DrainageCalculators

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.

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

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.

Calculate Cistern Size

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

Input Parameters

Collection Area

sf

Roof or surface area for rainwater collection

Type of roof or collection surface

in

Average annual rainfall at your location

Override default value if known (0-1)

Water Demand

What the harvested water will be used for

gal/day

Estimated daily water usage

System Parameters

in

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

%

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

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 TypeMinTypicalMax
Metal Roof0.90.950.98
Asphalt Shingles0.80.850.9
Clay/Concrete Tile0.750.80.85
Flat Membrane Roof0.850.90.95
Concrete Surface0.80.850.9
Green Roof0.20.350.5

Source: ARCSA/ASPE 63 (2013), Texas Water Development Board

Professional Review: For projects requiring regulatory approval or where public safety is involved, have your calculations reviewed and stamped by a licensed Professional Engineer (PE).

Was this calculator helpful?

Last verified: February 2026

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.