Residential Greywater Irrigation Capacity Planner

Introduction to residential greywater irrigation capacity

This residential greywater irrigation planner estimates whether water captured from showers, bathroom sinks, and laundry can meet a landscape irrigation target without overflowing storage or exceeding soil infiltration capacity. It combines average daily household supply with weekly plant demand, then models four weekly storage balances to show shortages, overflow, and end-of-week tank levels.

Use the greywater capacity estimate for early sizing conversations, seasonal what-if planning, or comparing irrigation schedules before building or modifying a reuse system. Local health rules and plumbing codes vary, so treat the results as a planning estimate and confirm eligible fixtures, filtration, storage retention, and discharge requirements with the relevant local authority.

How to use the residential greywater irrigation planner

For a household greywater irrigation estimate, enter fixture use, capture efficiency, irrigated area, plant water requirement, infiltration rate, event duration, irrigation days, tank size, and storage loss. Submit the form to see daily and weekly captured supply, demand coverage, a ponding or runoff warning, and a four-week storage simulation.

Why residential greywater reuse needs an irrigation water balance

Diverting showers, bathroom sinks, and laundry rinse cycles into a residential greywater system can reduce the potable water used outdoors. A collection tank and diverter alone, however, do not establish whether recovered water matches the landscape’s need or the soil’s ability to accept it. A system can accumulate unusable water when storage fills or irrigation events are too limited, while another can leave plants short because fixture use or diversion efficiency was assumed too optimistically. This Residential Greywater Irrigation Capacity Planner turns household water habits into daily gallons and compares them with landscape demand, helping identify whether capture, distribution, storage, or supplemental irrigation is the limiting part of the plan.

Residential greywater rules differ by location, including which fixtures may be connected and how water may be distributed. The available volume in this planner comes from showers, bathroom sinks, and laundry; kitchen fixtures are not inputs. Household occupancy, bathing habits, appliance water use, and diverter performance all affect supply. On the demand side, the irrigated area and the selected weekly water depth drive the gallons required. Use representative averages to explore feasibility, then obtain local design and permitting guidance for an installation.

Formulas translating greywater fixtures into irrigation supply

The greywater calculation starts with household fixture volumes. If occupants take showers per day, each lasting minutes at gallons per minute, daily shower volume is . Daily bathroom-sink volume is , where is sink-use minutes per person per day, and laundry contributes gallons per day, where is eligible weekly loads and is gallons per load. The calculator multiplies their sum by capture efficiency , expressed as a fraction, to obtain daily captured greywater :

Formula: G_d = e ⁢ N ⁢ s ⁢ d ⁢ f_s + N ⁢ m ⁢ f_b + L / 7 ⁢ g

Residential landscape demand is calculated from irrigated area in square feet and selected water depth in inches per week. Using 0.623 gallons per square foot for one inch of water, weekly demand is:

Formula: D_w = A ⁢ h ⁢ 0.623

The planner compares with to report demand coverage. It also compares average supply per planned irrigation day with the soil’s per-event intake limit: infiltration rate in inches per hour times event duration in hours, area , and 0.623. When supply assigned to an event is higher than that limit, the result warns of potential ponding or runoff.

Greywater tank storage and four-week irrigation simulation

The greywater storage simulation shows how a tank interacts with weekly household supply and scheduled landscape delivery. At the start of each simulated week, the previous tank balance is reduced by the entered daily storage-loss percentage compounded over seven days. The calculator then adds that week’s captured supply, delivers no more than the available water, weekly landscape demand, and total infiltration-limited capacity, and records overflow above the tank capacity. This is a weekly planning model rather than an hour-by-hour tank controller.

The four-row table reports supply, delivery, demand, ending storage, overflow, and any demand gap for each week. With unchanged inputs, the pattern can approach a repeating balance. A persistent overflow indicates that capture exceeds what the scheduled landscape application and tank can accommodate. A recurring demand gap indicates that available supply, infiltration capacity, or both are below the stated irrigation need. Compare several realistic schedules and storage sizes before relying on the result for a system decision.

Worked example: household greywater for a vegetable garden

Consider four occupants taking one nine-minute shower daily at 2.0 gallons per minute, with six eligible laundry loads per week at 30 gallons each. Bathroom sink use is seven minutes per person per day at 1.2 gallons per minute, and capture efficiency is 75 percent. These inputs produce about 98.5 gallons per day, or 689.4 gallons per week, of captured greywater. For a 700-square-foot garden requiring 1.5 inches per week, the calculated weekly demand is about 654.2 gallons.

If irrigation is scheduled four days each week for 1.25 hours per event and the soil infiltration rate is 0.4 inches per hour, average supply per event is about 172.4 gallons. The per-event infiltration capacity is about 218.1 gallons, so the calculator does not flag an infiltration exceedance. With a 150-gallon tank and a 3 percent daily loss setting, the four-week model can meet the stated weekly demand and retain a growing but capped balance; the exact storage and overflow entries in the table reflect the calculator’s weekly retention and tank-capacity steps. This example illustrates why a capacity result should be checked alongside tank overflow and local restrictions on storage and use.

Comparison of landscape types and typical water needs

Residential greywater irrigation limitations and planning considerations

This residential greywater irrigation calculator is a planning aid, not a code-compliant engineering design. Confirm local rules for allowed fixtures, filtration, distribution methods, and storage times before installing equipment. Greywater chemistry also matters: detergents, shampoos, and cleaning products can affect plants and soil, so product selection should suit the intended irrigation use. The calculation uses steady averages and does not model rainfall, changing seasons, individual irrigation-zone hydraulics, or day-by-day fixture timing. Revisit inputs as weather and landscape demand change, and use site-specific infiltration observations when available. A qualified designer or engineer can finalize piping, filtration, controls, and any required permits.