Rainwater Harvest Yield Calculator

Forecast rainwater collection totals and storage needs for better water planning.

sq ft
in
%
gal
gal
days

Quick Facts

Rule of Thumb
0.623
Gallons per sq ft per inch
Losses
10–30%
Efficiency reduces yield
Storage
Right Size
Match tanks to rainfall cycles
Decision Metric
Supply Days
Plan coverage by day

Your Results

Calculated
Harvest Yield
-
Total gallons collected
Storage Fill
-
Percent of storage used
Supply Days
-
Days of water supported
Daily Yield
-
Average gallons per day

Healthy Harvest Plan

Your defaults create a useful rainwater supply for the period.

Key Takeaways

  • This tool is built for scenario planning, not one-time guessing.
  • Use real baseline inputs before testing optimization scenarios.
  • Interpret outputs together to make stronger decisions.
  • Recalculate after meaningful context changes.
  • Consistency and execution quality usually beat aggressive one-off plans.

What This Calculator Measures

Estimate rainwater harvest yield based on roof area, rainfall, collection efficiency, and storage capacity.

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

The model converts rainfall and roof area into gallons collected so you can compare yield to storage needs.

How the Calculator Works

Yield = area × rainfall × 0.623 × efficiency
Storage fill: yield ÷ storage.
Supply days: yield ÷ daily use.
Daily yield: yield ÷ days.

Worked Example

  • 1200 sq ft with 3.2" rainfall yields ~2540 gallons before storage limits.
  • Efficiency reduces total yield.
  • Supply days shows how long storage can cover use.

How to Interpret Your Results

Result BandTypical MeaningRecommended Action
100%+Storage full.Consider overflow planning.
70–99%Healthy fill.Storage matches rainfall.
40–69%Partial fill.Consider additional capacity.
Below 40%Low yield.Increase collection area.

How to Use This Well

  1. Enter roof area and rainfall depth.
  2. Set collection efficiency.
  3. Add storage capacity and daily use.
  4. Review harvest yield and supply days.
  5. Adjust storage or capture area.

Optimization Playbook

  • Increase area: add gutters or capture zones.
  • Improve efficiency: reduce first-flush losses.
  • Right-size storage: avoid overflow waste.
  • Plan use: align usage with yield.

Scenario Planning Playbook

  • Baseline: typical monthly rainfall.
  • Dry month: reduce rainfall by 30%.
  • More storage: increase tank size by 500 gallons.
  • Decision rule: aim for 70% storage fill.

Common Mistakes to Avoid

  • Overestimating collection efficiency.
  • Ignoring storage overflow.
  • Assuming rainfall is constant.
  • Not aligning usage with yield.

Implementation Checklist

  1. Measure collection area.
  2. Track typical rainfall depth.
  3. Set realistic efficiency.
  4. Size storage to match yield.

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 the 0.623 factor?

It converts square feet and inches of rain to gallons.

Does efficiency include runoff loss?

Yes, it accounts for losses before the tank.

How much storage do I need?

Enough to hold the yield between rainy periods.

Related Calculators