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Calcimator

Rainwater Harvesting Calculator

Calculate annual rainwater collection potential from your roof. Determine tank sizing and whether collection meets your water needs.

Inputs

sq ft
inches
%
gal
gal/day

Results

Annual Collection

26,166 gal

≈ 3 tanker trucks

Days of Storage

50 days

Monthly Average2,181 gal
Annual Surplus/Deficit7,916 gal
First Flush Volume15 gal
How to Use This Calculator
  1. Measure your Roof Area Sq Ft — the catchment surface — and look up your Annual Rainfall (inches) from local weather records.
  2. Set Collection Efficiency (%) accounting for first-flush diversion, evaporation, and roof material losses (typically 80–90%).
  3. Enter Storage Tank Gallons and Daily Usage Gallons to calculate how many days of supply your tank provides.
  4. Review Annual Collection (gal) and Monthly Average to assess whether rainfall matches your seasonal demand.
  5. Check Days of Storage to evaluate whether your tank is sized for drought periods or just supplemental use.
  6. Use Surplus/Deficit (gal/yr) and First Flush Volume to design overflow routing and pre-tank filtration.

How the result changes with Roof Collection Area

Roof Collection AreaAnnual CollectionDays of Storage
2,09036,458 gal50 days
7,065123,242 gal50 days
13,035227,383 gal50 days
18,010314,166 gal50 days

What each input means

Roof Collection Area
Horizontal footprint of your roof area connected to gutters.
Annual Rainfall
Average annual rainfall for your location.
Collection Efficiency
Percentage of rain actually captured (losses from splash, first flush, etc.).
Storage Tank Capacity
Total rainwater storage tank capacity.
Daily Usage
Daily water usage from rainwater system (garden, livestock, laundry, etc.).

What each result means

Annual Collection
Total gallons collected per year.
Monthly Average
Average monthly collection (varies seasonally).
Days of Storage
How many days your tank lasts without rain.
Annual Surplus/Deficit
Positive means collection exceeds usage.
First Flush Volume
Gallons to divert per rain event for clean collection.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Roof Collection Area = 1500, Annual Rainfall = 35, Collection Efficiency = 80, Storage Tank Capacity = 2500 = 5 input(s) provided
  2. Calculate Annual Collection
    Annual Collection
    26166 = 26166
  3. Calculate Days of Storage
    Days of Storage
    50 = 50
  4. Calculate Monthly Average
    Monthly Average
    2181 = 2181
  5. Calculate Annual Surplus/Deficit
    Annual Surplus/Deficit
    7916 = 7916

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators.

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