Small Farm Rainwater Reserve Calculator

Calculate how much rainwater your farm roofs can capture, then see how many dry-season days of storage autonomy that gives you and what share of annual demand it covers.

Rainwater Reserve Outputs

Water Budget
Annual Harvest
0 L
Rain captured off the catchment area per year
Daily Demand
0 L
Household + irrigation use per day
Tank Autonomy
0 days
Days the full tank covers demand in the dry season
Annual Coverage
0%
Share of yearly demand met by harvested rain

Water Budget Components

How this small farm rainwater reserve calculator works

This calculator applies the standard rainwater-harvesting formula used for catchment planning: annual harvest equals catchment area times effective rainfall times a runoff coefficient. On a small farm the catchment is usually a barn, shed, greenhouse, or house roof rather than a single residential roof, and demand typically includes both household use and irrigation.

Annual harvest (L) = Roof area (m²) x [Annual rainfall (mm) − (First-flush loss (mm/event) x 12)] x Runoff coefficient x (1 − Downtime %)

One millimeter of rain falling on one square meter of catchment yields one liter of water before losses, so multiplying area (m²) by rainfall (mm) gives liters directly. The runoff coefficient (commonly 0.8-0.9 for metal or tile roofs) accounts for splash, evaporation, and gutter losses. First-flush diversion discards the first, dirtiest flow of each rain event to protect water quality, and collection downtime accounts for time the gutters or diverter are offline for maintenance or freezing.

Understanding the inputs

Enter catchment roof area in square meters and long-term average annual rainfall in millimeters for your site. The runoff coefficient is a value between 0 and 1 — 0.8-0.9 is typical for a metal farm roof, lower for rougher or absorbent surfaces. First-flush diversion is the volume discarded per rain event, in millimeters (2-4 mm is common). Tank size is your total storage capacity in liters. Household use and weekly irrigation use are the two main demand sources on a small farm, dry-season length estimates the stretch of the year with little rainfall, and collection downtime accounts for days the gutters or diverter are out of service.

Interpreting the results

Annual Harvest is the total rainwater your catchment can realistically capture in a typical year. Daily Demand combines household and irrigation use into one daily figure. Tank Autonomy shows how many days a full tank can supply that demand during the dry season, when no rain is topping it up. Annual Coverage shows what share of your total yearly demand the harvested rainwater can meet — a value below 100% means you will need a supplemental source, such as a well, municipal supply, or hauled water, for the rest of the year.

Frequently Asked Questions

What formula does this calculator use?
It applies the standard rainwater harvest formula: catchment area (square meters) x effective rainfall (millimeters) x a runoff coefficient, after subtracting first-flush losses and applying a collection-downtime factor. One millimeter of rain over one square meter equals one liter, so area times rainfall gives liters directly before those adjustments.
What runoff coefficient should I use for a farm roof?
Metal and tile roofs typically run 0.8 to 0.9 because they shed water efficiently with little absorption. Rougher surfaces such as wood shingles run lower, often 0.5 to 0.7. For a standard metal barn or shed roof, 0.85 to 0.9 is a reasonable default.
Why does the calculator subtract first-flush water?
The first water off any roof after a dry spell carries dust, pollen, bird droppings, and roof debris. Diverting that first flush, typically 1 to 4 millimeters of rain per event, before it reaches the tank protects stored water quality. This calculator deducts that volume, applied across up to 12 diversion events per year, from the harvest total.
How is tank autonomy calculated?
Tank autonomy divides your storage tank size in liters by your effective daily demand during the dry season, which combines household and irrigation use and scales up for the length of the dry period you enter. The result is the number of rain-free days a full tank can supply before running dry, which is the key number for sizing storage against your local dry season.