How this instrument works
'Showers use less water than baths' is common advice, but whether it's actually true for you depends on real numbers most people never measure: your shower's flow rate, how long you actually stay under it, how full you fill the tub, and even how much water your own body displaces once you're sitting in it. This calculator runs both sides of that comparison from numbers you enter rather than a generic rule of thumb.
Shower water use is straightforward — flow rate multiplied by minutes multiplied by how many times a week you shower. Bath water use is a little more involved: it's the tub's capacity multiplied by how full you fill it, plus the volume of water your body physically displaces once you're in the tub (estimated from body weight, using the standard assumption that water weighs about 8.35 lb per US gallon), plus an optional term for a faucet left running as a top-off while you soak.
The comparison genuinely can go either way. A quick, low-flow shower can use more water per week than an infrequent, partially-filled bath, or less than a daily, fully-filled one — this calculator reports whichever weekly total comes out ahead for your specific habits rather than asserting one is universally better, since the honest answer depends entirely on the numbers you plug in.
- Enter your Shower — effective flow rate, duration, and times per week.
- Enter your Bath — tub capacity, fraction filled, your body weight, and times per week.
- Optionally enter a Bath faucet flow rate and minutes if you tend to leave the tap running as a top-off while bathing.
- Read Shower — water used per week and Bath — water used per week side by side.
- Read Weekly water difference — a positive number means baths use more water that week; a negative number means baths actually use less than your shower habit does.
Worked example — a low-flow shower vs. a 75%-full tub
An 8-minute, 2.1 gal/min low-flow shower taken 7 times a week uses 2.1 x 8 x 7 = 117.6 gallons weekly. A 75%-filled 80-gallon tub for a 150 lb bather, also 7 times a week: body displacement is 150 ÷ 8.35 = 17.96 gallons, so each bath uses (80 x 0.75) + 17.96 = 77.96 gallons, and 7 baths total 545.75 gallons a week.
In this scenario, baths use 545.75 − 117.6 = 428.15 more gallons a week than showers — but that gap narrows or reverses entirely with different habits: a smaller half-filled 40-gallon tub used only 3 times a week for a 120 lb bather totals just 103.11 gallons weekly, barely double a daily 5-minute low-flow shower's 52.5 gallons.
Questions
Do showers always use less water than baths?
No — it depends on your actual flow rate, shower length, tub size, and fill level, not a universal rule. A long, high-flow shower taken daily can easily use more weekly water than a small tub filled partway a few times a week; this calculator exists precisely because the generic advice doesn't hold for every household's real numbers.
Why does body weight matter for bath water use?
Because a person sitting in a tub displaces their own body volume of water, meaning the tub doesn't need to be filled all the way to the same water level to reach the same depth around you — that displaced volume is water you didn't have to run from the tap. This calculator estimates it from body weight using the standard water-density assumption of about 8.35 lb per US gallon.
What counts as a 'low-flow' shower flow rate?
The EPA's WaterSense program certifies showerheads that use no more than 2.0 gallons per minute, compared to roughly 2.5 gpm for a typical standard showerhead. If you don't know your own shower's flow rate, a WaterSense-labeled fixture (if you have one) is a reasonable default around 2.0 gpm; an unlabeled older showerhead may run notably higher.
What's the faucet top-off term for, and when should I use it?
It's for the common habit of leaving the tap running for a few minutes while soaking, to keep bathwater warm as it cools — a real water cost that a simple 'tub capacity x fill percentage' calculation would otherwise miss entirely. Leave it at zero if you fill the tub once and turn the tap off, or enter your faucet's flow rate and how many minutes it typically runs if you do top it off.
Does this calculator account for water used to reheat or the energy cost of hot water?
No — it only compares water volume, in gallons, between the two routines. Energy use for heating that water is a related but separate question that depends on your water heater's efficiency and incoming water temperature, both outside the scope of this particular comparison.