Water Demand Calculator (household water use per day by fixture)

Understanding the household water demand calculator — concept and principle

This calculator estimates how much water a household uses per day, month and year, broken down by showers, toilets, taps, washing machine and dishwasher. Use it to size a water tank, compare with your water bill, or see which change would save the most.

Showers are the biggest use here, so they offer the biggest saving.

How it is calculated

Showers = people × minutes × flow (L/min)

Toilets = people × flushes × litres per flush

Taps = people × minutes × flow (L/min)

Washing machine = loads per week × litres per load ÷ 7; dishwasher likewise

Total per day = all uses + outdoor use

Per person = total ÷ people; US gallons = litres ÷ 3.785411784.
Per month = total × 30.4375 ÷ 1,000 m³; per year = total × 365.25 ÷ 1,000 m³.

The principle

Water use is the sum of flows and volumes. Fixtures that run continuously, such as showers and taps, use flow rate × time; fixtures that use a fixed amount per cycle, such as toilets and machines, use volume × number of uses. Weekly appliance loads are spread over 7 days. Default flows follow US federal fixture standards quoted by EPA WaterSense: showerheads 2.5 gal/min (about 9.5 L/min), toilets 1.6 gal (about 6 L) per flush and bathroom taps about 2 gal/min (7.6 L/min).

Worked example

Default inputs: 3 people; 8 min showers at 9.5 L/min; 5 flushes of 6 L; 5 min taps at 7.6 L/min; 4 washes of 100 L and 4 dishwasher loads of 12 L per week; no outdoor use.

Showers = 3 × 8 × 9.5 = 228 L; toilets = 3 × 5 × 6 = 90 L; taps = 3 × 5 × 7.6 = 114 L.
Washing machine = 4 × 100 ÷ 7 = 57 L; dishwasher = 4 × 12 ÷ 7 = 7 L.
Total = 228 + 90 + 114 + 57.1 + 6.9 = 496 L a day; per person 165 L or 43.7 US gal.
Per month = 496 × 30.4375 ÷ 1,000 = 15.1 m³; per year = 181.2 m³.
Toilet share = 90 ÷ 496 = 18 %.

Each point is the calculator re-run with only “Shower minutes per person per day” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Shower minutes per person per day	Household use per day (L)
2	325
4	382
6	439
8	496
10	553
12	610
16	724

How to read the result

Compare the monthly figure with your water bill, which is usually in cubic metres. The breakdown shows where saving pays off: here showers are the largest use, so a shorter shower or a lower-flow head saves the most. EPA cites an average of about 82 gallons (about 310 L) per person per day at home in the US, and toilets can be about 27 % of indoor use.

Common mistakes

Guessing shower flow; time how long it takes to fill a bucket.
Forgetting garden watering in summer.
Using the rated flow of an old fixture as if it were a new one.
Ignoring leaks, which do not show in the breakdown but do on the meter.

Keep in mind: The result depends entirely on the inputs. Measure flows and check the meter over a week for a reliable figure.

Frequently asked questions

How much water does one person use per day?

It varies widely by country and habits; EPA cites about 82 gallons a day at home in the US.

How do I measure my shower flow?

Fill a bucket for 10 seconds, measure the litres and multiply by 6 for litres per minute.

Which saves more, a new toilet or a new showerhead?

Compare the two lines in your breakdown; the larger use usually gives the bigger saving.

Sources: US EPA WaterSense, "Statistics and Facts" and "Indoor Water Use in the United States" · NIST Special Publication 811 (1 US gal = 3.785411784 L)

Checked 9 Nov 2026. US EPA WaterSense, “Statistics and Facts” (82 gallons per person per day at home) and “Indoor Water Use in the United States” (toilets up to 27% of indoor use; 1.6 gal per flush standard, 1.28 gal WaterSense; showerheads 2.5 gal/min; bathroom faucets about 2 gal/min; efficient clothes washers under 27 gal per load); 1 US gal = 3.785411784 L (NIST SP 811).

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