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Rainwater Catchment Yield Calculator

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Collected rainwater is 530 US gal.

Collected rainwater
US gal
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How to use this calculator

  1. Enter Catchment area
    Enter the surface area contributing runoff, in sq ft. For a roof, use the area that sends runoff into the collection system. 2
  2. Enter Rainfall depth
    Enter rainfall depth in inches from the forecast you are evaluating. The formula applies this depth to the entered catchment area. 2
  3. Enter Runoff coefficient
    Enter the captured share as a percentage of total rainfall. A higher coefficient represents a less absorbent surface. 2
  4. Read Collected rainwater

Collected rainwater (US gal) vs Catchment area (sq ft)

529870.52991100000

Collected rainwater (US gal) Catchment area (sq ft)

What this calculates

Estimate collected rainwater from the surface, rainfall depth, and captured share that you enter. A catchment area is the surface from which rainwater runoff is collected. 2

The result is a volume estimate for the entered conditions. Rain barrels conserve water and reduce storm water runoff. 1

Enter the values

Enter catchment area in sq ft, rainfall depth in in, and runoff coefficient as a percent. The calculator returns collected rainwater in US gal. 4

Use rainfall depth from the forecast you are evaluating. Keep the entries tied to the same catchment and rainfall conditions. 2

Read the collected yield

Read the returned value as collected rainwater in US gal. The result is a volume for the entries you provide. 4

Volume is the three-dimensional space occupied by matter or enclosed by a surface, measured in cubic units. 3

Follow the formula in stages

rainwater_volume = catchment_area * rainfall / 12 * runoff_coefficient / 100 * 12 * 12 * 12 / 231 2

Area stage divides rainfall by 12 to express inches as feet. Capture stage divides runoff_coefficient by 100 to apply its percentage. Conversion stage applies the remaining factors in displayed order. 2

In this formula, catchment_area is the entered surface area, and rainfall is its depth. runoff_coefficient is the captured percentage, and rainwater_volume is collected rainwater. 2

Choose the surface inputs

Enter runoff_coefficient as the percentage of rainfall captured from the surface. The formula divides that percentage by 100 before applying it. 2

Roofing materials affect the captured share. The cited source gives about 75 to 95 percent for a typical home with a sloped roof. 2

Worked examples

Mass.gov example: 1 inch on a 1,000 sq ft roof: Collected rainwater 623 US gal

See inputs and calculation

Starting values

Catchment area
1000 sq ft
Rainfall depth
1 in
Runoff coefficient
100 %
    1. Collected rainwater: 623 US gal
      Show arithmetic
      1000 * 1 / 12 * 100 / 100 * 12 ^ 3 / 231

    TAMU typical sloped-roof assumption: Collected rainwater 530 US gal

    See inputs and calculation

    Starting values

    Catchment area
    1000 sq ft
    Rainfall depth
    1 in
    Runoff coefficient
    85 %
      1. Collected rainwater: 530 US gal
        Show arithmetic
        1000 * 1 / 12 * 85 / 100 * 12 ^ 3 / 231
      Original example and source context

      Answer: "1000 sq ft, 1 in, 85 % → 529.8701298701299 US gal". 1

      Inputs: catchment area is 1000 sq ft, rainfall depth is 1 in, and runoff coefficient is 85 %. 2

      Area stage: 1000 * 1 / 12. Capture stage: 85 / 100. Conversion stage: apply * 12 * 12 * 12 / 231 to reach the quoted result. 2

      Connect yield with storage

      System planning starts with the volume of water that needs storage. That volume relates to catchment surface area and the runoff you want to capture. 2

      Use this estimate when comparing collected runoff with a storage need. The supplied material does not provide a tank-sizing equation, so this page does not calculate tank size. 2

      Compare rainfall scenarios

      To represent a city, enter its rainfall depth as the rainfall input. Keep catchment area and runoff coefficient tied to the surface being evaluated. 2

      No city name appears as a calculator input, and the supplied value rows have no city labels. This page therefore gives scenario calculations rather than city yields. 2

      Understand the quality scope

      Collected volume does not establish water quality. The supplied material addresses capture and surface absorbency, not quality guidance. 2

      Key facts

      • A gallon is 231 cubic inches. 4 4
      • Rain barrels conserve water and reduce storm water runoff. 1 1
      • The volume of water needing storage is the most important system-design factor. 2 2

      How this is calculated

      Estimate collected rainwater by multiplying catchment area by rainfall depth, applying the runoff coefficient, converting the resulting cubic inches using the exact NIST definition of a gallon as 231 cubic inches, and rounding the displayed gallon result.

