Determine whether you're measuring length, area, volume or mass.
When you measure your own height, you are measuring length. When you measure the size of the floor in your room, you are measuring area. If you measure the amount of liquid in a glass, this is volume. If you measure how much something weighs, this is mass.
Check conversion charts. If you already know the "imperial" measurement (the system used in the United States), you can compare the measurement in miles, for example, and convert it to kilometers (km).
Convert the measurement. As an example, let's say you need to convert 4 miles into kilometers. Since a conversion chart will tell you that 1 mile = 1.6 km, all you have to do is multiply 1.6 x 4. The answer is 6.4.
Round the numbers. Since you are only trying to estimate the measurement, you don't need an exact answer. You could round 1.6 to 1.5 so you can do this calculation in your head. (You can also draw a picture if this helps.)
For example, you wish to multiply 1.5 x 4. If you want to do this in your head, it can be easier to break it into two steps. First, you know that 1 x 4 would be 4.
Ask yourself next, what is .5 x 4? Since .5 means "one-half," what you are really asking is, "What is half of 4?" That's 2. Also fairly easy, if you remember that .5 means half of something.
Add these two results together. 4 + 2. That's 6. So your estimate is: "4 miles is about 6 kilometers." If you look above you'll see that your exact answer was 6.4. That's pretty close, so 6 is a good estimate.
This may sound complicated, but if you need or want to do conversions in your head, or very quickly with the help of a paper and pencil, this method will actually prove easier when you're in the "real world" outside the classroom.
Choose an object that has the length, area, mass or volume of a standard unit of measurement for each system. If you want to be even more clever, you can learn to estimate measurements just by looking at an object or area. For example, find an object that is 1 foot long (say, for example, part of your arm or leg) and when you look at the item you want to measure, picture how many times you could place that part of your arm alongside the thing before you reached the end. (Of course, with your arm it's easy since you can really do this! You can also choose to picture a ruler or yardstick.)
So, if you use your arm like a ruler and place it (or picture it) alongside a table 9 times, your estimate is that the table is 9 feet long.
Remember from your chart that 1 foot equals approximately .3 meters.
Multiply .3 x 9, since the table is around 9 feet long. What this really means is "What is one-third of 9?" since .3 is about a third.
So you break 9 up into 3 parts and ask "What is 9 divided by 3?" Since 3 x 3 = 9, 9 divided by 3 = 3. So 9 feet = around 3 meters.
So the final estimate is that the table is about 3 meters long.