120 m2 roof - how much rainwater can you collect?

    One roof, one formula, twelve months of free water. Enter your roof area, local rainfall and garden size to see how much rainwater you can collect, what tank size you need and how much you save on water bills.

    A 120 m2 roof in a moderate climate (800 mm annual rainfall) collects roughly 120 x 0.64 m3 of rainwater per year after accounting for the 0.8 runoff coefficient. That is enough to cover a medium garden through the entire growing season with a properly sized tank. The calculator below takes your 120 m2 roof area, asks for local rainfall and garden size, then recommends a tank capacity and shows how much of your watering bill disappears when you switch from mains water to what falls on your roof for free.

    Parameters

    Enter data for calculations

    Roof footprint, not total surface area

    Check local weather data for exact figures

    Lawn + beds that need watering

    Auto-select matches tank to your garden size

    US average: $3-6/m3

    Form progress0 / 4 fields

    💡 Fill in all required fields to unlock the calculate button

    How much free water is falling on your roof right now

    A 120 m2 roof in an area with 800 mm of annual rainfall collects 76,800 liters per year. That is 76.8 m3 of water - enough to cover a 100 m2 garden for the entire growing season and still have surplus left over for car washing. The formula is simple: roof area (m2) x rainfall (m) x 0.8 runoff coefficient. The calculator does the math, recommends a tank size and tells you how much of your garden watering you can replace with free rainwater.

    Quick start - what do the numbers mean

    Roof area: measure length x width of your house from above (the footprint). A two-story house with a 10 x 12 m footprint has a 120 m2 roof - regardless of roof slope.

    Runoff coefficient 0.8: not all rain reaches your tank. About 20% is lost to evaporation, gutter overflow and first-flush diversion. Tile and metal roofs achieve 0.8-0.9. Flat gravel roofs: 0.6-0.7.

    20 L/m2/week: standard garden watering rate during summer. Lawns need 20-25 L/m2/week, vegetable beds 15-30 L depending on crop.

    Six scenarios - from rain barrel to underground cistern

    Small garden, dry climate: Roof 80 m2, 400 mm rain, 30 m2 garden. Yield: 25,600 L/year. A 300 L rain barrel handles weekly demand (600 L/week). Coverage: 100% - plus surplus.
    Typical suburban home: Roof 120 m2, 800 mm rain, 100 m2 garden. Yield: 76,800 L/year. Recommended: 1000 L IBC tote. Demand: 2000 L/week. Coverage: 100%.
    Large garden, moderate rain: Roof 150 m2, 600 mm rain, 200 m2 garden. Yield: 72,000 L/year. Recommended: 2000 L cistern. Demand: 4000 L/week over 16 weeks = 64,000 L. Coverage: 100%.
    Apartment with balcony (no roof): Roof 0 m2. Zero yield. Rain barrels only work with guttered roof area. Consider a 200 L diverter barrel connected to a shared downpipe if your building allows it.
    Rainy climate, small roof: Roof 60 m2, 1200 mm rain, 50 m2 garden. Yield: 57,600 L/year. Even a small roof in a wet climate collects plenty. A 1000 L tank covers the full season.
    Whole-house rainwater system: Roof 180 m2, 1000 mm rain, toilets + garden. Yield: 144,000 L/year. With a 5000 L underground cistern, a pump and filtration, you can flush toilets and run the washing machine on rainwater year-round.

    How to use the rainwater calculator - step by step

    1. Roof area (m2) - measure your roof footprint from above. Include the garage if its gutters feed into the same tank.
    2. Annual rainfall - select the closest match to your region. Check local weather data for the exact figure (in mm per year).
    3. Garden area to water (m2) - only the parts you actually irrigate: lawn, flower beds, vegetable patches.
    4. Tank capacity - pick a size or let the calculator auto-select based on your garden area.
    5. Water price (optional) - enter your local rate per m3 to see how much rainwater saves you per year.

    Rainwater yield by roof size and rainfall

    Roof area 400 mm/year 600 mm/year 800 mm/year 1000 mm/year 1200 mm/year
    60 m2 19,200 L 28,800 L 38,400 L 48,000 L 57,600 L
    100 m2 32,000 L 48,000 L 64,000 L 80,000 L 96,000 L
    150 m2 48,000 L 72,000 L 96,000 L 120,000 L 144,000 L
    200 m2 64,000 L 96,000 L 128,000 L 160,000 L 192,000 L

    All values use a runoff coefficient of 0.8 (tile or metal roof). Flat gravel roofs: multiply by 0.75-0.85.

