Your Name:
I have split this activity up into three separate activities which cover the same ground but as separate files:
Part 1
Part 2
Part 3

The old intro material is only on the Part 1 page and it won’t print out. The table of units is on parts 1 and 2 but not on part 3 to save space and make it print on one sheet if you’re printing it out.

Group Activity:
Dimensional Analysis


Dimensional Analysis is a technique for solving problems. Particularly, this technique is suited to converting one kind of unit into another.

Converting one unit to another changes nothing: it just expresses a distance, a volume, a mass (etc) in different units. In effect, you are multiplying by one. Using it is the easiest way to find out how many miles someone from Canada means when they say that the hockey rink is about 42 km away. Here’s what you do:

          1 mi
42 km × ———————— = 26 mi  This works because 1 mi = 1.61 km
         1.61 km

Multiplying by any conversion factor is just like multiplying by one. You change the number but not the quantity.

The number of miles is directly proportional to the number of kilometers. The algebraic equation that leads to the math above is:

 x mi        1 mi
———————  =  ——————— which is:
 42 km       1.61 km
           1 mi
 x mi  =  ——————— × 42 km    so x mi = 26 mi
           1.61 km
Notice especially the way the units cancel each other out so 
that you are left with the units you want and you cancel the units you have.

These proportions are called unit factors because they are equal to the number one. Remember, multiplying by the number one does not change the numerical value! The unit factor is used to convert one unit to another unit as in the examples on this page. Pick the unit factor you use carefully as the units must cancel out. If the unit to be cancelled is not part of a fraction or is in the numerator then the unit factor must be written with that unit in the denominator. Here are more examples:

How many seconds are in one year?
        365 day    24 hr    60 min      60 s
1 yr ×  ——————— × ——————— × ——————— × ——————— = 31,536,000 s
         1 yr      1 day     1 hr      1 min
Notice that the unit you want to cancel goes on the opposite side of the
conversion factor from the unit you want to replace it: we want the unit
‘yr’ to cancel to be replaced by ‘day’.

Express 25 miles per hour as meters per second
   mi     1 km     1000 m     1 hr     1 min
25 ——— × ——————— × ——————— × ——————— × ——————— = 11 m/s
   hr   0.621 mi    1 km     60 min     60 s

Key Point

The math for all of this is really quite simple: multiply by the top, divide by the bottom. Do this for each conversion factor, cancelling units as you go. Do not record intermediate values from your calculator: do all calculations as one long series.

page break

Unit Factor Basic Skills

When it comes to knowing how to use the Unit Factor Process (also known as dimensional analisys) there are two basic skills. First, building a unit factor. Second, setting up and carrying out the calculation.

Building a Unit Factor

One quantity (like how much milk is left in the jug) can be expressed using different units: saying 1 quart is left is the same as saying 2 pints are left or saying 4 cups are left or even 950 mL are left. A unit equality gives the relationship between two units. It can be used to make two unit factors.

For example:

1 lb = 0.454 kg
can be written as either of the following:
      1 lb                  0.454 kg
     ———————                ———————
    0.454 kg                1 lb

This works because of some simple algebra:

                   1 lb          0.454 kg            1 lb
1 lb = 0.454 kg  ----------- = -----------       ----------- = 1
                   0.454 kg      0.454 kg           0.454 kg 
                divide both sides by 0.454 kg    0.454 kg/0.454 kg = 1!

The math above gives the unit factor on the left, but what about the one on the right? Can you figure out the necessary math to get that one?

Setting up Calculations

Which of the unit factors do you choose to solve a problem? It depends.
If you have a quantity expressed in kg then choose the unit factor on the left.

          1 lb
5.0 kg × ——————— = 11 lb
         0.454 kg
         0.454 kg 
11 lb × ——————— = 5.0 kg
          1 lb
Round to a convenient number of digits in the answer: only use as many digits as in the number you started with.

If you have a quantity expressed in lb then choose the unit factor on the right.

Why do you choose the unit factors as described in the examples above? You choose them that way so that you cancel out the units you are changing from and introduce the units you are changing to.

Here is a bigger example.

Conversion Steps





Convert 2 years to milliseconds (ms).
2 yr = 2 yr/1
 2 yr     365 day    24 hr      60 min      60 s      1,000 ms    
------ x -------- x -------- x -------- x -------- x ---------- = 6.3072 × 1010  ms
  1        1 yr      1 day       1 hr      1 min         1s     
Enter this into your calculator as 2 × 365 × 24 × 60 × 60 × 1,000 = 6.3072 × 1010

If it helps you to keep track of what you are doing then make a small chart like the one at left. The important thing to note in this example is that the unit to be cancelled out is always on the opposite side of the fraction bar.

