Units and unit conversion on the SAT test
SAT Subscore: Problem solving and data analysis
A unit is used to measure a quantity, while different kinds of quantities measured in different units.
Unit conversion changes a given quantity to a different unit of measurement, so that the relative amount does not change. Unit conversion is calculated with unit equivalencies.
Unit equivalencies show how the units of the same kind of quantity relate to each other.
Time Unit equivalencies measure time intervals that express duration (for example: 1 minute= 60 seconds).
Distance Unit equivalencies measure length (for example: 1 kilometer=1,000 meters ).
Mass Unit equivalencies measure weight of objects (for example: 1 kilogram=1,000 grams).
Volume Unit equivalencies measure capacity of an object or space in three dimensions (for example: 1 liter=1,000 milliliters).
Unit conversion is calculated by multiplication of the original quantity by a conversion factor of 1 and simplifying so that we are left with the desired units without the original units.
To Convert between 2 units a and c using a third unit b, we need to multiply by 2 conversion factors: a conversion factor between unit a and unit b and a conversion factor between unit b and unit c.
To Convert units that appear within rates, we need to construct one or two conversion factors to cancel the units that are given and not desired and create the units that are not given and desired.
Continue reading this page for detailed explanations and examples.
Unit conversion and unit equivalency definitions
A unit is used to measure a quantity, while different kinds of quantities measured in different units. For example: distance is measured in miles, weight is measured in kilograms and time is measured in hours. There can be many unites for measuring the same kind of quantity. For example: time can be measured in hours, minutes, or seconds.
Unit conversion changes a given quantity to a different unit of measurement, so that the relative amount does not change. For example: 0.5 hours can be converted to 30 minutes (the relative amount stayed the same). Unit conversion is calculated by multiplication of the original quantity by a conversion factor of 1 and simplifying so that we are left with the desired units without the original units.
Unit equivalencies show how the units of the same kind of quantity relate to each other. For example: 1 hour is equal to 60 minutes. To convert a given quantity to a desired unit, you first need to know the unit equivalency between the given unit and the desired unit.
Unit equivalencies lists
Below are lists of unit equivalences for time, distance, mass and volume.
Time Unit equivalencies:
Units of time measure time intervals that express duration.
1 minute= 60 seconds
1 hour= 60 minutes
1 day= 24 hours
1 week= 7 days
1 year= 12 months
1 year= 52 weeks
1 year= 365 days
Distance Unit equivalencies:
Units of distance measure length.
1 centimeter (cm)=10 millimeters (mm)
1 inch (in)= 2.54 centimeters (cm)
1 foot (ft)= 12 inches (in)
1 yard (yd)= 3 feet (ft)
1 meter (m)= 100 centimeters (cm)
1 kilometer (km)= 1,000 meters (m)
1 mile (mi)= 1.61 kilometers (km)
Mass Unit equivalencies:
Units of mass measure weight of objects.
1 gram (g)= 1,000 milligrams (mg)
1 pound = 453.59 grams (g)
1 kilogram (kg)= 2.2 pounds
1 kilogram (kg)= 1,000 grams (g)
1 ton (t)= 1,000 kilograms (kg)
Volume Unit equivalencies:
Units of volume measure capacity of an object or space in three dimensions, the objects can be fluids (like water) or bulk goods (like flour). Since volume is measured in three dimensions, the units of measure for volume are cubic units.
1 liter (l)= 1,000 milliliters
1 kiloliter= 1,000 liters
Calculating unit conversion
The unit conversion changes a given quantity to a different unit of measurement, so that the relative amount does not change.
The goal of the unit conversion is to transform the given quantity to desired units of measure, therefore we need to cancel the given units and create the desired units:
The given quantity has the given units in the nominator. To cancel the given units in the numerator we need to divide the given quantity by the given units.
The new quantity should have the desired units in the nominator. To create the desired units in the numerator we need to multiply the given quantity by the desired units.
Since the relative amount must stay the same, the only operation we can do is to multiply by 1 using unit equivalencies.
Therefore, unit conversion is calculated by multiplication of the original quantity by a conversion factor of 1 and simplifying so that we are left with the desired units without the original units.
Unit conversion steps
Remember to write the name of the unit near each value!
Step 1: Write the unit equivalence between the given units and the desired units.
Step 2: Write the conversion factor of 1 so that the desired units are in the numerator and the given unit are in the denominator.
Step 3: Write the conversion expression that is equal to the given quantity multiplied by the conversion factor of 1.
Step 4: Simplify the expression from step 3 so that you cancel out the given units and are left with the desired units.
Consider the following example:
Convert 2.7 meters (m) to centimeters (cm).
