Greywater reuse is both a conservation tool and a math problem
Diverting showers, bathroom sinks, and laundry rinse cycles into a greywater system can cut potable irrigation demand by hundreds of gallons per week. Yet many homeowners install a collection tank and a few valves without knowing whether the recovered water truly matches their landscape needs. Some systems sit idle because storage overflows, while others leave plants thirsty because they overestimate capture efficiency. The Residential Greywater Irrigation Capacity Planner brings rigor to these decisions. By translating household behavior into daily water volumes and comparing those volumes with landscape evapotranspiration requirements, the planner shows whether your system is undersized, oversized, or appropriately balanced. That insight helps you prioritize upgrades, adjust irrigation schedules, and justify the investment when discussing plans with local permitting offices.
Greywater regulations vary widely, but most jurisdictions allow gravity-fed or pump-assisted systems that irrigate ornamental plants and certain fruit trees. Households typically source greywater from showers, bathtubs, bathroom sinks, and laundry machines, excluding kitchen sinks and diaper washers because of higher pathogen loads. The volume available depends on how many people live in the home, their bathing habits, appliance efficiency, and whether diverter valves send every eligible gallon to the landscape. On the demand side, landscapes need more water during hot months, and the requirement depends on plant type, soil texture, and microclimate. The planner asks for representative averages so you can gauge feasibility before commissioning a more detailed design.
Formulas translating household water use into irrigation supply
Every shower adds gallons equal to duration times flow rate, while sink usage and laundry cycles contribute their own fractions. Suppose \(N\) occupants each take \(s\) showers per day, with an average duration \(d\) minutes and flow rate \(f_s\) gallons per minute. Shower volume per day is \(N \times s \times d \times f_s\). Add bathroom sink use \(N \times m \times f_b\) and laundry flow \((L \times g)/7\) where \(L\) is loads per week and \(g\) is gallons per load. Multiply the sum by the capture efficiency \(e\) to account for imperfect diversion. The total daily captured greywater \(G_d\) feeds the irrigation supply:
Landscape demand is calculated from area \(A\) in square feet and water depth \(h\) in inches per week. Because one inch of water over one square foot equals 0.623 gallons, weekly irrigation demand \(D_w\) is:
The planner compares \(7 \times G_d\) with \(D_w\) to produce a coverage percentage. It also checks the irrigation schedule against soil infiltration. The maximum volume that can infiltrate during one irrigation event is the product of infiltration rate \(i\) in inches per hour, event duration \(t\) in hours, area \(A\), and 0.623. If per-event supply exceeds that limit, the tool flags the risk of runoff or ponding so you can lengthen the event or split it into more frequent cycles.
Storage modeling and the four-week simulation
Greywater storage tanks buffer day-to-day fluctuations between supply and irrigation demand. However, storing water for too long can lead to odor and pathogen growth, so many codes limit retention time to 24 hours unless the tank is treated. This planner assumes you have a permitted tank sized to your jurisdiction’s rules and allows you to test whether the volume is adequate. The simulation applies a daily loss percentage to represent evaporation, absorption, or intentional drain-down. At the start of each week, stored water is reduced by that loss factor, then new weekly supply is added. The system can only deliver as much water as remains after losses, up to the lesser of landscape demand and the infiltration-limited capacity across all planned irrigation days. Any excess beyond storage capacity is counted as overflow, highlighting wasted potential that might justify larger tanks or additional irrigation zones.
The weekly table summarizes supply, deliveries, demand gaps, and ending storage levels over a four-week horizon. Because the inputs are averages, the numbers repeat after a few weeks in steady state. Nonetheless, the visualization helps you see whether storage trends toward overflow or depletion. If storage grows every week, your greywater system is producing more than the landscape can absorb; you may need more irrigation days, higher-flow emitters, or a backup discharge plan. If storage drops to zero quickly and demand gaps persist, consider adding potable irrigation during peak months or expanding capture sources by including bathroom sinks or upgrading to high-efficiency fixtures that still meet code but yield more reusable water.
Worked example: Matching a family’s showers to a vegetable garden
Assume a household of four captures greywater from daily showers (nine minutes each at 2.0 gpm) and laundry (six loads per week at 30 gallons each). Bathroom sink usage averages seven minutes per person per day at 1.2 gpm. With diverter valves capturing 75 percent of eligible water, the system yields approximately 482 gallons per week. The family irrigates a 700 square-foot raised-bed vegetable garden that needs 1.5 inches of water per week, or about 654 gallons. They schedule irrigation four times per week, running a drip zone for 1.25 hours each time. The soil infiltration rate is 0.4 inches per hour, the tank holds 150 gallons, and losses are 3 percent per day.
After entering these numbers, the planner reports that greywater supply covers roughly 74 percent of demand. Each irrigation event delivers 120 gallons, but the infiltration limit is about 175 gallons, so there is room to extend runtime slightly. The four-week simulation shows storage dropping to near zero after each irrigation cycle, with no overflow but a weekly shortfall of about 172 gallons. The family can respond by adding a fifth irrigation day, capturing water from an additional bathroom sink, or supplementing with rainwater during shoulder seasons. Because storage never exceeds capacity, investing in a larger tank would not materially improve coverage unless supply increases.
Comparison of landscape types and typical water needs
| Landscape type | Water requirement (inches/week) | Notes |
|---|---|---|
| Xeric native shrubs | 0.25 – 0.5 | Often satisfied by rain except during prolonged drought; greywater can be supplemental. |
| Mixed ornamental beds | 0.75 – 1.0 | Typical for turf replacements with perennials and small shrubs. |
| Vegetable gardens | 1.5 – 2.0 | Higher demand during fruiting; drip irrigation helps apply water slowly. |
| Fruit trees | 1.0 – 1.5 | Requirements vary with canopy size; basins or subsurface emitters prevent runoff. |
Limitations and planning considerations
This tool provides a planning snapshot rather than a code-compliant engineering design. Always consult local regulations on allowable fixtures, filtration requirements, and maximum storage times. The calculator assumes that greywater quality remains suitable for irrigation; in reality, detergents, shampoos, and cleaning agents affect soil microbiology. Choose plant-friendly, low-sodium products when operating a reuse system. The simulation treats supply and demand as steady averages, so it does not model seasonal fluctuations or rainfall events. Consider adjusting inputs quarterly to reflect summer versus winter irrigation needs. Finally, be mindful that infiltration rates can change with soil compaction or mulching; field measurements with a double-ring infiltrometer yield more accurate data than textbook values. Use the planner to size your system conceptually, then engage a landscape designer or engineer to finalize piping, filtration, and control strategies.