Understanding your result
The headline is the hot water used per day, with the gallons per person and the energy to heat it in the sub-line. The bars show where the hot water goes. The stats give the daily gallons, the per-person figure and the energy in kilowatt-hours. The table lists each use, the total per day and per year, the energy in BTU, kWh and therms, and the share taken by showers.
The energy row is the useful heat, before the water heater’s losses; the water heater running cost calculator takes the gallons, divides by the heater’s efficiency and prices them. The notes show what a minute less in each shower, or a 2.0 GPM head, saves in gallons a day.
Daily use decides the running cost; the busiest hour decides the tank size, which the water heater tank size calculator works out, and the fixtures running at once decide a tankless unit, which the tankless size calculator sizes. The recovery time calculator shows how quickly a tank catches up between showers.
How we calculate this
The hot share follows from mixing: the shower water is a blend of hot water at the heater setting and cold water at the groundwater temperature, and the share of hot is the fraction of the temperature gap it must close. The dishwasher and laundry figures are the US Department of Energy’s per-load hot water from its water heater sizing guidance; the bath and sink allowances are our estimates. Heating water takes 8.34 BTU per gallon per degree Fahrenheit, and the conversions to kWh and therms are on the unit conversions page; the fuel energy contents are on the fuel energy content page.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Shower flow | 2.0 GPM | EPA WaterSense; federal maximum 2.5 |
| Shower temperature | 105 °F | Typical comfortable shower |
| Dishwasher | 6 gal per load | US DOE water heater sizing guidance |
| Clothes washer | 7 gal per warm or hot load | US DOE water heater sizing guidance |
| Bath | 20 gal of hot water | Our estimate |
| Sinks, hand washing, cleaning | 2 gal per person a day | Our estimate |
| Groundwater, setting | 55 °F, 120 °F | Typical mid-US; DOE recommended setting |
Assumptions last reviewed October 8, 2026.
The calculator does not count the water wasted waiting for hot water at distant taps, recirculation losses, hot water for a hydronic heating system, or the moisture-heavy uses of a hot tub. The guide to water heater running costs uses these gallons to compare heater types, and the guide to water heater sizing explains why daily use and peak-hour use are different questions.
Two worked examples
A family of four
Four people, four 8-minute showers a day at 2.0 GPM and 105 °F, one dishwasher load, one warm laundry load, 55 °F groundwater, 120 °F setting.
- Hot share: (105 − 55) ÷ (120 − 55) = 77%
- Showers: 4 × 8 × 2.0 × 0.77 = 49.2 gal
- Dishwasher 6 + laundry 7 + sinks 8 = 21 gal
- Total: 70 gallons a day, 17.6 per person
- Energy: 70.2 × 8.34 × 65 = 38,072 BTU (11.2 kWh)
Showers are 70% of the hot water. One minute less in each shower saves about 6 gallons a day, more than switching the laundry to cold.
A couple with long showers
Two people, two 10-minute showers a day at 2.5 GPM, a dishwasher every other day, a warm wash every other day.
- Showers: 2 × 10 × 2.5 × 0.77 = 38.5 gal
- Dishwasher 3 + laundry 3.5 + sinks 4 = 10.5 gal
- Total: 49 gallons a day, 24.5 per person
- Energy: 26,542 BTU (7.8 kWh)
A 2.0 GPM head would save 7.7 gallons of hot water a day, about 16%, with no change in habits. The per-person figure is high because of the older 2.5 GPM head and long showers.
Where to find your inputs
Showers and length. Count a typical day; time a shower or two.
Shower flow. Stamped on the head; or time how long it takes to fill a one-gallon bucket and divide 60 by the seconds.
Groundwater. Measure cold tap water after running it for a minute; colder in winter.
Heater setting. On the dial or display of the water heater; 120 °F is the DOE recommendation.
Common mistakes
- Treating the shower as all hot water. About three-quarters of it is, at typical temperatures.
- Using the old washer figure. Modern washers use 7 gallons of hot water a warm load, not 25.
- Ignoring the season. Colder winter groundwater raises the hot share and the energy.
- Counting cold laundry. A cold wash uses no hot water.
- Using daily use to size a tank. Tanks are sized by the busiest hour.
- Setting the heater above 120 °F. It wastes energy and raises the scald risk.
Questions people ask
- How much hot water does a family use per day?
- For a family of four taking four 8-minute showers at 2 GPM, running the dishwasher and one warm laundry load, about 70 gallons a day, of which about 49 is shower water. The Department of Energy's water heater test assumes 38 to 84 gallons a day depending on the draw pattern, with about 55 gallons for a medium-use household. Long showers, baths and hot laundry push the figure higher.
- How much of a shower is hot water?
- The share that has to come from the heater to bring cold groundwater up to the shower temperature. At a 105 °F shower, 55 °F groundwater and a 120 °F setting, it is (105 − 55) ÷ (120 − 55), or about 77%. In winter, with colder groundwater, the share rises; in the South, with 70 °F groundwater, it falls to about 70%. An 8-minute shower at 2 GPM uses about 12 gallons of hot water at 77%.
- How much energy does it take to heat a day's hot water?
- 8.34 BTU per gallon per degree. Seventy gallons heated through 65 °F (55 to 120 °F) takes about 38,000 BTU, which is 11.2 kWh or 0.38 therms before the heater's losses. The water heater running cost calculator divides that by the heater's efficiency and multiplies by your energy price; for a gas tank it is about 80 cents a day at 2025 average prices.
- Does a low-flow shower head save much?
- Yes, because showers dominate. Replacing a 2.5 GPM head with a 2.0 GPM WaterSense head on four 8-minute showers a day saves about 12 gallons of hot water a day at a 77% hot share, about 4,500 gallons a year. With an electric tank at 17 cents per kWh that is around $85 a year; with gas around $35. WaterSense heads must also meet a spray performance standard so the shower still feels good.
- How much hot water does a dishwasher or washing machine use?
- The DOE's figures are about 6 gallons of hot water per dishwasher load and 7 per warm or hot clothes washer load for current machines. Modern dishwashers heat their own water to washing temperature, and modern washers use much less water than older top-loaders, which could take 25 gallons or more of hot water a load. Cold-water laundry uses none.
- What is a typical hot water use per person?
- About 15 to 25 gallons a day per person in most households, with showers the biggest share. A household where everyone takes long showers can reach 30 or more per person; a household with low-flow heads and short showers 12 to 15. The calculator gives the figure for your inputs, which is the best number to use for comparing water heaters.