(Editor's note: The original version of this column erred in the definition of a Btu. The calculations have been adjusted.)

Dear Johnnie:
How much does it cost to take a bath in a standard sized tub, using Longmont water and a gas water heater? -- Kathleen

Dear Kathleen: I'll start with a standard-sized tub, which is about 60 inches long, 30 inches wide and 14 inches deep and has a volume of roughly 14.58 cubic feet.

Assuming you fill the tub to the brim, it will hold roughly 109 gallons. But it won't hold that much with you in it. A 150-pound person will displace about 18 gallons of water. Keeping your 10-pound noggin above the water means you will displace 16.8 gallons and the tub would hold 92.2 gallons. Kathleen, only you know how much water you would displace.

So, how much does 92.2 gallons of water cost in Longmont? I spoke with Bill Powell, customer service manager for the city's Public Works & Natural Resources Department. He confirmed a figure I have used in the past: $2.91 per 1,000 gallons. (Prices vary based on use, but he said this works as a good average.) At that price, 92.2 gallons would cost you about 26.83 cents.

Determining the cost of heating the water is a little trickier, but still doable.

First of all, how hot do you want your water? I chose 104 degrees Fahrenheit, based on a quick Internet search about safe bathing temperatures. Powell concurred that you don't want to go hotter than that.

But to determine the cost of heating the water, you have to know how cold the water is to start with. I was surprised by Powell's answer. This time of year, Longmont tap water comes in at 45 degrees F. So to get your tub full of hot water, you need to raise the temperature of 92.2 gallons of water by 59 degrees. Since one British thermal unit is required to raise the temperature of a pound of water by 1 degree F, and a gallon weighs about 8.3 pounds, then you would need about 45,150 Btu to heat your water.

My next stop was Xcel Energy, our natural gas supplier, to see what they charge for a Btu these days. Turns out they charge by the Therm, or 100,000 Btu. So, heating your water will take .4515 Btu.

Xcel currently charges 45.17 cents per Therm. So multiplying the .4515 by the cost per them gave me 20.39 cents.

Before I give you the total, you must know that there are many variables -- the temperature of the tub before you put hot water in it; what material the tub is made of, the actual size of your tub and how high you fill it; water heater size and efficiency; taxes and fees; how much of you there is to put in the water; air temperature; personal preference for water temperature, and so on. You get the idea.

But, in the example I provide, the cost for a hot bath in Longmont is ... drum roll ... 47.22 cents.

Many thanks to Michelle Aguayo of Xcel; to Steve Moskal of Steve's Plumbing, who confirmed my cost figures; and to Bill Powell, who told me that my "math holds water."