Free Tool
Fill in the form and click Calculate
Your mulch basin dimensions, annual water savings, and state permit status will appear here.
Daily volume = (loads per week ÷ 7) × gallons per load based on your washer type. The calculator gives you daily, monthly, and annual totals.
Basin dimensions are calculated from daily volume ÷ number of zones × soil absorption factor. Clay soils need larger surface area; sandy soils can use smaller, deeper basins.
Annual savings = annual greywater volume × your water rate per 1,000 gallons. This assumes you'd otherwise use the same volume of potable water for irrigation.
| Soil Type | Absorption Rate | Common Locations | Basin Sizing |
|---|---|---|---|
| Sandy / gravelly | >2 in/hr | Desert Southwest, coastal areas, river deltas | Smaller basins OK — 3 sq ft per 10 gpd |
| Sandy loam / loam | 0.5–2 in/hr | Most garden soils, amended beds, Pacific NW | Medium basins — 5 sq ft per 10 gpd |
| Clay loam | 0.1–0.5 in/hr | Midwest, parts of Southeast, river valleys | Larger basins — 8 sq ft per 10 gpd |
| Clay / caliche | <0.1 in/hr | Phoenix area, parts of Texas, California Central Valley | Largest — 12+ sq ft per 10 gpd, wider & shallower |
Not sure of your soil type? Dig a hole 12 inches deep, fill with water, and time how long it takes to drain. Use the absorption rate table above to match your observation.
For clay soils, the calculator recommends wider, shallower basins (more surface area to compensate for slow downward drainage). For sandy soils, deeper and narrower is acceptable. In all cases, the top 3–4 inches should be wood chip mulch to prevent evaporation and odor, with the water outlet buried in the mulch.
Distributing your daily greywater across 3–6 irrigation zones rather than sending it all to one spot gives the soil time to recover between applications. Front-loading machines are typically plumbed to allow up to 8 distribution zones; top-loaders with more water pressure can handle more zones. Each zone gets a proportional share of the daily volume — the calculator divides evenly.
These calculations assume year-round use. In practice, many homeowners redirect to sewer during rainy seasons or when the landscape is dormant. This doesn't change basin sizing (size for peak use), but your actual annual savings will be lower if you use greywater only during dry months. Systems in Texas, Arizona, and Southern California can realistically run 10–12 months of the year. Washington and Oregon homeowners should plan for 5–7 months of active use.