In short: a typical bath costs roughly 50¢ to $1 depending on tub size, water heater type and local rates — and heating the water usually costs more than the water itself. Enter your own tub size, heater and rates above for a figure specific to your home.
These are planning estimates from typical fixture volumes and heating assumptions. Your metered water bill and utility rate sheet are the true measure — use them to fine-tune the inputs.
How the cost is calculated
A bath's cost has two parts: the water itself, billed by volume, and the energy to heat it from your incoming water temperature up to bath temperature. The calculator adds them together:
total cost = (gallons ÷ 1,000) × water rate + (gallons ÷ 1,000) × sewer rate + heating energy cost
heating energy cost = [(mass × 4,186 J/kg·K × ΔT) ÷ 3,600,000] ÷ efficiency × energy rate
ΔT is the bath temperature minus the incoming water temperature, in Kelvin. Gas heating energy is converted to therms (1 therm = 29.3 kWh) before applying the gas rate.
Worked example. A 35-gallon bath filled from 60°F to 100°F (a 40°F rise) needs about 3.42 kWh of heat — 132.5 kg of water (35 gal × 3.785 L/gal) heated through 22.2 Kelvin at 4,186 J/kg·K, or about 12.3 million joules, divided by 3.6 million J/kWh. At 100% electric efficiency and $0.16/kWh, that's about $0.55 in heating energy, plus $0.21 for the water at $6.00 per 1,000 gallons — a total of about $0.76 per bath.
Cost by tub size and heater type
The defaults above use the highlighted row. Switching from a standard electric tank to an efficient gas or tankless unit is the single biggest lever on the heating share of the bill.
| Tub size | Electric (~100%) | Gas, standard tank (~60%) | Gas, high-efficiency (~90%) |
|---|---|---|---|
| Compact tub — 20 gal | $0.43 | $0.25 | $0.21 |
| Standard tub — 35 gal | $0.76 | $0.44 | $0.37 |
| Deep soak — 50 gal | $1.08 | $0.63 | $0.52 |
Typical U.S. residential rates
Water, sewer and energy rates vary enormously by utility and region — always use your own bill or rate sheet in the calculator above. These broad national ranges are a starting point if you don't have yours handy.
| Rate | Typical range |
|---|---|
| Water | $4–$12 / 1,000 gal |
| Sewer (where billed separately) | $3–$10 / 1,000 gal |
| Electricity | $0.12–$0.30 / kWh |
| Natural gas | $0.90–$1.60 / therm |
Source: U.S. Energy Information Administration (EIA) residential price data and typical AWWA-member utility water/sewer rates. For your own bill, try the water bill calculator.
Why heating usually costs more than the water
Water has a high specific heat — it takes 4,186 joules to raise one kilogram by one degree Celsius (or Kelvin). A 35-gallon bath is about 132 kg of water, so raising it 40°F takes roughly 3.4 kWh of thermal energy. At typical electric rates that's more than double the cost of the water itself, even though the water is what you're billed for by volume.
Natural gas is usually cheaper per unit of heat delivered than electricity, so gas water heaters tend to cost less to run — but a standard tank heater only converts about 60% of the fuel's energy into hot water, with the rest lost up the flue and through standby heat loss. High-efficiency and tankless gas units close much of that gap. The bigger the temperature rise — colder incoming water, hotter bath — the more heating costs, regardless of fuel.
Worked scenarios
Typical electric
$0.76
35-gal bath, 60°F→100°F, $6.00/1,000 gal water, $0.16/kWh, 100% efficient electric tank.
Standard gas tank
$0.51
40-gal bath, 60°F→100°F, $6.00/1,000 gal water, $1.20/therm gas, ~60% efficient.
Higher-cost market
$1.17
35-gal bath, 60°F→100°F, $12.00/1,000 gal water, $0.22/kWh electric.
What else affects the result
Incoming water temperature swings. Groundwater and municipal supply run colder in winter — and much colder from a well in cold climates — so the same bath can cost noticeably more to heat in January than in July.
Standby heat loss. This calculator only counts the energy to raise the water you use; it doesn't include the ongoing energy a storage tank spends keeping water hot around the clock, which adds a baseline cost beyond any single bath.
Heater age and condition. Sediment buildup, an aging burner, or a failing thermostat can push real-world efficiency below the nameplate figure used here. Heat-pump water heaters can exceed 100% apparent efficiency (they move heat rather than generate it) and will typically cost less than this model shows.
Tiered or seasonal water rates. Some utilities charge more per gallon above a threshold, or more in summer. A single bath rarely crosses a tier on its own, but for a full monthly bill try the water bill calculator.
Frequently asked questions
How much does an average bath cost?
For a typical 35-gallon bath heated from 60°F to 100°F, expect roughly $0.75 to $1.00 with an electric water heater, or about half that with an efficient gas heater — mostly driven by your local water and energy rates. Enter your own numbers above for a figure specific to your home.
Does heating the water cost more than the water itself?
Usually, yes. At typical U.S. rates, heating a bath from 60°F to 100°F with an electric water heater costs about two to three times as much as the water itself. Water has a high specific heat, so raising its temperature takes real energy — gas heating is often cheaper per unit of heat, but standard tank heaters lose a share of that heat up the flue.
Is a bath more expensive than a shower?
It depends on tub size, shower length, and flow rate. A full 35-gallon bath usually costs more than a quick 5-minute shower, but a long, hot shower at a high flow rate can cost more than a smaller bath. Use the comparison figure above, or the bath vs. shower water usage calculator, for numbers based on your own habits.
Does my water heater type change the cost?
Yes. A standard-efficiency gas tank water heater (around 60% efficient) typically costs less to run than an electric tank at similar rates, because natural gas is usually cheaper per unit of heat delivered. High-efficiency and tankless units lose less heat and can lower the cost further.
Why does the calculator ask for both incoming and bath temperature?
The cost of heating depends on the temperature rise, not the bath temperature alone. Incoming water is colder in winter and in some regions year-round, so the same bath can cost noticeably more to heat in January than in July.
How can I lower the cost of a bath?
Fill a smaller tub or to a lower level, lower your water heater's thermostat slightly, or switch to a more efficient heater. Since heating usually costs more than the water, the biggest savings come from reducing the temperature rise or improving heater efficiency, not just using less water.
Sources & references
- U.S. Department of Energy (DOE), Office of Energy Efficiency & Renewable Energy. Energy Saver: Water Heating.
- U.S. Environmental Protection Agency (EPA). WaterSense: Statistics and Facts.
- U.S. Energy Information Administration (EIA). Electricity and Natural Gas residential price data.
Sarah Bennett, MS
Water Quality & Sustainability Specialist · Portland, Oregon
Environmental scientist (MS, Water Resources) with 9+ years in residential water quality, treatment, and usage efficiency, and 150+ completed assessments. Sarah writes and reviews the treatment, usage, and cost calculators on watercalculator.us. Read full bio
MS Environmental Science, Water Resources (2018) · BS Environmental Engineering (2016)