    Tank size guide

    Tank type Capacity Best for Price range
    Rain barrel 200-300 L (55-80 gal) Garden under 30 m2 $30-80
    Large barrel 500 L (130 gal) Garden 30-50 m2 $60-120
    IBC tote 1000 L (265 gal) Garden 50-100 m2 $50-150
    Above-ground cistern 2000-5000 L Garden 100+ m2 $200-600
    Underground tank 3000-10,000 L Whole-house system $1,500-5,000+

    Practical examples

    Starter setup (80 m2 roof, 800 mm rain, 40 m2 garden): Yield: 51,200 L/year. Demand: 800 L/week x 16 weeks = 12,800 L. A 500 L barrel covers 100%. Savings at $4.50/m3: $57.60/year.
    Family home (120 m2 roof, 600 mm rain, 100 m2 garden): Yield: 57,600 L/year. Demand: 2000 L/week x 16 = 32,000 L. 1000 L IBC tote recommended. Coverage: 100%. Savings: $144/year.
    Large property (200 m2 roof, 1000 mm rain, 300 m2 garden): Yield: 160,000 L/year. Demand: 6000 L/week x 16 = 96,000 L. 2000 L cistern. Coverage: 100%. Savings: $432/year.
    Dry climate challenge (100 m2 roof, 400 mm rain, 150 m2 garden): Yield: 32,000 L/year. Demand: 3000 L/week x 16 = 48,000 L. Coverage: 67%. Rainwater saves $96/year but you still need mains water for the rest.
    Wet climate bonus (150 m2 roof, 1200 mm rain, 80 m2 garden): Yield: 144,000 L/year. Demand: 1600 L/week x 16 = 25,600 L. Coverage: 100% with 118,400 L surplus. Use it for toilet flushing, laundry or car washing.

    FAQ

    How big a rain tank do I need?
    It depends on your garden size. Rule of thumb: 1000 liters per 50 m2 of watered area. A 30 m2 garden needs a 300-500 L barrel. A 100 m2 garden needs 1000 L. Over 150 m2 - consider 2000 L or two linked tanks.
    Can I drink collected rainwater?
    Not without treatment. Roof runoff contains dust, bird droppings, leaf tannins and potentially chemical residues. For drinking, you need sediment filtration, UV sterilization and ideally a carbon filter. For garden watering, car washing and toilet flushing, untreated rainwater is fine.
    What is a first-flush diverter?
    A device that discards the first 1-2 liters of rainfall per m2 of roof. The initial runoff carries the most dirt, pollen and bird droppings. After the first flush drains away, clean water flows into your tank. Cost: $15-40. Essential for any system.
    Does roof type affect collection?
    Yes. Metal and tile roofs have a runoff coefficient of 0.8-0.9 (most water reaches the tank). Flat gravel roofs: 0.6-0.7 (more absorption). Thatched/green roofs: 0.3-0.5 (most water absorbed). The calculator uses 0.8 as the standard for hard roofing.
    What happens to the tank in winter?
    Above-ground tanks can freeze and crack in cold climates. Options: (1) drain before the first frost and reconnect in spring, (2) insulate with foam wrap, (3) install an underground tank below the frost line (no freezing, but higher cost). Most rain barrels are designed for seasonal use only.
    How fast does a rain barrel fill up?
    A 120 m2 roof in 10 mm of rain (a moderate shower) collects 960 liters (120 x 0.01 x 0.8 = 0.96 m3). That fills a 300 L barrel 3 times over from a single rain event. The limiting factor is tank size, not rainfall.

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    See also

    Rainwater from a 60 m2 Roof - Annual Yield and Tank SizeRainwater from a 80 m2 Roof - Annual Yield and Tank SizeRainwater from a 100 m2 Roof - Annual Yield and Tank SizeRainwater from a 150 m2 Roof - Annual Yield and Tank SizeRainwater from a 180 m2 Roof - Annual Yield and Tank SizeRainwater from a 200 m2 Roof - Annual Yield and Tank SizeRainwater from a 250 m2 Roof - Annual Yield and Tank Size

    Calculator verified by the LiczGrupa.pl team

    Content, formulas and results have been reviewed for accuracy and relevance by our team of specialists.

    Natalia Skrzek

    Reviewed by: Natalia Skrzek

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