Formula Used
One millimetre of rainfall over one square metre equals one litre. Monthly simulation captures seasonal supply and demand differences. Actual performance still depends on local weather variability.
How to Use
- Choose the calculation mode and preferred units.
- Enter one or more catchment areas and materials.
- Provide monthly rainfall and estimated rainy days.
- Adjust efficiencies, first flush, and uncertainty.
- Enter household, irrigation, or commercial demand.
- Describe the existing tank or enter dimensions.
- Add local water charges and project costs.
- Submit to review storage, overflow, shortages, and savings.
Example Data
| Input | Example | Purpose |
|---|---|---|
| Roof area | 120 m² metal roof | Defines the main collection surface. |
| Annual rainfall | 990 mm | Represents total yearly precipitation. |
| System efficiencies | 95%, 92%, 97% | Models gutter, filter, and conveyance losses. |
| Tank capacity | 8,000 L | Limits monthly stored water. |
| Daily demand | About 370 L/day | Combines household and garden uses. |
| Water tariff | £2.50/m³ | Estimates avoided mains-water cost. |
Frequently Asked Questions
How accurate is this calculator?
It provides planning estimates from user-entered rainfall and demand. Weather variability can change real collection significantly. Use several years of rainfall records when possible.
What runoff coefficient should I use?
Smooth metal roofs often have high coefficients. Rough or absorbent surfaces collect less water. Choose custom values from reliable local guidance.
Why are rainy days required?
First-flush water is usually discarded for each event. More rainy days can increase total first-flush losses. Enter realistic event counts for better estimates.
Can I use annual rainfall only?
Yes, select annual rainfall scaled monthly mode. The entered annual total scales the monthly pattern. Monthly records still provide better storage modelling.
How is recommended tank capacity calculated?
The calculator tests increasing capacities against your reliability target. It then adds your selected reserve percentage. This remains a planning estimate, not engineering approval.
What does tank adequacy mean?
It compares entered capacity with recommended modelled capacity. Values above one hundred percent meet the estimate. Lower values indicate possible shortages.
Can several tanks be included?
Yes, enter the number of identical tanks. Capacity or dimensions apply to each tank. Their combined volume is used during simulation.
Is harvested rainwater safe to drink?
Untreated rainwater may contain biological or chemical contaminants. Drinking systems need appropriate filtration and disinfection. Follow local health and plumbing requirements.
Why can overflow occur with shortages?
Rain may arrive when the tank is already full. Later dry periods can still empty storage. Larger tanks or reduced demand may help.
Does payback include every project cost?
No, it uses entered installation and maintenance costs. Pump energy and replacement parts may add expenses. Include them within your estimates where relevant.
Should professionals review the final design?
Yes, especially for drinking water or large tanks. Structural loads, overflow routes, and plumbing need review. Check local regulations before construction or connection.
Important Guidance
Results are estimates and not construction approval. Untreated rainwater should not be assumed potable. Verify filtration, disinfection, overflow, backflow, and plumbing requirements locally.
Large tanks can impose significant structural loads. Confirm foundations, access, mosquito protection, and safe maintenance. Professional assessment is recommended for complex installations.