How this is calculated
Daily draw is people times the per-person figure, plus what animals, garden and workshop take. Storage is that times days of autonomy, divided by 0.9, because the last tenth of a tank sits below the outlet or above the fill line.
Pressure-tank drawdown is a separate sum: pump flow times minimum run time, to stop the motor short-cycling itself to death. The shell you buy is larger, because only part of a diaphragm tank empties between the switch pressures.
Worked example: four people at 124 liters each. Daily draw is 496 liters, a week 3,472, and seven days of autonomy needs 496 × 7 ÷ 0.9 = 3,858 liters of tank. A pump giving 38 liters a minute with a one-minute run time needs 38 liters of drawdown; at 30/50 that is a 123 liter shell.
What a household actually uses in a day
Most of the gap between the American and the European figure is bookkeeping. The 82 gallons includes outdoor use, which EPA WaterSense puts at more than 30 percent of a household's total, and the European per-resident figures leave that out.
| Where the figure comes from | Per person, per day | Four people, per day | Four people, per week |
|---|---|---|---|
| United States, all use at home — USGS 2015 | 82 gal / 310 L | 328 gal / 1,242 L | 2,296 gal / 8,691 L |
| United States, indoor metered — REU2016 | 58.6 gal / 222 L | 234 gal / 887 L | 1,641 gal / 6,211 L |
| Britain — Energy Saving Trust, 2013 | 37.5 gal / 142 L | 150 gal / 568 L | 1,050 gal / 3,976 L |
| Europe, resident average — EurEau, 2022 | 32.8 gal / 124 L | 131 gal / 496 L | 917 gal / 3,472 L |
| England and Wales, metered homes — April 2024 to March 2025 | 32.2 gal / 122 L | 129 gal / 488 L | 902 gal / 3,416 L |
| Germany — BDEW, 2023 | 32.0 gal / 121 L | 128 gal / 484 L | 895 gal / 3,388 L |
| Off-grid planning target, plumbed and careful | 20 gal / 76 L | 80 gal / 303 L | 560 gal / 2,120 L |
| Off-grid planning target, water hauled | 8 gal / 30 L | 32 gal / 121 L | 224 gal / 848 L |
| Emergency ration — Ready.gov minimum | 1 gal / 3.8 L | 4 gal / 15 L | 28 gal / 106 L |
The first six lines are measured averages from the study named. The two off-grid lines are planning targets rather than measurements, and Ready.gov's line is a stored-water minimum for drinking and sanitation, well below what a plumbed house runs on.
Two studies on two continents landed on the same five flushes per person per day, which makes the toilet the one fixture whose printed volume goes straight onto the bill.
American use has fallen for thirty years without anyone choosing it. Toilets went to a 1.6 gallon federal maximum in January 1994, measured clothes washer loads fell from 41 gallons to 31 between the 1999 and 2016 studies, and indoor use per household dropped 22 percent over the same span.
Leaks come first in any retrofit. The 2016 study measured 7.9 gallons a person a day lost to leaks, and a third of homes leaked more than that, some at 600 gallons a day.
European use has fallen too. Germany measured 147 liters a person a day in 1990 and 121 in 2023, an 18 percent drop on BDEW's numbers. The British split puts showers first at 25 percent of household water and toilets at 22.
Metering moves the number more than any fixture. England and Wales report 122 liters a person in metered homes against 175 in unmetered ones, April 2024 to March 2025.
