Estimate how much rainwater you can harvest from your roof or catchment area, how many tanks it fills, and what irrigation area it can support.
Enter your catchment area (roof footprint in m²), rainfall in mm, roof material (determines runoff coefficient: metal 0.9, tile 0.8, concrete 0.7, bare ground 0.3), and tank capacity. Get harvested litres, tank fills, and irrigation area.
A homeowner with a 100 m² metal roof in a 600 mm annual rainfall region installed a 5,000 L tank expecting it to be plenty. But in a single 50 mm storm, the roof captured 4,500 L — nearly filling the tank. In a wet month (150 mm), that's 13,500 L, overflowing 2.7 tanks. They needed either multiple tanks or a larger one. The calculator shows: area × rainfall × 0.9 = harvested litres.
✓ Expert Reviewed: This calculator and its content have been reviewed by agricultural experts. Formulas are based on standard extension service recommendations. For site-specific advice, consult your local agronomist.
Measure your roof or catchment area (length × width in metres). Enter the expected rainfall (per storm, per month, or annual — be consistent). Select roof material to apply the correct runoff coefficient. Enter your tank capacity. The result shows harvested litres and how many tanks that fills.
The Rainfall Harvest Calculator helps homeowners and small farmers design rainwater collection systems. It uses the simple but powerful formula: Harvested water = Catchment area × Rainfall × Runoff coefficient. With 1 m² × 1 mm = 1 L, the math is straightforward and reliable for planning.
Harvested water (L) = Catchment area (m²) × Rainfall (mm) × Runoff coefficient. The 1:1 ratio (1 m² × 1 mm = 1 L) is exact because 1 mm over 1 m² = 1 litre. Runoff coefficients: metal roof 0.9 (some splash loss), tile 0.8 (rough surface absorbs more), concrete 0.7, bare soil 0.3 (much infiltration). Subtract 5-10% for first-flush diversion.
100 m² metal roof, 50 mm rain, coefficient 0.9: Harvested = 100 × 50 × 0.9 = 4,500 L. A 5,000 L tank fills 0.9 times (90%). This supports 450 m² of garden irrigation at 10 mm depth. Annually at 600 mm rainfall: 100 × 600 × 0.9 = 54,000 L = 54,000 L/year.
Common mistakes: using roof footprint instead of actual catchment area (gutter direction matters), forgetting to apply a runoff coefficient (overestimates by 10-30%), not sizing the tank for worst-case dry periods, and ignoring first-flush diversion (divert the first 1-2 mm to wash off roof debris).
Rule of thumb: tank capacity (L) ≈ catchment area (m²) × monthly rainfall (mm) × 0.8. For a 100 m² roof in a region with 100 mm average monthly rain, design for 100 × 100 × 0.8 = 8,000 L. Also size for your dry period: if you go 60 days without rain and use 500 L/day, you need 30,000 L. Larger tanks reduce overflow but cost more upfront.
The runoff coefficient is the fraction of rainfall that actually runs off into your tank. Smooth surfaces (metal roofs) shed 90% of rain. Rough surfaces (tile, thatch) absorb and hold more, delivering 70-80%. Bare ground infiltrates most rain, yielding only 20-30%. Using the right coefficient prevents overestimating your harvest.
Roof-harvested rainwater is not potable without treatment. It may contain bird droppings, dust, roof material leachate, and algae. For drinking: add a UV filter, use food-grade tanks, install a first-flush diverter, and test annually. For garden irrigation, livestock watering, and toilet flushing, untreated rainwater is perfectly safe.
Exactly 1 litre per square metre per millimetre of rain. This is a handy mental shortcut: 100 m² roof × 1 mm = 100 L. A 10 mm rain event = 1,000 L on that roof. This 1:1:1 relationship (m² × mm = L) makes rainwater math easy.
Overflow depends on tank size vs. harvest volume. If your tank holds 5,000 L but a storm harvests 8,000 L, 3,000 L overflows (37.5%). Design: either add multiple tanks, use overflow for garden irrigation automatically, or redirect to a swale/infiltration trench. Overflow water is wasted — plan to use it before the next rain.