People will tell you a shower is cheaper than a bath as though it settled the matter. It does not, because the cost of either one depends almost entirely on how much hot water you draw, and a long power shower can swallow more than a bath while a quick rinse uses a fraction of it. Once you see where the money actually goes, you can work out which is cheaper in your own bathroom rather than repeating a rule of thumb that may not hold.
You are paying to heat the water, not to use it
The water itself is cheap, and on an unmetered supply it costs nothing extra at all. What you pay for is the energy that lifts cold mains water, often around ten degrees in winter, up to a comfortable washing temperature of roughly forty to forty-five degrees. Heating a litre of water by thirty-five degrees takes about 0.04 kilowatt hours, so the sum that matters is simply litres of hot water multiplied by the price of whatever heats it. A gas combi heats that water at the price of gas; an electric shower or an immersion heats it at the much higher price of electricity. That single difference, gas against electricity, often matters more than bath against shower.
A bath, costed
A typical bath holds somewhere around eighty litres of water at bathing temperature once you have mixed the hot and cold. Heating eighty litres by thirty-five degrees works out at roughly three and a quarter kilowatt hours. Fed by a gas combi at an example price of 7p per kilowatt hour, that is about 23p of gas. Heat the same bath from an immersion heater or an electric cylinder at an example 30p per kilowatt hour and it climbs to nearer a pound. A deeper bath, a hotter bath or one topped up halfway through all push the figure up, because every extra litre is more water to heat. The depth you run is the dial you actually control.
A shower depends entirely on which kind you have
There is no single shower cost, because the three common types behave very differently. A mixer shower fed by a combi boiler draws hot water at whatever rate the head allows, often eight to twelve litres a minute, and heats it with gas. A pumped power shower pushes far more water through, sometimes twelve to fifteen litres a minute, which is where the long power shower quietly overtakes the bath. An electric shower heats cold water on the spot using a heating element, runs on electricity, and restricts its own flow to perhaps five or six litres a minute so the element can keep up.
Work a few through at the same example prices. A brisk five-minute combi mixer shower at eight litres a minute uses forty litres, about 1.6 kilowatt hours of gas, near 11p. Stretch that to a ten-minute power shower at thirteen litres a minute and you are at one hundred and thirty litres, more than the bath, costing around 38p of gas. An eight-minute electric shower at six litres a minute uses less water but heats it with pricey electricity: about 1.1 kilowatt hours, near 34p. So the cheapest of the lot is the short gas-fed shower, and the dearest can be either the long power shower or the electric one, depending on the day. You can run your own figures through the running-cost calculator with the flow rate and price that match your home.
Time and flow beat the bath-or-shower question
What those examples show is that the choice between a bath and a shower is the wrong place to start. A four-minute shower will almost always beat a bath; a fifteen-minute one will almost always lose to it. The two numbers that decide the cost are how many litres your shower puts out each minute and how long you stand under it, and most people have never checked the first. Hold a measuring jug or a bucket under the running head for ten seconds, multiply by six, and you have your litres a minute. Combine that with the minutes you actually spend and you can rank your own habits without guessing.
Metered water adds a second, smaller bill
If your home is on a water meter, every litre carries a charge for the water coming in and the wastewater going out, so the litres count twice: once for the energy to heat them and again on the water bill. The water charge is usually the smaller of the two, but it tilts things further towards whichever option uses fewer litres, which is generally the short shower. On an unmetered supply the water is a fixed cost no matter how much you use, so only the heating energy varies. Knowing which you are on tells you how much the litres matter beyond the energy.
It changes again across a whole household
One person deciding between a bath and a shower is a small sum; a family repeating the choice twice a day is where it adds up. Four people each taking a short combi shower will, between them, usually use less hot water than four separate baths, which is the everyday case for showers winning. The picture flips the other way too. Two small children sharing one bath use far less hot water than two showers, and an adult who runs a fifteen-minute power shower every morning may be the most expensive person in the house without realising it. A shared, slightly shallower bath also steams up and reheats the bathroom less than a string of back-to-back showers. The thing to do is look at the household's real routine, count the long showers and the deep baths separately, and aim the savings at the biggest users rather than at whoever happens to prefer a bath.
The cheap ways to cut either one
The levers are the same whichever you prefer. Run baths a little shallower and a touch cooler. Keep showers short, and fit a low-flow or aerating showerhead on a combi or power shower so each minute draws fewer litres without the spray feeling weak, though note these do little on an electric shower that already limits its own flow. Turning a combi boiler's hot-water flow temperature down slightly, or not setting taps to scalding and then adding cold, avoids heating water hotter than you ever use. The wider picture of stored versus instant hot water, cylinders and immersions sits in the hot water savings guide, and the specific economics of heating on the spot are covered in the electric shower cost guide.
So which is cheaper?
For most households a short shower from a gas combi is the cheapest way to get clean, a bath sits in the middle, and a long power shower or a daily electric shower can cost as much as or more than the bath people assume they are avoiding. The fuel doing the heating matters as much as the method, and the litres matter most of all. Rather than carry around a fixed belief about baths and showers, measure your shower's flow once, keep an eye on the minutes, and let your own numbers settle the argument.