Rainfall Harvest Calculator
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.
Results
How to Use This Calculator
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.
About This Tool
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.
Applicability: Suitable for irrigation planning in North America and Europe, adaptable to semi-arid and Mediterranean climates worldwide.
How the Calculation Works
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.
Real-World Example
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 & Tips
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).
Reviewed by Agricalc Editorial Team — Agricultural engineers and agronomists with 10+ years of field experience. Our team verifies every calculator formula against peer-reviewed extension publications. For site-specific advice, consult your local agronomist.
References
- USDA NRCS — Rainwater Harvesting
- FAO — Rainwater Harvesting for Agriculture
- Oregon State Extension — Rainwater Harvesting
Frequently Asked Questions
What size rainwater tank do I need?
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.
What is a runoff coefficient and why does it matter?
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.
Can I drink rainwater from my roof?
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.
How much water falls on my roof per mm of rain?
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.
How much overflow can I expect?
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.
