Stormwater Cistern Buffer Calculator

Estimate stormwater cistern buffer using rainfall and drawdown.

sq ft
in
gal
gal
%
%

Quick Facts

Capture
Efficiency
Efficiency reduces loss
Drawdown
Use
Drawdown clears capacity
Overflow
Risk
Overflow risk from events
Decision Metric
Buffer
Buffer days

Your Results

Calculated
Runoff Volume
-
Event runoff volume
Captured Volume
-
Captured runoff
Buffer Days
-
Days of buffer
Overflow Risk
-
Overflow percent

Cistern Plan

Your defaults show a steady stormwater buffer.

What This Calculator Measures

Estimate stormwater cistern buffer using roof area, rainfall, and drawdown.

By combining practical inputs into a structured model, this calculator helps you move from vague estimation to clear planning actions you can execute consistently.

This calculator estimates stormwater cistern buffer days.

How to Use This Well

  1. Enter roof area and rainfall.
  2. Add cistern capacity and drawdown.
  3. Set overflow and efficiency.
  4. Review buffer days.
  5. Adjust capacity or drawdown.

Formula Breakdown

Runoff = roof x rainfall x 0.623
Captured: runoff x efficiency.
Buffer: captured / drawdown.
Overflow: (runoff - capacity) / runoff.

Worked Example

  • 2,000 sq ft with 1 in rain = 1,246 gal.
  • Captured 85% = 1,059 gal.
  • Buffer about 5.3 days at 200 gal/day.

Interpretation Guide

RangeMeaningAction
5+ daysStrong.Great buffer.
3-5 daysGood.Solid coverage.
1-3 daysModerate.Increase drawdown.
Under 1Low.Expand capacity.

Optimization Playbook

  • Increase capacity: reduce overflow.
  • Improve drawdown: free space faster.
  • Maintain filters: keep efficiency high.
  • Track storms: size for events.

Scenario Planning

  • Baseline: current capacity.
  • More rainfall: add 0.5 in.
  • Higher drawdown: add 50 gal/day.
  • Decision rule: keep buffer above 3 days.

Common Mistakes to Avoid

  • Ignoring efficiency losses.
  • Underestimating overflow risk.
  • Not planning drawdown.
  • Skipping filter maintenance.

Implementation Checklist

  1. Measure roof area.
  2. Estimate capacity.
  3. Plan drawdown.
  4. Review after storms.

Measurement Notes

Treat this calculator as a directional planning instrument. Output quality improves when your inputs are anchored to recent real data instead of one-off assumptions.

Run multiple scenarios, document what changed, and keep the decision tied to trends, not a single result snapshot.

FAQ

What is drawdown?

Water use or release between storms.

How do I measure efficiency?

Estimate capture after losses and filters.

Should I plan for overflow?

Yes, include overflow management.

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