Shower & Bath Cost Calculator (water + heating)

Understanding the shower and bath cost calculator — concept and principle

Every shower costs twice: once for the water and sewer charge, and again for the energy that heats the water. The heating part is usually the bigger one. This calculator estimates both for a single shower, the water and energy used, and the yearly cost of a daily shower, from flow rate, minutes, temperature rise, prices and heater efficiency.

Heating the water costs more than the water itself: $0.28 vs $0.22 per shower.

How it is calculated

Water (L) = flow (L/min) × minutes
Energy (kWh) = litres × 4.186 × temperature rise (K) ÷ 3,600 ÷ (efficiency ÷ 100)
Cost per shower = litres ÷ 1,000 × water price per m³ + energy × energy price per kWh
Yearly, daily shower = cost per shower × 365

Temperature rise is the difference between the cold supply and the water you shower in; 10 °C mains water heated to 40 °C is a 30 K rise. The water price should include sewer charges if they are billed per m³. Heater efficiency covers losses in the boiler or water heater.

The principle

Heating water needs energy equal to mass × specific heat × temperature change. Water's specific heat is about 4.186 kJ per kg per kelvin, and one litre weighs about one kilogram, so each litre warmed by 1 K needs 4.186 kJ. Dividing kJ by 3,600 converts it to kWh. A heater with 90% efficiency must use 1 ÷ 0.9 times that amount of fuel energy. For a bath, multiply the bath volume instead: a full bath typically holds 80–150 L.

Worked example

Inputs: 9 L/min, 8 minutes, 30 K, water $3/m³, energy $0.10/kWh, 90% efficiency.
Water: 9 × 8 = 72 L.
Heat delivered: 72 × 4.186 × 30 ÷ 3,600 = 2.51 kWh.
Energy bought: 2.51 ÷ 0.90 = 2.79 kWh.
Cost: 72 ÷ 1,000 × $3 = $0.216 water + 2.79 × $0.10 = $0.279 energy, total $0.495, shown as $0.50.
Yearly: $0.495 × 365 = $180.70.

Each point is the calculator re-run with only “Heater efficiency (%)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Heater efficiency (%)	Cost per shower
22.5	$1.33
45	$0.77
67.5	$0.59
90	$0.50
112	$0.44
135	$0.40
180	$0.36

How to read the result

Compare the water and energy parts to see where to save. Cutting minutes reduces both. A lower-flow shower head reduces both too; the US WaterSense label requires 2.0 gallons per minute (about 7.6 L/min) or less. A slightly cooler shower lowers the temperature rise, and the energy part falls in proportion.

Common mistakes

Using the water heater's thermostat setting instead of the mixed shower temperature.
Forgetting sewer charges in the water price.
Using an electricity price when the water is heated by gas.

Keep in mind: Flow and temperature vary in real showers, and pipe and storage-tank losses are not included. It is not financial advice.

Frequently asked questions

Is a shower cheaper than a bath?

Compare litres: a 72 L shower uses less than a typical 80–150 L bath. A long, high-flow shower can use more.

How do I measure my shower's flow?

Time how long it takes to fill a bucket of known volume and divide litres by minutes.

Why does winter raise the cost?

Colder supply water means a bigger temperature rise and more energy per litre.

Sources: NIST Chemistry WebBook — thermophysical properties of water · U.S. EPA WaterSense — showerheads specification

Checked 8 Nov 2026. Typical coverage rates and standard formulas. Always check the product label and local building rules; buy 5–10% extra for waste.

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