Cistern Sizing Calculator

Calculate cistern sizing for rainwater harvesting. Determines collection potential, storage requirements, and annual water balance using ASABE S597.1 and ARCSA guidelines.

Calculate Cistern Size

For educational purposes only. Not a substitute for professional engineering judgment.

Input Parameters

Collection Area

Roof or surface area for rainwater collection

Collection Surface Type

Type of roof or collection surface

Annual Rainfall

Average annual rainfall at your location

Custom Runoff Coefficient

Override default value if known (0-1)

Water Demand

Primary Water Use

What the harvested water will be used for

Average Daily Demand

gal/day

Estimated daily water usage

System Parameters

First Flush Diversion Depth

Initial rainfall diverted for water quality (0.04-0.08 in typical)

System Losses

Losses from gutters, filters, etc. (3-10% typical)

Include cost analysis

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