Sizing the tank by days of autonomy
Days of autonomy is a risk decision more than a water one. On a well with mains backup, two or three days covers the outages people actually get; on rain catchment the tank has to bridge the dry season.
| Days without a refill | Four people, off-grid target | Four people, European average | Four people, American average |
|---|---|---|---|
| 1 day | 80 gal / 303 L | 131 gal / 496 L | 328 gal / 1,242 L |
| 2 days | 160 gal / 606 L | 262 gal / 992 L | 656 gal / 2,483 L |
| 3 days | 240 gal / 908 L | 393 gal / 1,488 L | 984 gal / 3,725 L |
| 5 days | 400 gal / 1,514 L | 655 gal / 2,480 L | 1,640 gal / 6,208 L |
| 7 days | 560 gal / 2,120 L | 917 gal / 3,472 L | 2,296 gal / 8,691 L |
| 14 days | 1,120 gal / 4,240 L | 1,834 gal / 6,944 L | 4,592 gal / 17,383 L |
| 30 days | 2,400 gal / 9,085 L | 3,931 gal / 14,880 L | 9,840 gal / 37,248 L |
| 90 days | 7,200 gal / 27,255 L | 11,793 gal / 44,640 L | 29,520 gal / 111,745 L |
Raw draw. Divide by 0.9 for the tank you actually buy, which covers dead volume below the outlet and freeboard above the fill line. For what a roof can collect against these numbers, run the water catchment calculator, then the water storage sizing guide.
What the pump has to push against
Total dynamic head is everything resisting the pump, written as a column of water: depth to the standing level, the extra it falls while pumping, the climb to the tank, the pressure held, and pipe friction. A pump curve is read at that head.
Power follows from physics: water power in kilowatts is flow in liters per second times head in meters times 9.81, over a thousand. Divide that by wire-to-water efficiency for the watts the wire has to carry.
| Flow | 100 ft / 30 m | 200 ft / 61 m | 300 ft / 91 m | 400 ft / 122 m |
|---|---|---|---|---|
| 5 GPM / 19 L-min | 210 W | 420 W | 630 W | 840 W |
| 10 GPM / 38 L-min | 420 W | 840 W | 1,260 W | 1,680 W |
| 15 GPM / 57 L-min | 630 W | 1,260 W | 1,890 W | 2,520 W |
| 20 GPM / 76 L-min | 840 W | 1,680 W | 2,520 W | 3,360 W |
Input watts at an assumed 45 percent wire-to-water efficiency, a working figure for a four-inch submersible rather than a published one: read your own pump curve and motor data before buying. Start-up surge is a separate, larger number: see the well pump calculator.
Pressure-tank drawdown
A pressure tank lets the pump run in usable bursts instead of stuttering. Amtrol's own Well-X-Trol sheet sizes on one minute of run time up to three-quarters horsepower and two minutes at three-quarters and above, so drawdown equals flow times that run time.
| Tank shell | Drawdown at 30/50 | Drawdown at 40/60 | Drawdown at 50/70 |
|---|---|---|---|
| 10.3 gal / 39 L | 3.2 gal / 12 L | 2.8 gal / 11 L | 2.4 gal / 9 L |
| 14 gal / 53 L | 4.3 gal / 16 L | 3.8 gal / 14 L | 3.3 gal / 12 L |
| 20 gal / 76 L | 6.2 gal / 23 L | 5.4 gal / 20 L | 4.7 gal / 18 L |
| 26 gal / 98 L | 8.0 gal / 30 L | 7.0 gal / 26 L | 6.1 gal / 23 L |
| 32 gal / 121 L | 9.9 gal / 37 L | 8.6 gal / 33 L | 7.6 gal / 29 L |
| 44 gal / 167 L | 13.6 gal / 51 L | 11.8 gal / 45 L | 10.4 gal / 39 L |
| 62 gal / 235 L | 19.2 gal / 73 L | 16.6 gal / 63 L | 14.6 gal / 55 L |
| 81 gal / 307 L | 25.0 gal / 95 L | 21.7 gal / 82 L | 19.1 gal / 72 L |
| 119 gal / 450 L | 36.8 gal / 139 L | 31.9 gal / 121 L | 28.1 gal / 106 L |
Amtrol Well-X-Trol drawdown, sheet MC 7025 (04/14), models WX-104 to WX-350. Liters converted.
