# Rainwater Harvesting Tank Calculator

> Estimate how much rainwater a roof can collect in a year and size a storage tank to cover your daily demand through the longest dry spell.

Category: Construction · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/rainwater-harvesting-tank-calculator

One millimetre of rain on one square metre of roof is one litre. Reduce it for runoff and collection losses, then size the tank to carry you through the dry period.

Also called: Rain tank size calculator, Rainwater yield calculator.

## Inputs
- Roof catchment area (plan) (unit m²; default 100)
- Average annual rainfall (unit mm; default 800)
- Runoff coefficient (default 0.8)
- Collection efficiency (default 0.9)
- Daily water demand (unit L/day; default 150)
- Longest dry period to cover (unit days; default 30)

## Results
- Tank size for the dry period
- Tank size
- Rainwater collected per year
- Demand per year
- Share of demand met over a year

## Formula

```
Yield (L) = Area (m²) × Rainfall (mm) × Runoff × Efficiency;  Tank = Daily demand × Dry days
```

Where:
- Area: roof plan area
- Runoff: runoff coefficient of the roof surface
- Efficiency: collection and filter efficiency

## Worked example
1. Yield = 100 × 800 × 0.8 × 0.9 = 57,600 L a year.
2. Demand = 150 × 365 = 54,750 L, so rain can meet all of it on average.
3. Tank = 150 L × 30 days = 4,500 L.

**Result:** A 100 m² roof in an 800 mm rainfall area yields about 57,600 L a year; a 4,500 L tank covers 30 dry days at 150 L a day.

## What the result means

If the yearly yield is below demand, plan for a mains top-up. A monthly water balance with local rainfall records gives a better tank size than the dry-period rule.

## Assumptions and limits
- Rainfall is an annual average; yields vary year to year.
- The tank is full at the start of the dry period.
- Seasonal patterns and overflow are not modelled.
- Water quality and treatment for drinking use are outside the scope of this calculator.

## Questions

### Do I use the roof’s sloped area?

No. Use the plan (footprint) area: rain falls vertically, so the horizontal projection is what catches it.

### Why is my tank smaller than the yearly yield?

The tank only needs to hold water between refills, not a year’s supply.

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_Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials._

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/rainwater-harvesting-tank-calculator