Here is an example in which you must cancel units both in the numerator and the denominator:
Convert 2.88 × 104 km/hr to mi/s

 28,800 km     1 hr       1 min     0.621 mi
------------ x -------- x -------- x -------- = 4.97 mi/s
  1 hr          60 min      60 s       1 km      
Type into your calculator: 2.88e4 ÷ 60 ÷ 60 × 0.621 =

Unit Factor Conversions

Linear Measure
2.54 cm = 1 in
12 in = 1 ft
5280 ft = 1 mi
0.3048 m = 1 ft
1 km = 0.621 mi
1 furlong = 660 ft
3 ft = 1 yd
0.0394 in = 1 mm
1 stone = 14 lbs
1 lb = 0.4536 kg
1 oz = 16 drams
1 oz = 28.3 g
16 oz = 1 lb
2,000 lbs = 1 ton
1 oz = 28,350 mg
1 carat = 0.2 g
Volumetric Measure
1 cup = 16 tablespoons
2 cups = 1 pint
2 pints = 1 quart
1 gal = 4 quarts
1 gal = 3.78 L
1 mL = 1 cm3
1 m3 = 1000 L
1 cup = 236.6 mL
1,000 ms = 1 s
60 s = 1 min
60 min = 1 hr
24 hr = 1 day
365 day = 1 yr
14 days = 1 fortnight
10 yr = 1 decade
100 yr = 1 century

Do the following conversions using the unit factor process. Show your work for each step and cancel out units as you work. See the examples on the first page for how you should show your work! Express your answers in scientific notation when appropriate. Do all work on a separate piece of paper and number each problem clearly.

Basic Conversions
  1. Convert 4,200 ft to furlongs
  2. Convert 116 cm to ft
  3. Convert 64 ft to m
  4. Convert 2,512 yd to mi
  5. Convert 13.6 stone to kg
  6. Convert 175 drams to g
  1. Convert 0.875 kg to oz
  2. Convert 54 km to ft
  3. Convert 42 tbsp to quarts
  4. Convert 0.5 L to pints
  5. Convert 788,400,000 seconds to years
  6. Convert 1,250 cm3 to quarts

Thinking about Conversions

The following problems ask you to convert from one metric unit from another. Even if you know the proper metric conversion, do the calculation using the units given in the tables above. After you have finished all of the math, answer the questions below.

  1. Convert 1 km to m
  2. Convert 1 g to mg
  3. Convert 1 L to mL
  4. Convert 1 kg to g
  1. Convert 1 m to mm
  2. Convert 1 cm to mm
  3. Convert 1 m to cm
  4. Convert 1 m3 to cm3

  1. The answers to numbers 1 - 5 in this section were all very close to the same number. What was it?
  2. Using what you may remember about the metric system and the answer to the previous question, write correct (that is, exact) conversions between km and m, g and mg, L and mL, kg and g, and m and mm.
  3. What are the exact conversions between cm and mm and between m and cm?
  4. Why do you think there are so many more cubic centimeters (cm3) in a cubic meter (m3) than cm in a m?

page break
Units with Remainders

Some traditional units are not given in decimal form when there are whole units and parts of units. For example, weights for babies and produce are often given in this form—6 lbs 12 oz—rather than this one—6.75 lbs. Write the answers to the following conversions using this convention, which is also used for time units. For example, 2.5 hours is 2 hours 30 minutes.

To convert 10 lbs 2.5 oz to a decimal expressed in pounds you convert the 2.5 oz to pounds and add the result to the 10 lbs.
            1 lb
2.5 oz × --------- = 0.156 lb
           16 oz 
So the result is 10. 156 lb.

To convert in the other direction works like this:
14.64 lbs can be written using both pounds and ounces by converting just the 0.64 lbs to oz.
            16 oz
0.64 lb × --------- = 10.24 oz
             1 lb 
So the result is 14 lb 10.2 oz.
  1. Convert 5 lb 2 oz to decimal pounds
  2. Convert 12 lb 15 oz to decimal pounds
  3. Convert 7.88 pounds to pounds and ounces
  4. Convert 19.27 pounds to pounds and ounces
  1. Convert 2.75 hours to hours and minutes
  2. Convert 1.32 hours to hours and minutes
  3. Convert 3 hours 42 minutes to decimal hours
  4. Convert 7 hours 19 minutes to decimal hours

One reason this is important is that it enables much quicker arithmetic. Try the following problems to see why. Show your work for each step of each calculation. Do not try to do it in your head!

  1. Add the following weights:
    (3 lbs 3 oz) + (2 lbs 14 oz)
  2. What is the total weight of 7 items that weigh 4 lbs 9 oz each?
  3. How many blocks of classes can fit into 5 hours if blocks are 1 hr 15 minutes long?
  4. Convert 5.3 kg to pounds and ounces.
  1. Convert 3 lb 7 oz to kg
  2. Add the following weights:
    (2.8 lbs) + (5.9 lbs)
  3. What is the total weight of 9 items that weigh 4.25 pounds each?
  4. What is the total amount of time required to run 3 blocks of classes that are 1.625 hours long?


