Estimate a rainwater tank size in gallons or litres from roof area, surface, annual rainfall and daily demand. Account for first-flush diversion and collection losses, then compare a storage estimate with the monthly supply/demand balance. Annual collection potential is different from the amount a tank can supply during a dry spell.
Calculate Cistern Size
For educational purposes only. Not a substitute for professional engineering judgment.
Rainwater Harvesting Overview
Cistern sizing balances rainwater supply with water demand to determine optimal storage. Key factors include:
- Collection Area - Roof area that captures rainfall
- Runoff Coefficient - Fraction of rain that becomes runoff
- Daily Demand - How much water you need each day
- Storage - Tank size to bridge dry periods
Collection Surface Runoff Coefficients
| Surface Type | Min | Typical | Max |
|---|---|---|---|
| Metal Roof | 0.9 | 0.95 | 0.98 |
| Asphalt Shingles | 0.8 | 0.85 | 0.9 |
| Clay/Concrete Tile | 0.75 | 0.8 | 0.85 |
| Flat Membrane Roof | 0.85 | 0.9 | 0.95 |
| Concrete Surface | 0.8 | 0.85 | 0.9 |
| Green Roof | 0.2 | 0.35 | 0.5 |
Source: ARCSA/ASPE 63 (2013), Texas Water Development Board
How cistern sizing works
Sizing a cistern is a water-balance problem: estimate annual supply from the roof, compare it to annual demand, then provide enough storage to ride through dry periods. The Texas Water Development Board manual, Chapter 4 explains this supply, demand and storage approach. The monthly pattern and storage assumptions used here provide a preliminary estimate.
1. Annual collection potential
The gross volume your catchment can yield in a year:
Vannual = P × A × k × C
- P — annual rainfall depth (in or mm)
- A — catchment (roof) area (sf or m²)
- k — depth-to-volume conversion: 0.623 gal per inch-square-foot (US), or 1.0 L per mm-square-metre (metric)
- C — runoff (collection) coefficient for the surface, 0–1
2. Effective collection after losses
First-flush diversion and system losses (gutters, leaf screens, filters) reduce the usable volume:
Veff = (Vannual − Vff loss) × (1 − loss%)
The first-flush loss is the per-event diverted volume (dff × A × k) multiplied by the assumed number of rain events per year (~70). System losses default to about 5%.
3. Demand and supply/demand ratio
Dannual = Ddaily × 365 ratio = Veff ÷ Dannual
A ratio of 1.0 means estimated annual collection equals annual demand. Timing, overflow and dry periods still affect how much demand stored rainwater can meet.
4. Recommended tank size
The preliminary tank estimate is the larger of average monthly demand and twice the largest monthly supply deficit. The monthly simulation uses that tank starting at 50% full. The reported storage recommendation is the larger of the preliminary estimate and a 21-day demand buffer:
Vtank = max( Ddaily × 21 days, preliminary tank estimate )
The tool lists nearby standard tank options. The monthly simulation can use a different capacity from the final recommendation; it is not a daily reliability simulation.
Collection surface runoff coefficients
The runoff coefficient (C) is the fraction of rain hitting the surface that actually reaches the tank. Smooth, impervious roofs capture the most; vegetated green roofs retain much of the rainfall.
| Collection surface | Min | Typical | Max |
|---|---|---|---|
| Metal roof (standing seam, corrugated) | 0.90 | 0.95 | 0.98 |
| Flat membrane roof (TPO, EPDM) | 0.85 | 0.90 | 0.95 |
| Asphalt / composition shingles | 0.80 | 0.85 | 0.90 |
| Concrete surface | 0.80 | 0.85 | 0.90 |
| Clay / concrete tile | 0.75 | 0.80 | 0.85 |
| Green roof (after retention) | 0.20 | 0.35 | 0.50 |
Source: ARCSA/ASPE 63 (2013) Rainwater Catchment System Design; Texas Water Development Board Rainwater Harvesting Manual. Tile roofs sit lower because of their textured profile; green roofs vary widely with media depth and antecedent moisture.
