{
  "slug": "roof-rainwater-harvest",
  "title": "Rainwater Harvest Calculator",
  "heading": "Rainfall to Gallons Calculator (Roof Catchment)",
  "category": "other",
  "url": "https://www.revenuelab.fyi/toolbox/roof-rainwater-harvest",
  "summary": "Convert rainfall on your roof footprint into gallons collectible in a rain barrel or cistern.",
  "description": "Every square foot of roof collects roughly 0.62 gallons of water per inch of rain that falls on it, since one inch of rain over one square foot equals 0.623 gallons. Multiply that by your roof's horizontal footprint (not the sloped surface area) and a collection efficiency factor that accounts for splash-out, evaporation, first-flush diversion, and gutter overflow during heavy storms, and you get a realistic harvestable volume. This is the standard sizing calculation used by rainwater harvesting installers to size cisterns and by stormwater engineers to estimate runoff reduction from a rain barrel program. Most residential setups run 75-85% efficiency; a well-designed system with first-flush diverters and screened gutters can hit 90%.",
  "formula": "Gallons = roof footprint (sq ft) × rainfall (in) × 0.623 × collection efficiency.",
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=roof-rainwater-harvest",
  "inputs": [
    {
      "id": "footprint",
      "label": "Roof footprint area",
      "kind": "number",
      "hint": null,
      "default": 1800,
      "unit": "sq ft",
      "min": 50,
      "max": null
    },
    {
      "id": "rainfall",
      "label": "Rainfall amount",
      "kind": "number",
      "hint": null,
      "default": 1,
      "unit": "in",
      "min": 0.01,
      "max": null
    },
    {
      "id": "efficiency",
      "label": "Collection efficiency",
      "kind": "number",
      "hint": null,
      "default": 80,
      "unit": "%",
      "min": 30,
      "max": 95
    },
    {
      "id": "barrelSize",
      "label": "Storage capacity",
      "kind": "number",
      "hint": null,
      "default": 55,
      "unit": "gal",
      "min": 10,
      "max": null
    }
  ],
  "outputs": [
    {
      "id": "gallons",
      "label": "Gallons collectible",
      "format": "number",
      "hint": null,
      "primary": true
    },
    {
      "id": "perInch",
      "label": "Gallons per inch of rain",
      "format": "number",
      "hint": null,
      "primary": false
    },
    {
      "id": "barrelsNeeded",
      "label": "Barrels needed to capture it all",
      "format": "number",
      "hint": null,
      "primary": false
    },
    {
      "id": "overflow",
      "label": "Overflow past first barrel",
      "format": "number",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Roof footprint area: 1800 sq ft",
      "Rainfall amount: 1 in",
      "Collection efficiency: 80 %",
      "Storage capacity: 55 gal"
    ],
    "outputs": [
      "Gallons collectible: 897",
      "Gallons per inch of rain: 897",
      "Barrels needed to capture it all: 17",
      "Overflow past first barrel: 842"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter roof footprint area (sq ft).",
      "Enter rainfall amount (in).",
      "Enter collection efficiency (%).",
      "Enter storage capacity (gal).",
      "Read your gallons collectible on the right — it updates as you type.",
      "Hit Share to keep the scenario or send it to someone."
    ]
  },
  "scenarios": [
    {
      "name": "Conservative",
      "description": "Lower-end numbers — what if things land soft?",
      "values": {
        "footprint": 1100,
        "rainfall": 0.6,
        "efficiency": 50,
        "barrelSize": 35
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "footprint": 1800,
        "rainfall": 1,
        "efficiency": 80,
        "barrelSize": 55
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "footprint": 2900,
        "rainfall": 1.6,
        "efficiency": 95,
        "barrelSize": 90
      }
    }
  ],
  "limitations": [
    "Estimates assume standard, average conditions — local rules, pricing, and materials vary.",
    "Results are rounded for readability; add a buffer before ordering, booking, or committing.",
    "Double-check anything with a real cost attached against a local quote."
  ],
  "faq": [
    {
      "q": "Why use roof footprint instead of the actual sloped roof area?",
      "a": "Rain falls vertically, so the amount collected depends on the horizontal area it lands over, not the pitched surface area of the shingles. A steeply pitched roof has more material but the same catchment as a flat roof with the same footprint."
    },
    {
      "q": "What lowers collection efficiency below 100%?",
      "a": "Wind-driven splash-out during heavy rain, evaporation off hot shingles before runoff reaches the gutter, first-flush diverters intentionally discarding the dirtiest initial water, clogged gutters, and overflow once storage is full all cut into the theoretical maximum."
    },
    {
      "q": "How much rain does it take to fill a standard 55-gallon barrel from an average roof?",
      "a": "On a 1,800 sq ft footprint at 80% efficiency you collect about 897 gallons per inch of rain, meaning even a tenth of an inch (roughly 90 gallons) can fill a single 55-gallon barrel — most systems need multiple barrels or a cistern to capture a full storm."
    },
    {
      "q": "Is harvested rainwater safe to drink?",
      "a": "Not without treatment. Roof runoff picks up dust, bird droppings, and roofing material residue (especially from asphalt shingles), so it's fine for irrigation and non-potable uses but needs filtration and disinfection for drinking water in most jurisdictions."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/evapotranspiration-irrigation",
    "https://www.revenuelab.fyi/toolbox/snow-load-roof"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — Rainwater Harvest Calculator (https://www.revenuelab.fyi/toolbox/roof-rainwater-harvest)"
}