JJ.2 Use inches, feet, and yards to estimate lengths
What are inches, feet, and yards?
Inches, feet, and yards are standard units of length in the U.S. customary measurement system. They help us describe how long, tall, or wide an object is.
- An inch is about the length from the tip of your thumb to the first knuckle.
- A foot is about the length of a standard ruler.
- A yard is about the length from your nose to your fingertips when your arm is stretched out to the side.
These units are related: 1 foot equals 12 inches, and 1 yard equals 3 feet or 36 inches.
Choosing the best unit for estimation
Estimation is making a careful guess about a measurement. We choose the unit that lets us use a simple, friendly number, avoiding very large or very small counts.
- Use inches for small things: a pencil, a book, or a smartphone.
- Use feet for medium-sized things: a desk, a room's height, or a sofa.
- Use yards for larger things: the length of a car, a driveway, or a baseball bat.
Think: "Would it take hundreds of this unit, or just a few?" Choose the unit that gives you a number that's easy to think about.
Using benchmarks to estimate
A benchmark is a known measurement you can use to compare and estimate the length of an unknown object.
- A standard sheet of paper is about 1 foot long and 11 inches tall.
- A standard classroom door is about 7 feet tall.
- The height of a doorknob is usually about 3 feet from the floor.
- A baseball bat is about 1 yard long.
Memorize a few personal benchmarks, like your own height in feet or your hand span in inches, to make estimating easier anywhere you go.
The estimation process in action
Follow a clear process to make a reliable estimate: identify a benchmark, compare, choose a unit, and state your estimate.
- Step 1: Choose a benchmark. You know a standard ruler is 1 foot.
- Step 2: Compare mentally. How many rulers would fit across the whiteboard? It looks like about 4.
- Step 3: Choose the unit. Feet are appropriate for this size.
- Step 4: State the estimate. "I estimate the whiteboard is about 4 feet wide."
An estimate is not a guess. It is a thoughtful measurement using logic and known information. It’s okay if your estimate is different from a friend’s, as long as you can explain your reasoning.