Here are some fun trivia questions.

  1. Are you older than 1 million (1,000,000 or 1 × 106) seconds?
  2. A billion is a number that is 1,000 times a million (1,000,000,000 or 1 × 109). How long would you have to live to be older than a billion seconds?

page break

Combining Units

So far we have concentrated on cancelling units when using dimesional analysis. Units can also be combined according to the rules of algebra. For example, 12x multiplied by 9/4x equals 27 because the x cancels out and 12/4 × 9 is 27. If the problem had been 2x multiplied by 5x then the answer is 10x2. The x variables combine to be written together as x2 instead of x · x. Here are some examples with actual units:

Area                        Density
5 cm × 2 cm = 10 cm2         5 g 
Volume                     ------ = 1.67 g/cm3
1 m × 2 m × 3 m = 6 m3       3 cm3 

The general idea is that if a unit does not cancel out during the calculation, it is still there at the end. Combine like terms and pay attention to whether the unit is in the numerator (on top) or in the denominator (on the bottom).

Here are two examples of problems solved by using combined units.

 Convert 42 ft2 to m2
 42 ft2   42 ft·ft    0.3048 m      0.3048 m   
------ = --------- x ---------- x ----------  = 3.9 m2
  1         1           1 ft          1 ft
Enter this into your calculator as 42 × 0.3048 × 0.3048 = 3.90192768

 Find the volume in mL of 63 g of gold, which has a density of 19.3 g/mL
 63 g       1 mL       
------ x ---------   = 3.26 mL
  1        19.3 g       
Enter this into your calculator as 63 ÷ 19.3 = 3.264248705

Areas and Volumes

Areas and volumes are examples of combined units, for the most part. An acre is an example of a unit of area which is not a combined unit. A gallon is similarly a unit of volume which is not a combined unit. Square feet (ft2) are really (feet) times (feet) because that’s how area is calculated. Similarly, a cubic meter (m3) can be imagined as a cubic space one meter on a side. For area units you must convert with two conversion factors. For volume, use three conversion factors.

  1. Convert 1 m3 to cm3
  2. Convert 1 ft2 to in2
  3. Convert 1 mi3 to km3
  4. Convert 1,000 mL (or 1 L) to in3
  1. Convert 1 mi2 to km2
  2. Convert 1 gal to cm3
  3. 1 acre is 43,560 ft2; convert 1 acre to m2
  4. Convert 42,000,000 gal to m3

Common units of speed are miles per hour (mi/hr), kilometers per hour (km/hr), meters per second (m/s), centimeters per second (cm/s), kilometers per second (km/s), and miles per second (mi/s). Show work as always. There is an example of this type of calculation in the introduction.

  1. Highway speed limit: 65 mi/hr
    Convert to km/hr
  2. Speed of Light 3.00 × 108 m/s
    Convert to mi/hr
  3. Speed of Voyager 1 Spacecraft: 17.0 km/s
    Convert to mi/hr
  4. Speed of Voyager 1 Spacecraft: 17.0 km/s
    How many miles does it travel 10 minutes?
  1. Speed of a fast snail: 0.28 cm/s
    Convert to mi/hr
  2. Speed of Sound in Air: 343 m/s
    Convert to mi/hr
  3. Speed of Sound in Water: 3,310 mi/hr
    Convert to m/s
  4. Speed of the Space Shuttle in orbit: 17580 mi/hr Convert to km/s
  5. How long does it take the Space Shuttle to travel 500 mi?

Density is often taught using the formula D = m/V. Since students frequently make mistakes using this formula a better way to proceed is to use dimensional analysis, as in the example shown above. Some useful data: the density of aluminum is 2.70 g/mL; the density of lead is 11.3 g/cm3; the density of water is 1 g/mL.

  1. Find the mass of 52 mL of aluminum
  2. Find the volume of 117 g of lead
  1. Find the mass of 1 in3 of lead
  2. Find the volume of 5 oz. of aluminum
Note: Some of the problems in this packet are borrowed from the Chang textbook referenced on the bibliography page.
Metrics Units are treated in a separate activity.
Additional Dimensional Analysis Problems
First Dimensional Analysis Homework
Activity: Dimensional Analysis with Word Problems
Second Dimensional Analysis Homework
After that homework, practice your skills with the problems on this page from Science by Jones:
Square and Cubic Units Homework
Density, Part 1
Density: Additional Problems
Last updated: Sep 03, 2020 Home
Copyright and Terms of Use
Today is