Category Archives: Measurement

How Much Does a Gallon of Water Weigh? Easy Calculation

Question: How much does a gallon of water weigh?

Ever wonder how much a gallon of water weighs? A US gallon is 8.34 lbs or 3.78 kg. (David Mulder)

Ever wonder how much a gallon of water weighs? A US gallon is 8.34 lbs or 3.78 kg. (David Mulder)

Answer: A US gallon of water weighs 8.34 lbs or 3.78 kg at 62 °F (17 °C). An imperial gallon (UK) weighs 10.022 lbs or 4.546 kg, at its most dense temperature, which is 2.20456 lbs / L at 4 °C or 39 °F.

Ballpark Calculation (for all temperatures)

1 gallon is about 3.75 liters
1 liters = 1 kilogram (density of water is 1 kg/liter)
1 kg is about 2.2 lbs
so, 3.75 kg is about 8.25 lbs and 1 gallon is about 8.25 lbs

Three Different Units of Gallon

It makes a difference which unit of gallon you are using. There are two US definitions of gallon. The US liquid gallon (most commonly used) is defined as 231 cubic inches, which is 3.785 liters, and weighs 8.344 lbs at its highest density.

The US dry gallon is defined as 1/8th of a US bushel, which is 268.8025 cubic inches or 4.405 L. The weight of a US dry gallon is 9.711 lbs of water at its highest density.

The UK gallon or imperial gallon originally was defined as 10 lbs of water, but the modern definition is exactly 4.54609 L or 10.02 lbs of water at its maximum density.

Effect of Water Temperature on Weight

Cold water is more dense than ice or than warm water or liquid just above freezing. This is an unusual property of the substance, resulting from hydrogen bonding. So, a gallon of warm water would weigh slightly less than a gallon of cold water. The exact difference would depend on the temperatures in question, but it doesn’t affect the value by much.

Of course, gravity affects weight too, so a gallon of water (or anything else) would weigh more on Jupiter than on Earth, while it would weigh less on the Moon than Earth.

Easy Way To Remember the Weight of Water

While you’ll want to keep the exact volume definitions in mind for scientific calculations, for everyday use you can remember the weight of water using the simple rhyme: A pint’s a pound, the world round.

The saying refers to the rough equivalence between 16 fluid ounces (a pint) and 16 ounces avoirdupois weight. A gallon is 4 quarts or 8 points, so a gallon weighs approximately 8 lbs.

Why There Is No Degree in Kelvin Temperature

This thermometer registers temperature in Celsius and Fahrenheit, which both use the degree symbol. Kelvin doesn't use the symbol because it's an absolute temperature scale with a defined endpoint. (image: Ged Carroll)

This thermometer registers temperature in Celsius and Fahrenheit, which both use the degree symbol. Kelvin doesn’t use the degree because it’s an absolute temperature scale with a defined endpoint. (image: Ged Carroll)

When you write a temperature using the Celsius, Fahrenheit, or Rankine scales, you include a degree symbol. Have you ever wondered why there is no degree in Kelvin?

The answer has to do with the definition of a degree. A degree is a change in temperature that is measured against a scale. The size of a degree is arbitrary. For example, originally the Celsius scale was set such that 0°C was the freezing point of water, while 100°C was the boiling point of water. Why water? Because it’s familiar and makes sense to people. But, the temperature scale could as easily have been designed around the freezing and boiling points of methane. You could even design your own temperature scale around what you consider to feel hot and cold. If you did so, your scale would use degrees. The old Celsius scale and your personal scale are examples of relative temperature scales. (Before you get all feisty… yes, the modern Celsius scale is based on absolute zero and the triple point of water, but there are still reasons why it uses degrees. We’ll get to that in a moment.)

Why Kelvin Doesn’t Have Degrees

Kelvin is different because it’s an absolute scale. 0K is absolute zero — the point at which gas molecules have no thermal energy. There’s no negative temperature on the Kelvin temperature scale. It’s not just about the endpoint, though. Temperature is a measure of the amount of energy contained by molecules. The Kelvin unit reflects this, where doubling the Kelvin temperature means you doubled the thermal energy.