Example: collection, demand and storage buffer
Enter a 2,000 ft² metal roof, 36 in/yr rainfall, 50 gal/day irrigation demand, 0.04 in first flush and 5% system losses. The calculator assumes 70 rain events per year.
- Runoff coefficient (metal roof) C = 0.95
- Annual potential = 36 × 2,000 × 0.623 × 0.95 ≈ 42,600 gal/yr
- Annual demand = 50 × 365 = 18,250 gal/yr
- First-flush loss = 0.04 × 2,000 × 0.623 × 70 = 3,488.8 gal/yr
- Collection after losses = (42,613.2 − 3,488.8) × 0.95 ≈ 37,168 gal/yr; annual supply/demand ratio ≈ 2.04
- 21-day storage buffer = 50 gal/day × 21 days = 1,050 gal. The final recommendation also considers the monthly estimate.
These calculations use the tool's stated assumptions, not measured roof performance. TWDB's collection example explains the approximate 0.62 gal per inch-square-foot conversion and collection losses. Annual supply exceeding demand does not guarantee uninterrupted supply.
Frequently asked questions
How big a cistern do I need for rainwater harvesting?
Start with roof area, rainfall and daily demand. This calculator compares a 21-day demand buffer with a preliminary storage estimate based on average monthly demand and monthly deficits. It also displays a monthly water balance. The result is an initial estimate; annual rainfall and a monthly pattern do not establish reliability through a particular dry spell.
How much rainwater can my roof collect?
Annual collection = annual rainfall x catchment area x conversion factor x runoff coefficient. In US units, 1 inch of rain on 1 square foot yields about 0.623 gallons; in metric, 1 mm of rain on 1 square metre yields exactly 1 litre. For example, a 2,000 sf metal roof (runoff coefficient 0.95) under 36 in/yr of rain collects roughly 36 x 2,000 x 0.623 x 0.95 ≈ 42,600 gallons of gross potential per year, before first-flush and system losses.
What is a first flush diverter and why does it matter?
The first flush is the initial sheet of runoff that washes dust, pollen, bird droppings and debris off the roof at the start of a storm. A first-flush diverter routes that dirtier water away from the tank to protect water quality. Typical diversion depths are about 0.04-0.08 in (roughly 1-2 mm) of rainfall per event. The diverted volume is a real collection loss, so the calculator subtracts it across an assumed ~70 rain events per year.
Can I use harvested rainwater for drinking?
Not without proper treatment. Untreated roof runoff is suitable for irrigation, toilet flushing and laundry, but potable use requires filtration and disinfection that meet your local health code. The calculator flags potable use as needing treatment and does not model that treatment. Always confirm allowable end uses and any cross-connection and backflow requirements with your local jurisdiction.
What This Solves
Calculates cistern storage requirements for rainwater harvesting based on roof collection area, local rainfall patterns, and projected water demand.
Best Used When
- You are designing a rainwater harvesting system for irrigation, toilet flushing, or non-potable use
- You need to balance monthly rainfall supply against projected demand to size the tank
- You want to estimate annual water savings and payback period for a cistern system
Do NOT Use When
- You need a stormwater detention facility to attenuate peak flows — Use Pond Sizing Calculator
- You need to size a dry well for stormwater infiltration rather than storage for reuse — Use Dry Well Calculator
Key Assumptions
- Collection efficiency accounts for first-flush diversion and gutter losses (typically 80-90%)
- Monthly rainfall data represents average conditions over the analysis period
- Demand is constant or follows a known seasonal pattern
- The cistern is watertight with no leakage losses
- Overflow is directed to an appropriate discharge point
Input Quality Notes
Use local monthly rainfall data from NOAA Atlas 14 or equivalent. Demand estimates should be based on actual usage patterns, not theoretical values. Collection area should exclude non-contributing roof sections.
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Last verified: February 2026