Read across for the pattern: with the air precharge set two psi below cut-in, a tank returns about 31 percent of its shell at 30/50, 27 percent at 40/60 and 23 percent at 50/70. Raising pressure for a better shower costs drawdown, and drawdown is motor life.
- Set the air precharge two psi below cut-in, tank drained. Amtrol ships tanks at a 38 psi factory precharge, which suits a 40/60 switch and needs bleeding down for 30/50.
- A waterlogged tank has lost its air charge; the pump then cycles every few seconds with a tap open.
- Arithmetic gives the minimum shell, and the manufacturer's chart usually gives the next size up. A 10 GPM pump at 30/50 works out at a 32 gallon shell; Amtrol's quick chart picks its 34 gallon model.
Pipe sizes and what they carry
Pipe is sized by velocity. The Copper Tube Handbook tells designers not to exceed 8 ft/s (2.4 m/s) in cold water or 5 ft/s (1.5 m/s) in hot water up to 140°F, to keep noise down and erosion-corrosion off the inside of the fittings.
| Nominal size | Inside diameter | At 5 ft/s (1.5 m/s) | At 8 ft/s (2.4 m/s) |
|---|---|---|---|
| ½ in | 0.545 in / 13.8 mm | 3.6 GPM / 13.8 L-min | 5.8 GPM / 22.0 L-min |
| ¾ in | 0.785 in / 19.9 mm | 7.5 GPM / 28.5 L-min | 12.1 GPM / 45.7 L-min |
| 1 in | 1.025 in / 26.0 mm | 12.9 GPM / 48.7 L-min | 20.6 GPM / 77.9 L-min |
| 1¼ in | 1.265 in / 32.1 mm | 19.6 GPM / 74.1 L-min | 31.3 GPM / 118 L-min |
| 1½ in | 1.505 in / 38.2 mm | 27.7 GPM / 105 L-min | 44.4 GPM / 168 L-min |
| 2 in | 1.985 in / 50.4 mm | 48.2 GPM / 182 L-min | 77.2 GPM / 292 L-min |
Type L copper, inside diameters from the Copper Tube Handbook. GPM is 2.448 times bore in inches squared times feet per second.
A 10 GPM pump wants 1 in supply and ¾ in branches; ½ in on a long run eats the head you sized for.
European pipe is sold by outside diameter: 25 mm and 32 mm polyethylene carry roughly what 1 in and 1¼ in copper do.
Livestock, garden and the seasonal load
Animals move the total more than any fixture choice, and hardest in the week you can least afford it. Between 70°F and 90°F a growing steer's intake rises by three-fifths, and a house of broilers trebles.
| Animal | At 70°F / 21°C | At 90°F / 32°C |
|---|---|---|
| Beef cow, pregnant, 1,110 lb | 8.7 gal / 33 L | — |
| Beef cow, lactating, 900 lb | 16.9 gal / 64 L | 16.2 gal / 61 L |
| Growing steer, 600 lb | 7.8 gal / 30 L | 12.7 gal / 48 L |
| Mature bull, 1,400 lb | 11.7 gal / 44 L | 19.0 gal / 72 L |
| Dairy cow, 50 lb milk a day | 23–27 gal / 87–102 L | — |
| Dairy cow, 100 lb milk a day | 35–41 gal / 132–155 L | — |
| Horse, mature, idle | 8.2 gal / 31 L | 12.7 gal / 48 L |
| Horse, mature, working | 10.8 gal / 41 L | 21.7 gal / 82 L |
| Ewe, lactating, 175 lb and up | 2.0–3.0 gal / 7.6–11.4 L | — |
| Pig, finishing, 100–250 lb | 2–3 gal / 7.6–11.4 L | — |
| Sow, lactating | 5–8 gal / 19–30 L | — |
| Laying hens, per 100 birds | 5.6 gal / 21 L | 10.6 gal / 40 L |
| Broilers growing, per 100 birds | 4.2 gal / 16 L | 13.2 gal / 50 L |
| Turkeys, heavy males, per 100 | 14.5 gal / 55 L | 47.6 gal / 180 L |
Cattle, horses, sheep and swine from NDSU Extension AS1763, Tables 1 to 5; horse rows are quoted at 68°F, then 86°F idle and 95°F working, and the sheep and swine ranges carry no temperature. Poultry from University of Kentucky extension Table 12.1, at 68°F and 89.6°F. Lactating beef cows are the one line that falls in heat, because intake tracks milk yield and yield drops off.
