Advanced options
What The Bath Cost Calculator Works Out
- Cost per bath: the heat to lift the bath's hot water from about 10C mains to a 40C bath (litres x 4.186 kJ per degree, converted to kWh) priced by your heating fuel, plus the metered water. 150 litres needs about 5.2 kWh of heat, which a gas boiler delivers for under 50p while an immersion at the current Ofgem price cap electricity rate charges around GBP 1.40 for the same warmth.
- Metered water: the hot water plus the cold top-up (about 1.4 times the hot volume in total) at a combined supply and sewerage rate of around 0.4p per litre, roughly 85p on a standard bath. Unmetered homes pay nothing extra per bath.
- Per year: the cost per bath multiplied by baths a week and 52. The four-bath-a-week habit totals a modest but real couple of hundred pounds a year, and most bathers have never priced it.
- vs an 8-minute shower: a standard mixer shower's 8 to 10 litres a minute means the 8-minute wash uses half a bath's water and heat, but the same arithmetic convicts the 15-minute power-shower reverie of out-drinking the tub.
- Temperature rise (advanced): mains water is around 10C in winter and 15C to 18C in summer, so a summer bath needs less heat; a very hot bath needs more.
- Gas rate, boiler efficiency and electricity rate (advanced): both fuel prices default to the current Ofgem price cap unit rates; edit them for your tariff, and set the boiler or cylinder efficiency. Combis and modern condensing boilers reach 85% to 92%; an old cylinder with poor lagging loses heat on the water's journey and effectively runs lower.
- Metered water rate (advanced): set your own combined supply and sewerage charge per litre from your bill, or 0 if you are on unmetered rates.
- Heat in the water and per month (advanced): the kilowatt-hours actually in the bath, the fuel needed to supply them, and the monthly total.
- Shower comparison (advanced): a shower of the length you choose at 9 litres a minute, heated and metered the same way as the bath, and the break-even length at which a shower costs exactly what the bath does. Flow rate times duration is the only equation in the bathroom, which is why four-minute-shower campaigns and flow-restricting heads target exactly those two factors.