Rainwater Cistern First-Flush Diverter & Dry-Spell Autonomy Calculator

Size rainwater cistern storage, first-flush diverter gallons, and dry-spell days.

Net Harvestable Cistern Water
1,394.47 gallons
First-Flush Diverter Capacity 36 gallons
Dry-Spell Supply Autonomy 39.84 days
3" PVC Diverter Pipe Length (0.37 gal/ft) 98.09 ft of 3" pipe
Inputs
sq ft
Roof catchment footprint area in square feet.
in
Rainfall depth over the storm or month in inches.
gal/day
Daily household or garden water draw from cistern (gal/day).
Results & Graph
Net Harvestable Cistern Water
1,394.47 gallons
First-Flush Diverter Capacity
36 gallons
Dry-Spell Supply Autonomy
39.84 days
3" PVC Diverter Pipe Length (0.37 gal/ft)
98.09 ft of 3" pipe

Net Harvestable Cistern Water

Graph 1 • Roof Catchment Sq Ft Sensitivity Recalculates the domain equation across 50% to 150% of the primary input while holding other parameters constant.
0 gallons 1.1k gallons 2.3k gallons 50%: 697.24 gallons 697.2 50% 75%: 1,045.86 gallons 1.0k 75% 100%: 1,394.47 gallons 1.4k 100% 125%: 1,743.09 gallons 1.7k 125% 150%: 2,091.71 gallons 2.1k 150%
Graph 2 • Primary & Supporting Metric Comparison Compares the primary output alongside supporting domain benchmarks.
Net Harvestable Cistern Water 1,394.47 gallons First-Flush Diverter Capacity 36 gallons Dry-Spell Supply Autonomy 39.84 days 3" PVC Diverter Pipe Length (0.37 gal/ft) 98.09 ft of 3" pipe
Graph 3 • Exact Output Table & Units Displays each calculated output in its exact domain unit.
Net Harvestable Cistern Water1,394.47 gallonsFirst-Flush Diverter Capacity36 gallonsDry-Spell Supply Autonomy39.84 days3" PVC Diverter Pipe Length (0.37 gal/ft)98.09 ft of 3" pipe
Summary: From 1.5" of rain on a 1800 sq ft roof (1683 gal gross), diverting 36 gal first-flush yields 1394 net gallons (39.8 days of supply).
Summary: From 1.5" of rain on a 1800 sq ft roof (1683 gal gross), diverting 36 gal first-flush yields 1394 net gallons (39.8 days of supply).
⚡ Send result to: Board Foot & Framing →Concrete & Sq Ft →Recipe & Cooking →
Interactive chart & math engine powered by CalculatorGraph

Formula & Mathematical Methodology: Rainwater Cistern & Flush Formula

HarvestedGal = RoofSqFt × Rainfallin × 0.6233 × 0.85
FirstFlushGal = RoofSqFt × 0.02
HarvestedGal = RoofSqFt × Rainfall_in × 0.6233 × 0.85; FirstFlushGal = RoofSqFt × 0.02
  • roofCatchmentSqFt: Roof Catchment Sq Ft (sq ft)
  • rainfallDepthInches: Rainfall Depth Inches (in)
  • dailyWaterUsageGal: Daily Water Usage Gal (gal/day)

Rounding: Physical package counts round up where appropriate; continuous measurements round to 2 decimal places.

Worked Example: Practical Benchmark: Rainwater Cistern & Flush

Size rainwater cistern storage, first-flush diverter gallons, and dry-spell days.

  1. Enter your project dimensions, machining speeds, or recipe weights.
  2. Compute the governing equation: HarvestedGal = RoofSqFt × Rainfall_in × 0.6233 × 0.85; FirstFlushGal = RoofSqFt × 0.02.
  3. Review the primary result, supporting dimensions, and 3 SVG visualizations.
Result: Computes Rainwater Cistern & Flush via HarvestedGal = RoofSqFt × Rainfall_in × 0.6233 × 0.85; FirstFlushGal = RoofSqFt × 0.02.

Frequently Asked Questions

How does the Rainwater Cistern First-Flush Diverter & Dry-Spell Autonomy Calculator calculate results?

It evaluates HarvestedGal = RoofSqFt × Rainfall_in × 0.6233 × 0.85; FirstFlushGal = RoofSqFt × 0.02 directly in your browser and plots three custom SVG visualizations.

Can I compare different material dimensions or batch sizes side by side?

Yes. Click "Compare (0/3)" in the results toolbar to pin up to 3 scenarios side by side.

Can I print a clean shop or kitchen reference sheet?

Yes. Click the Print button in the results toolbar for a distraction-free worksheet.

Construction & Home · Landscaping & Outdoor

Rainwater Cistern First-Flush Diverter & Dry-Spell Autonomy Calculator: inputs, method, and how to read the result

Size rainwater cistern storage, first-flush diverter gallons, and dry-spell days.

Choose inputs for the case you are checking

The defaults are illustrative. Replace them with values for your case and keep the units shown beside each field.

  • Roof Catchment Sq Ft: Roof catchment footprint area in square feet.
  • Rainfall Depth Inches: Rainfall depth over the storm or month in inches.
  • Daily Water Usage Gal: Daily household or garden water draw from cistern (gal/day).

What the calculation includes

With the default values shown, Net Harvestable Cistern Water is 1,394.47 gallons. The calculation follows Rainwater Cistern & Flush Formula; the formula and variable definitions above show how the inputs combine.

One-input check: With the other defaults held constant, changing Roof Catchment Sq Ft from 1,800 sq ft to 1,980 sq ft changes Net Harvestable Cistern Water from 1,394.47 gallons to 1,533.92 gallons. This is a comparison inside the calculator's stated model, not a prediction.

Where to use caution

  • Uses standard North American & metric trade dimensions, Machinery’s Handbook formulas, and culinary weight ratios.
  • Verify local municipal building codes, tool manufacturer limits, and food-safety time/temperature tables before execution.

Scope: Trade, Shop & Culinary Science Reference Standard Check the assumptions and limitations against your situation before relying on the estimate.

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