The garden runs on the same arithmetic: one millimeter over one square meter is one liter, so 25 mm across 100 m² is 2,500 liters, and one inch over 1,000 square feet is 623 gallons.
Quick answers
How many gallons of water does one person use in a week?
About 574 gallons, 2,173 liters, at the 82 gallons a day EPA WaterSense quotes for all use at home. Metered indoor use alone was 58.6 gallons a day in 2016, so 410 a week.
How many liters does a European household use per day?
About 124 liters a resident on the EurEau 2022 figure: 496 liters a day for four people, 3,472 a week. Germany measured 121 liters in 2023, England and Wales 137 over 2024 to 2025.
What size water tank do I need for a week off-grid?
Daily draw times seven, divided by 0.9 for the volume you cannot use. Four people at the careful 20 gallon target need about 622 gallons, 2,356 liters; four at the American average need 2,551 gallons.
How do I size a pressure tank for a well pump?
Drawdown equals pump flow times minimum run time: one minute up to three-quarters horsepower, two above. A 10 GPM pump at 30/50 needs 10 gallons of drawdown, which is a 32 gallon shell.
How much water does a toilet flush use?
1.6 gallons, 6.1 liters, under the American federal maximum since January 1994, and 1.28 gallons with a WaterSense label. British regulations have capped a single flush at 6 liters since 1999.
Sources
- American per-fixture and per-person figures. Water Research Foundation, Residential End Uses of Water, Version 2, April 2016; 23,749 homes, 138 gallons per household per day, 58.6 per person. The 1999 edition supplies the pre-1994 flush volume of 3.65 gallons.
- 82 gallons a person a day at home. EPA WaterSense, citing the US Geological Survey's Estimated Use of Water in the United States in 2015. Outdoor use is more than 30 percent of the household total on the same page.
- Fixture limits. EPA WaterSense: showerheads 2.0 gpm against the federal 2.5, faucets 1.5 gpm, toilets 1.28 gpf against the federal 1.6. EPA published a draft 1.2 gpm faucet revision in December 2024. ENERGY STAR: dishwashers 3.2 gallons a cycle, clothes washers 14 gallons a load against 20 for a standard machine.
- British end uses and per-event volumes. Energy Saving Trust, At Home with Water, 2013; 86,171 responses. Per-event volumes are the calculator's stated modeling assumptions.
- European norms. EurEau 2022, 124 liters, quoted by the European Environment Agency; BDEW for Germany, 121 liters in 2023 against 147 in 1990; Discover Water for England and Wales, 137 liters, April 2024 to March 2025.
- Drawdown and run time. Amtrol Well-X-Trol specification and quick sizing chart, MC 7025 (04/14).
- Pipe. Copper Development Association, Copper Tube Handbook, publication A4015: velocity limits and type L inside diameters.
- Livestock intake. NDSU Extension AS1763 (April 2024), drawn from NRC nutrient requirement tables; poultry from University of Kentucky poultry extension, Table 12.1.
- Emergency ration. Ready.gov: at least one gallon per person per day for drinking and sanitation.
Every figure was re-checked against these sources in September 2026.
Go deeper
Storage is half the question; the other half is what refills the tank. Size a roof with the water catchment calculator, pick a source in off-grid water source choice, power the pump in the well pump calculator, clean the output with filtration methods, and cut the biggest indoor use in the composting toilet comparison. More under water.