Ecology & Environment
Greywater Reuse Calculator
Estimate daily shower and laundry greywater, captured supply, reuse demand, usable daily reuse, surplus or shortfall, storage coverage, and annual potable-water offset.
Daily shower greywater supply (L/day)-
Average daily laundry supply (L/day)-
Gross recoverable source water (L/day)-
Captured daily greywater (L/day)-
Entered reuse demand (L/day)-
Supply-limited daily reuse (L/day)-
Captured supply minus demand (L/day)-
Storage for entered coverage (L)-
Annual potable-water offset (m³/year)-
Decision view
Daily greywater source-to-reuse pathway
Daily greywater source-to-reuse pathwayShower and laundry supply pass through recovery, the demand cap, storage coverage, and annual potable offset.
Exact scenario comparisonCapture and treatment recovery (%) changes while all other entered assumptions remain constant.
| Capture and treatment recovery (%) | Daily shower greywater supply (L/day) | Average daily laundry supply (L/day) | Gross recoverable source water (L/day) | Captured daily greywater (L/day) | Entered reuse demand (L/day) | Supply-limited daily reuse (L/day) | Captured supply minus demand (L/day) | Storage for entered coverage (L) | Annual potable-water offset (m³/year) |
|---|
How to use Greywater Reuse Calculator
- Enter occupants and recoverable shower water per person-day.
- Enter weekly laundry loads and recoverable litres per load.
- Enter recovery efficiency, permitted demands, and storage days.
- Use the flow balance to identify whether supply or demand is limiting.
Calculator guide
Understanding Greywater Reuse Calculator
Greywater reuse is limited by whichever is smaller: recoverable supply or permitted demand. This calculator normalizes shower and laundry sources to a daily basis, applies treatment recovery, and keeps storage sizing separate from annual potable-water offset.
Calculation method
How the calculation works
Average shower and laundry source volumes to a daily basis, apply recovery efficiency, cap reuse by entered demand, and size a simple storage coverage volume. Convert every source to litres per day, apply the entered capture efficiency, cap usable reuse at daily demand, then multiply the usable flow by storage days and 365 days.
Detailed calculation process
Balance daily greywater supply against permitted reuse demand
The default case has four occupants, six laundry loads per week, 85% recovery, 200 L/day of total demand, and one day of storage coverage.
General formula: S_s = Nq_s; S_l = Lq_l/7; S_c = eta(S_s+S_l); D = D_i+D_o; R = min(S_c,D); V = Rt; O = 365R/1000 Shower and laundry volumes are first expressed on the same daily basis. Capture efficiency reduces gross source water, usable reuse cannot exceed either captured supply or demand, and litres are divided by 1,000 when annual offset is reported in cubic metres.
What each symbol means
Worked substitution with the default inputs
The default system captures and reuses 169.271 L/day, needs 169.271 L for one storage day, and offsets about 61.784 m3/year while remaining supply-limited.
Water pathway
See source water split, recovery loss, demand, and storage
The diagram reconciles every daily litre before the annual conversion.
Worked situations
Practical examples
- Default gross supply is 199.142857 L/day.
- After 85% recovery, captured supply is 169.271429 L/day.
- Because demand is 200 L/day, all captured supply is usable.
Better inputs
Useful tips
- Check whether local rules permit each intended end use.
- Keep blackwater and prohibited kitchen sources out of the estimate.
- Avoid treating calculated storage volume as a complete tank specification.
Before relying on the result
Limitations and common mistakes
- Daily averages do not model the timing mismatch between generation and demand.
- Treatment losses, seasonal irrigation, drawdown limits, stagnation, overflow, and tank dead volume are simplified.
- Plumbing, public-health, cross-connection, and permitting requirements need qualified local review.
Reference
Key terms
- Captured supply
- Gross recoverable source water after the entered recovery efficiency.
- Supply-limited reuse
- Usable reuse capped by the available treated water.
- Potable offset
- Modeled mains-water demand displaced by reused greywater.
Important note
Calculated from the entered environmental values using the displayed model. Measurement quality, local conditions, system boundaries, and source data affect interpretation.
Frequently asked questions
Why divide weekly laundry water by seven?
Every supply and demand must share the same daily time basis before comparison.
Why is usable reuse not the captured supply in every case?
When demand is lower than supply, the model caps reuse at demand.
Does storage eliminate a daily shortfall?
Not in this average balance; storage shifts timing but does not create additional water.
Is annual offset guaranteed?
No. It assumes the modeled daily reuse persists for 365 days.