A degree of the Celsius scale may seem like Kelvin, just bumped up 273, but if you double a Celsius temperature, you don’t double the thermal energy. Doubling 20°C gives you 40°C, which feels a lot hotter, but isn’t very meaningful from the thermodynamic perspective. Even worse, what if you double -40°C? Do you get -80°C or -20°C? You can see why scientific formulas often ask for Kelvin temperature.

Why Does the Rankine Scale Use Degrees?

Zero on both the Kelvin temperature scale and the Rankine scale is absolute zero, so you might be wondering why Rankine temperature is reported in degrees. It has to do with the size of the unit. The Rankine degree (temperature interval) is defined to equal a Fahrenheit degree. A temperature of 459.67 °R exactly equals 0 °F. Fahrenheit, like Celsius, has degrees defined around the freezing and boiling points of water. It’s nit-picky, right? To be honest, some people consider Rankine a unit, just like Kelvin, but the National Institute of Standards and Technology (NIST) advises against using the degree when citing Rankine temperatures.

Why Use the Kelvin Scale?

William Thomson - Lord Kelvin

William Thomson – Lord Kelvin

Aside from avoiding mathematical problems, like you might get dealing with negative numbers in other temperature scales, there are two other reasons to use the Kelvin scale. First, K is the approved SI unit (metric unit), so it’s the standard scale all over the world. Yes, the United States uses Fahrenheit, but that’s mainly because the degree size is so handy for expressing body temperature and weather. For science, even Americans use Celsius and Kelvin.

The second reason is that the Kelvin scale may be used to express both extremely cold and hot temperatures on the same (positive numerical value scale). If you want to compare the temperature of liquid nitrogen and red-hot iron, the Kelvin scale is the way to go. Now, when you’re talking about the temperature of the Sun, it really doesn’t matter whether you use Celsius or Kelvin, because that 273 degree difference is negligible.

Who Was Kelvin?

The Kelvin temperature scale is named for Sir William Thomson, who was also known as Lord Kelvin of Scotland or 1st Baron Kelvin. Kelvin invented the temperature scale that bears his name in 1848.

Physical Constants, Prefixes, and Conversion Factors

Here are some useful physical constants, conversion factors, and unit prefixes. They are used in many calculations in chemistry, as well as in physics and other sciences.

Useful Constants

Acceleration of Gravity9.806 m/s2
Avogadro’s Number6.022 x 1023
Electronic Charge1.602 x 10-19 C
Faraday Constant9.6485 x 104 J/V
Gas Constant0.08206 L·atm/(mol·K)

8.314 J/(mol·K)

8.314 x 107 g·cm2/(s2·mol·K)

Planck’s Constant6.626 x 10-34 J·s
Speed of Light2.998 x 108 m/s
p3.14159
e2.718
ln x2.3026 log x
2.3026 R19.14 J/(mol·K)
2.3026 RT (at 25°C)5.708 kJ/mol

Common Conversion Factors

QuantitySI UnitOther UnitConversion Factor
Energyjoulecalorie
erg
1 cal = 4.184 J
1 erg = 10-7 J
Forcenewtondyne1 dyn = 10-5 N
LengthmetreAngstrom1 Å = 10-10 m = 10-8 cm = 10-1 nm
Masskilogrampound1 lb = 0.453592 kg
Pressurepascalbar
atmosphere
mm Hg
lb/in2
1 bar = 105 Pa
1 atm = 1.01325 x 105 Pa
1 mm Hg = 133.322 Pa
1 lb/in2 = 6894.8 Pa
TemperaturekelvinCelsius
Fahrenheit
1°C = 1 K
1°F = 5/9 K
Volumecubic metrelitre
gallon (U.S.)
gallon (U.K.)
cubic inch
1 L = 1 dm3 = 10-3 m3
1 gal (U.S.) = 3.7854 x 10-3 m3
1 gal (U.K.) = 4.5641 x 10-3 m3
1 in3 = 1.6387 x 10-6 m3

SI Unit Prefixes

FactorsPrefixSymbol
1012teraT
199gigaG
106megaM
103kilok
102hectoh
101decada
10-1decid
10-2centic
10-3millim
10-6microµ
10-9nanon
10-12picop
10-15femtof
10-18attoa

 

SI Units Word Search Puzzle

This SI units word search puzzle contains vocabulary words dealing with base and derived SI units used in science. These terms can be found horizontally, vertically, and diagonally. Click the image for full-size or download the PDF and give it a try.

