Estimate how much rain your roof can harvest, how long a full storage tank lasts on its own, and how much of a dry season your reserve can cover.
Rainwater Reserve Outputs
Water Budget
Annual Harvest
0 L
Rain captured off the roof per year, net of first-flush losses
Daily Demand
0 L
Household + irrigation use per day
Tank Autonomy
0 days
Days a full tank alone covers demand with zero rain
Dry-Season Coverage
0%
Share of dry-season demand a full tank alone can supply
Water Budget Components
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How the Dry Climate Rainwater Reserve Calculator works
This calculator applies the standard rainwater-harvesting yield formula used for sizing catchment systems: Volume (L) = Roof area (m²) × Rainfall (mm) × Runoff coefficient, which holds because one millimeter of rain falling on one square meter of surface equals exactly one liter of water before losses. It nets out an estimated first-flush loss, then compares the resulting harvest against your household's daily water use and storage tank size to show how far stored water alone can stretch through a dry season.
Understanding the inputs
Enter your catchment/roof area in square meters and your site's average annual rainfall in millimeters (a local climate normal is a good source). The runoff coefficient accounts for water lost to evaporation, overspray, and absorption before it reaches the tank — use roughly 0.8–0.9 for metal or tile roofs and 0.7–0.8 for shingle roofs. First-flush diversion is the small depth of dirty initial runoff discarded at the start of each storm to protect water quality; the calculator assumes about 12 qualifying rain events per year when totaling this loss. Storage tank size, household water use, and irrigation use describe demand, and dry-season length sets how many months stored water needs to cover with little or no rain.
Interpreting the results
Annual Harvest is the volume your roof can realistically deliver to the tank each year. Daily Demand combines household and irrigation use into one daily draw-down rate. Tank Autonomy divides tank size by that daily demand to show how many rain-free days a full tank alone can cover. Dry-Season Coverage is the check specific to dry climates: it compares full tank capacity against total demand over the entire dry season, assuming no rain falls during that stretch, so a low percentage signals that storage capacity or catchment area may need to grow.
Frequently Asked Questions
How is the annual rainwater harvest calculated?
Annual harvest uses the standard rainwater yield formula: roof area in square meters times annual rainfall in millimeters times the runoff coefficient, since one millimeter of rain over one square meter equals one liter. The calculator also subtracts an estimated annual first-flush loss (first-flush depth times about 12 qualifying rain events per year) before applying the runoff coefficient, since the first flush of a storm is normally diverted away from storage.
What runoff coefficient should I use?
The runoff coefficient is the share of rain landing on the catchment that actually reaches the tank; the rest is lost to evaporation, absorption, and overspray. Metal or tile roofs typically run 0.8 to 0.9, while shingle or textured roofs run closer to 0.7 to 0.8, and the value should always stay below 1.
What is the difference between tank autonomy and dry-season coverage?
Tank autonomy is how many days a full tank can meet total household plus irrigation demand with zero rainfall, calculated as tank size divided by daily demand. Dry-season coverage instead compares that same full tank capacity against total demand over the entire dry season, a conservative check on whether stored water alone, with no rain at all, gets you through the driest stretch.
Why subtract a first-flush volume from the rainfall?
The first flush of runoff from any roof carries accumulated dust, pollen, and debris and is normally diverted away from the tank rather than collected. Diverting a small depth at the start of each rain event improves stored water quality at a small cost to total harvested volume.