      Collected rainwater

      A × R ÷ 12 × runoff_coefficient ÷ 100 × 12 ^ 3 ÷ 231
      • A = Catchment area (sq ft)
      • R = Rainfall depth (in)
      • runoff_coefficient = Runoff coefficient (%)
      Understand each part

      These names are shorthand for the exact calculations below, not different formulas.

      1. Catchment area (A)

        The roof or other impervious collection area in square feet.

        See the definition of Acatchment_area
      2. Rainfall depth (R)

        The rainfall depth in inches.

        See the definition of Rrainfall
      3. Runoff coefficient (C)

        The captured fraction of rainfall expressed as a percentage.

        See the definition of Crunoff_coefficient / 100
      4. Collected rainwater (V)

        The estimated collected volume in U.S. gallons.

        See the definition of VA * 144 * R * runoff_coefficient / 100 / 231
      Follow the calculation with your values
      1. Collected rainwater: 530 US gal
        Show arithmetic
        1000 * 1 / 12 * 85 / 100 * 12 ^ 3 / 231
      Full formulas, symbols and sources

      rainwater_volume = catchment_area * rainfall / 12 * runoff_coefficient / 100 * 12 ^ 3 / 231

      catchment_area
      Catchment area (sq ft)
      rainfall
      Rainfall depth (in)
      runoff_coefficient
      Runoff coefficient (%)
      rainwater_volume
      Collected rainwater (US gal)

        Frequently asked questions

        How do I use a rainwater harvesting calculator?
        Provide the catchment area, rainfall depth, and runoff coefficient in their displayed units. Then read the returned collected-rainwater value as the estimate for those entries. 2
        What is the rainwater harvesting formula?
        The formula multiplies area and rainfall, applies the captured percentage, and converts the resulting volume to US gallons. Its divisions and multiplications preserve the unit changes shown in the formula. 2
        What runoff coefficient should I use for my roof?
        Use a runoff coefficient to estimate the share of rain your roof can capture. The coefficient depends on the roofing materials, and a less absorbent surface has a higher coefficient. 2
        How much rainwater can my roof collect?
        A Massachusetts rule of thumb gives 623 gallons from 1 inch of rain on a 1000 sq ft roof. Your page estimate can differ because it also applies the runoff coefficient you enter. 1
        How does rainwater catchment work?
        Rain falls on an impervious catchment surface, and its runoff is collected. A larger surface changes the volume available under the entered rainfall and coefficient. 2
        What factors affect harvest potential?
        Harvest potential depends on catchment surface area, rainfall amount, and the share of runoff captured. The storage volume you need also matters when designing the system. 2
        How large should my rainwater tank be?
        Size a tank around the volume of water that needs storage, not yield alone. The cited material links that storage volume to catchment area and the runoff you want to capture, but this page has no tank-size input. 2
        When should I use a rainwater harvesting calculator?
        Use the calculator when you need a volume estimate for entered surface and rainfall conditions. It can frame storage planning because system design depends on the volume needing storage. 2
        Can I compare rainwater yield by city?
        Use a city's rainfall depth as the rainfall input when comparing places. The calculator has no city field, so it cannot produce a city result without that value. 2

        Limitations & common mistakes

        • The page accepts catchment area from 1 to 100000 sq ft, rainfall depth from 0.01 to 100 in, and runoff coefficient from 0 to 100 %; these ranges are the page's choices, not properties of the formula.
        • Outside those choices, the calculator refuses to answer and shows: "Catchment area and rainfall must be positive, and the runoff coefficient must be between 0 and 100 percent."
        • Results are computed from the numbers you enter and are only as exact as those numbers; they are not a substitute for a measurement or for a professional's judgement where one is required.

        By Editorial Team, Assembled from the cited primary sources; not yet expert-reviewed

        Last updated

        Sources current as of

        How we review

        Formula, sources, and review

        This calculation follows the formula published on this page and rests on 4 cited sources, listed under Sources below. Last checked .

        How we review

        Sources & methodology

        ConstantValueUnitSource
        rainwater_catchment_yield_c1122
        rainwater_catchment_yield_c232
        rainwater_catchment_yield_c32314

        Sources