SI Units Word SearchIf you would like some help finding some of the terms, take a peek at the solution to the SI Unit word search puzzle. You can download the PDF of the completed puzzle. Enjoy!

Here is a rundown on the terms used.

Ampere – Base unit of electrical current
Symbol: A

Becquerel – Derived unit of radioactive activity
Symbol: Bq
Base units: sec-1

Candela – Base unit of luminous intensity
Symbol: cd

Coulomb – Derived unit of electric charge
Symbol: C
Base Units: A·s

Farad – Derived unit of electrical capacitance
Symbol: F
Base Units: m-2·kg-1·s4·A2

Henry – Derived unit of magnetic inductance.
Symbol: H
Base Units: m2·kg·s-2·A-2

Hertz – Derived unit of frequency
Symbol: Hz
Base Units: s-1

Joule – Derived unit of energy
Symbol: J
Base units: m2·kg·s-2

Kelvin – Base unit of temperature
Symbol: K

Kilogram – Base unit of mass
Symbol: kg

Lux – Derived unit of illuminance
Symbol: lx
Base units: m-2·cd

Meter – Base unit of length
Symbol: m

Mole – Base unit of quantity
Symbol: mol

Newton – Derived unit of force
Symbol: N
Base units: m·kg·s-2

Ohm – Derived unit of electrical resistance
Symbol: Ω
Base units: m2·kg·s-3·A-2

Pascal – Derived unit of pressure
Symbol – Pa
Base units: m-1·kg·s-2

Second – Base unit of time
Symbol – s

Tesla – Derived unit of magnetic flux density
Symbol: T
Base units: kg·s-2·A-1

Volt – Derived unit of electrical potential difference
Symbol: V
Base units: m2·kg·s-3·A-1

Watt – Derived unit of power
Symbol: W
Base units: m2·kg·s-3

Weber – Derived unit of magnetic flux
Symbol: Wb
Base units: m2·kg·s-2·A-1

Printable Protractors

Have you ever needed a protractor but couldn’t find one? Here’s a pair of printable protractors available whenever you want one.

These printable protractors have the added benefit to be transparent images which makes them extremely useful to print on transparencies. This allows you to look through the protractor while you use it. Click the image to open the full-sized image. Alternatively, the PDF files provide protractors with clearer text and sharper lines.

The first is the typical 180° protractor with a six-inch ruler along the bottom.
Download the PDF of this protractor.

Printable Protractor and RulerThe second is a round protractor for the full 360° experience.
Download the PDF of this protractor.

Printable Protractor 360

Significant Figures – Multiplication Worksheet

Significant figures are the scientist’s preferred method of expressing uncertainty in their measurements. For new students, learning the rules of significant figures is easy—applying them is the problem.

Significant Figures - Multiplication WorksheetThis significant figures multiplication worksheet contains 20 different multiplication and division problems for the student to calculate the solution to the correct number of significant figures.

Download the PDF of this worksheet or click the above image to print as many copies as you need.

The answer key can be found on the next page. (Page 2)

For other Significant Figure Worksheets, check out:
Significant Figures Worksheet
Significant Figures – Addition Worksheet

Significant Figures Worksheet PDF – Addition Practice

Significant figures are the scientist’s preferred method of expressing uncertainty in their measurements. For new students, learning the rules of significant figures is easy—applying them is the problem.

This significant figures worksheet PDF contains 20 different addition and subtraction problems for the student to calculate the solution to the correct number of significant figures.

Significant Figures - Addition WorksheetDownload the PDF of this worksheet or click the above image to print as many copies as you need.

The answer key can be found on the next page. (Page 2)

For other Significant Figure Worksheets, check out:
Significant Figures Worksheet
Significant Figures – Multiplication Worksheet