One millimetre of rain falling on one square metre equals one litre before losses. In US units, one inch over one square foot is about 0.623 US gallons before losses.
Plan the whole path
Rainwater projects work best when this topic is connected to roof drainage, storage, intended use and safe overflow. A component that performs well by itself can still create a weak system when it is inaccessible, undersized, incompatible with other parts or used outside its approval.
Begin with a sketch showing elevations, pipe routes, isolation points, maintenance access and the route water follows when something is blocked. For larger, buried, indoor or potable systems, use qualified local professionals and the current rules adopted by the authority having jurisdiction.
Planning steps
- Measure only the sections that reach the collection point.
- Use local rainfall records for the period being planned.
- Apply a runoff factor for wetting, splash and conveyance losses.
- Subtract first-flush diversion and expected overflow.
- Test several dry and wet scenarios.
Cost and performance factors
Costs vary with tank size, material, delivery access, excavation, structural support, plumbing, power, treatment, permits and restoration. Compare complete scopes. A cheaper component can be more expensive over time if it clogs often, wastes collected water or requires difficult access.
Performance should be judged against measured demand and local rainfall timing. Annual averages are useful for screening an idea, but month-by-month or event-based analysis is more reliable for major investments.
Risks and limits
- Calculator output is an estimate, not a guarantee.
- Snow storage and spring melt may not behave like ordinary rainfall.
- Blocked gutters or undersized downpipes can sharply reduce capture.
- A large theoretical yield does not mean the tank can store it.
Questions to resolve before purchase
- What end use is approved and what water quality does it require?
- What happens during the longest likely dry period?
- What happens when the tank, filter or pipe is full or blocked?
- Who will inspect and maintain this part of the system?
Document the final system
Keep a diagram, product manuals, permit records, test results where applicable, valve positions, winter procedures and a maintenance log. Clear records help future owners, tenants and contractors avoid unsafe alterations.