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### Exact Weight of One Gallon of Water

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Date: 08/15/2001 at 14:50:20
From: Davis
Subject: Exact weight of one gallon of water?

What is the exact weight of one gallon of water?

The best answer I can get is 8.33 or 8.34 - it depends what source I
go to.

I have looked up this problem on different sites and have used search
engines, but the resources available only give me decimals up to
hundreds. I'd like to get it as close as I can.

I work in the fire service and this information would be very helpful
to me.

Thanks,
Davis
```

```
Date: 08/15/2001 at 15:46:55
From: Doctor Rick
Subject: Re: Exact weight of one gallon of water?

Hi, Davis.

Your question can be stated: "What is the density of water in pounds
per gallon?" The density varies with temperature and pressure, though
only slightly. Here is a reference that gives the density of water in
grams per milliliter at various temperatures and (I presume) 1
atmosphere pressure:

Density / Vapor Pressure of Water (North Carolina State University)
http://www.ncsu.edu/chemistry/resource/H2Odensity_vp.html

At 70 degrees F (close to 21 degrees C) the density is 0.9979955 g/mL.
We can convert this to lb/gal as follows:

0.9979955 g   1000 mL   3.785411784 L      1 lb
----------- * ------- * ------------- * ----------- = 8.328676 lb/gal
1 mL        1 L        1 gal        453.59237 g

I got the exact conversion factors from "How many? A Dictionary of
Units of Measurement," by Russ Rowlett:

http://www.unc.edu/~rowlett/units/

The density of water varies from 0.9991026 g/mL at 15 C to 0.9956502
g/mL at 30 C according to the NCSU site, so the figure I have
calculated will vary from 8.337915 to 8.309104 lb.gal over this
temperature range. You can see why figures of 8.3 and 8.4 are given:
to be more accurate requires knowledge of the temperature.

- Doctor Rick, The Math Forum
http://mathforum.org/dr.math/
```

```
Date: 08/15/2001 at 16:00:59
From: Doctor Rob
Subject: Re: Exact weight of one gallon of water?

Thanks for writing to Ask Dr. Math, Davis!

The weight will vary some with the temperature of the water.  To
begin, we know that by definitions,

1 gallon = 231 in^3*(2.54 cm/in)^4/(1 cm^3/ml),
= 3785.411784 ml,

exactly. Now it is just a matter of multiplying by the density d in
g/ml of water at the temperature you want, and then converting to
pounds by dividing by exactly 453.59237 g/lb. Thus the answer to 12
significant figures is

1 gallon of water weighs 8.34540445202*d pounds.

Here is a Web site that will allow you to learn the value of d for
the temperature:

Measurement: Frequently asked questions - Fred Senese
http://antoine.frostburg.edu/chem/senese/101/measurement/faq.shtml

Select the last question under Density: "How do I find water's density
at different temperatures?" Enter the temperature in degrees Celsius
and see the density d. Use this and the formula above to get the
temperature-adjusted weight in pounds. Since d can only be measured to
level of accuracy.

I hope this helps.

- Doctor Rob, The Math Forum
http://mathforum.org/dr.math/
```
Associated Topics:
High School Physics/Chemistry
Middle School Terms/Units of Measurement

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