Calculate the gallons and dollars you save by reusing household greywater, plus system payback time and the landscape area your greywater can irrigate.
Greywater Reuse Savings Formula
The calculator estimates the greywater your household produces, then converts the portion you reuse into gallons and dollars saved. The quick estimate uses total indoor water use:
GW = P * IU * (GS/100)
The detailed estimate builds the same number from individual sources:
GW = (N * t * F) + (L * V / 7) + (P * S)
Annual savings come from the reused share priced at your utility rate:
AS = (GW * (E/100) * 365 / 1000) * (WR + SR)
Payback and irrigation coverage use:
PB = (C - R) / (AS - M)
A = (7 * GWd) / (0.623 * D)
- GW is greywater produced (gal/day)
- P is people in the household
- IU is indoor water use (gal/person/day)
- GS is the share of indoor use that is greywater (%)
- N is showers per day, t is minutes per shower, F is showerhead flow (gal/min)
- L is laundry loads per week, V is gallons per load
- S is bathroom sink use (gal/person/day)
- E is reuse efficiency (%), the share of collected greywater actually put to use
- AS is annual savings ($/yr), WR and SR are water and sewer rates ($/1,000 gal)
- PB is simple payback (years), C is system cost, R is rebates, M is annual maintenance
- A is irrigable area (sq ft), GWd is greywater used (gal/day), D is plant water demand (in/week)
The savings mode applies the first three formulas and reports daily, monthly, and annual gallons and dollars, with an optional cap at your real outdoor watering need. The payback mode applies the fourth formula to an installed cost. The irrigation mode applies the fifth formula, which converts weekly gallons into landscape area using the rule that one inch of water over one square foot equals 0.623 gallons.
Typical Greywater Output by Source
Use this table to sanity check your inputs. Efficient fixtures cut greywater volume, which also lowers how much you can reuse.
| Source | Standard fixture | Efficient fixture |
|---|---|---|
| Shower (8 minutes) | 20 gal (2.5 gal/min) | 14.4 gal (1.8 gal/min) |
| Laundry (per load) | 30 to 40 gal | 14 to 20 gal |
| Bathroom sink (per person/day) | 4 to 6 gal | 2 to 3 gal |
| Household of 4 (per day) | 120 to 200 gal | 80 to 120 gal |
Landscape Area Your Greywater Can Irrigate
Savings only count when greywater replaces water you would have purchased, so match your reuse to real irrigation demand. This table shows how much landscape different daily greywater volumes can support at three plant demand levels, using A = 7 x gallons per day / (0.623 x inches per week).
| Greywater (gal/day) | Low demand (0.5 in/wk) | Moderate (1.0 in/wk) | High (1.5 in/wk) |
|---|---|---|---|
| 50 | 1,124 sq ft | 562 sq ft | 375 sq ft |
| 100 | 2,247 sq ft | 1,124 sq ft | 749 sq ft |
| 150 | 3,371 sq ft | 1,685 sq ft | 1,124 sq ft |
| 200 | 4,494 sq ft | 2,247 sq ft | 1,498 sq ft |
Low demand fits established natives and drought tolerant beds. High demand fits lawns and vegetables in warm weather. If the area you can irrigate is smaller than the area your greywater could cover, use the outdoor watering cap in the calculator so the savings stay honest.
Greywater Reuse Savings Example Problems
Example 1: Annual savings for a family of four. The household has 4 people using 60 gallons per person per day indoors, with 60% of that water being greywater. The system reuses 80% of what it collects and water costs $6.00 per 1,000 gallons.
Greywater produced: GW = 4 x 60 x 0.60 = 144 gal/day. Reused: 144 x 0.80 = 115.2 gal/day. Annual volume: 115.2 x 365 = 42,048 gallons. Annual savings: (42,048 / 1,000) x $6.00 = $252.29 per year, or about $21 per month. At a 4% annual rate increase, that compounds to roughly $3,029 over 10 years.
Example 2: How much garden 100 gallons per day can water. A laundry and shower system delivers 100 gal/day to plants with moderate demand of 1.0 inch per week.
Weekly volume: 100 x 7 = 700 gallons. Area: 700 / (0.623 x 1.0) = 1,124 sq ft, roughly a 34 ft by 34 ft planted area. If your plants only need 0.5 inch per week, the same water stretches to about 2,247 sq ft.
Greywater Reuse Savings FAQ
Does reusing greywater lower my sewer bill too? Usually not directly, and this is the most common reason savings estimates run high. Most utilities bill sewer on the metered water entering your home, and greywater reuse does not change that meter reading; it replaces outdoor water you would have bought separately. Some utilities set sewer charges from winter average use, which already excludes summer irrigation. Only add a sewer rate in the calculator if the water you are offsetting is genuinely billed for sewer, for example when treated greywater replaces toilet flushing water inside the home.
Which household water counts as greywater? Showers, bathtubs, bathroom sinks, and laundry are greywater in most plumbing codes. Toilets are blackwater and never count. Kitchen sinks and dishwashers are excluded in most US codes because grease and food waste make the water too high strength for simple reuse, although a few states treat kitchen water as dark greywater with extra requirements.
Is greywater reuse legal, and what does a system cost? Rules vary by state and city. Many states allow simple laundry to landscape systems without a permit because they do not cut into existing drain plumbing; these typically cost $300 to $1,500 installed. Branched drain systems run about $1,500 to $4,000, and pumped or treated systems range from $3,000 to $25,000 and usually need permits. Check your local code before building, and use the payback mode to test whether the system cost makes sense against your annual savings.
