment Area x Rainfall = 200 m² x 700 mm | 1,40,000 L |
Step 2 — Runoff Captured
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Runoff captured | Gross Rainfall x Runoff Coefficient = 1,40,000 L x 0.8 | 1,12,000 L |
Step 3 — First Flush Loss
| Calculation | Formula / Substitution | Result |
|---|---|---|
| First flush loss | Catchment Area x First Flush Depth = 200 m² x 2 mm | 400 L |
| Water after first flush | Runoff Captured − First Flush Loss = 1,12,000 − 400 | 1,11,600 L |
Step 4 — Net Harvestable Water
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Net harvest | Water After First Flush x System Efficiency = 1,11,600 x 85% | 94,860 L |
Step 5 — Stored Water and Overflow
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Max stored at one time | min(Net Harvest, Tank Capacity) = min(94,860, 8000) | 8,000 L |
| Overflow | Net Harvest − Tank Capacity = 94,860 − 8000 | 86,860 L |
Step 6 — Demand Coverage
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Annual demand | Daily Demand x Days in Period = 300 x 365 | 1,09,500 L |
| Demand coverage | (Net Harvest ÷ Period Demand) x 100 = (94,860 ÷ 1,09,500) x 100 | 86.6% |
| Backup water still needed | Period Demand − Net Harvest = 1,09,500 − 94,860 | 14,640 L |
Step 7 — Days of Supply From Stored Water
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Days of supply | Stored Water ÷ Daily Demand = 8,000 ÷ 300 | 26.7 days |
Step 8 — Recommended Backup Tank Size
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Calculated backup tank | Daily Demand x Dry Days x (1 + Safety Factor) = 300 x 7 x (1 + 10%) | 2,310 L |
| Practical rounded tank size | Round up to the next practical tank size | 2,500 L |
Therefore, this catchment can harvest approximately 94,860 L per year (effective capture 67.8%), covering about 86.6% of the entered yearly demand, with a recommended backup tank of 2,500 L.
Actual yield can vary with roof slope, gutter layout, leaf guards, maintenance, and real rainfall timing. Cross-check the recommended tank against the Reference Tables in the complete Rainwater Harvesting Calculator.
This page keeps things focused on the calculation above. For the full construction guide — verification checklist, reference tables, usage steps, tips, common mistakes, and limitations.See the complete Rainwater Harvesting Calculator.