Step 1: Writing the unit equivalence between meters and centimeters:
We need to convert 2.7 meters, therefore we should write 1 meter in centimeters: 1 meter= x centimeters.
We know that 1 meter= 100 centimeters
Step 2: Writing the conversion factor of 1 so that the centimeters are in the numerator meters are in the denominator:
100 centimeters / 1 meter= 1
Step 3: Writing the conversion expression that is equal to the given quantity multiplied by the conversion factor of 1:
2.7 meters = 2.7 meters * 100 centimeters / 1 meter
Step 4: Simplifying the expression from step 3 canceling meters and leaving centimeters:
2.7 meters * 100 centimeters / 1 meter= 2.7*100 centimeters = 270 centimeters.
Converting between 2 units using a third unit
In these questions we are asked to convert unit a to unit c, while we are given 2 unit equivalencies: the unit equivalency between unit a and unit b and the unit equivalency between unit b and unit c. Therefore, to convert from unit a to unit c we must include their unit equivalency with unit b.
To Convert between 2 units using a third unit, we need to solve using the steps introduced above. In these questions we need to multiply by 2 conversion factors: a conversion factor between unit a and unit b and a conversion factor between unit b and unit c.
We are given unit a in the numerator and we need to be left with unit c in the numerator. To do so we must divide by unit a and multiply by unit c, therefore we need unit c in the numerator and unit a in the denominator. Since we are not given the direct unit equivalency between unit a and unit c, we must include their unit equivalencies with unit b. We know that unit b must cancel out, therefore we write unit b in the numerator and the denominator:
unit c unit b
_______ * ________
Unit b unit a
Consider the following example:
1 kilometer (km)= 1,000 meters (m)
1 mile (mi)= 1.61 kilometers (km)
How many meters are in 1 mile?
Step 1: Writing the unit equivalence between miles and meters:
We need to convert miles to meters, but we are given the conversion factor between miles and kilometers and the conversion factor between kilometers and meters.
1 kilometer (km)= 1,000 meters (m)
1 mile (mi)= 1.61 kilometers (km)
Step 2: Writing the 2 conversion factors of 1:
We know that we must divide by miles and multiply be meters (so that miles unit will cancel out and meters unit will be created).
meters
_______ * _______
miles
We don’t need kilometers in the answer and we don’t have them in the given quantity, but we must use them in the conversion factors. Therefore, we must create kilometers and then cancel them out, to do so we need to multiply and divide by kilometers.
kilometers meters
___________ * _________
miles kilometers
Now we can put values from the given unit equivalencies into the conversion factors:
1 kilometer (km)= 1,000 meters (m)
1 mile (mi)= 1.61 kilometers (km)
1.61 kilometers 1,000 meters
________________ * _____________
1 miles 1 kilometers
Step 3: Writing the conversion expression that is equal to the given quantity multiplied by 2 conversion factors of 1:
1 mile= 1 mile 1.61 kilometers 1,000 meters = 1.61 * 1,000 = 1,610 meters
* _______________ * ______________
1 mile 1 kilometers
Converting units that appear within rates
Each rate is composed from 2 units, one in the numerator and the other in the denominator. Remember to write the name of the units near the values in the numerator and the denominator of the rate.
To Convert units that appear within rates, we need to solve using the steps introduced above. In these questions we need to construct one or two conversion factors to cancel the units that are given and not desired and create the units that are not given and desired.
Consider the following example:
Convert 1 dollars per pound to cents per kilograms.
Step 1: Writing the unit equivalence between the units:
1 kilogram (kg)= 2.2 pounds
1 dollar= 100 cents
Step 2: Writing the conversion factors of 1:
In this question we need to convert between 2 rates and each rate is composed from different units. Therefore, we need to cancel 2 units that appear in numerator and the denominator of the given rate (dollars per pounds) and create 2 units that appear in the numerator and the denominator of the desired rate (cents per kilograms).
pounds cents
________ * ___________
dollars kilograms
We know the unit equivalence between dollars and cents and between kilograms and pounds:
2.2 pounds 100 cents
____________ * ____________
1 kilogram 1 dollar
Step 3: Writing the conversion expression that is equal to the given quantity multiplied by the conversion factors of 1:
1 dollar 1 dollar 2.2 pound 100 cents
________ = __________ * ___________ * __________
1 pound 1 pound 1 kilogram 1 dollar
Step 4: Simplifying the expression from step 3 canceling dollars and pounds and leaving cents and kilograms:
1 dollar 2.2 pounds 100 cents 2.2*100 cents 220 cents
_________ * _____________ * ___________ = _______________ = ____________
1 pound 1 kilogram 1 dollar 1 kilogram 1 kilogram