Estimating a length means making a reasonable guess before measuring. Good estimation uses benchmark references: a fingernail is about 1 centimeter wide; a dollar bill is about 6 inches long; a classroom door is about 2 meters tall. A useful estimate is close enough to be meaningful — within about 20–30% of the actual value. Estimating before measuring builds number sense and checks against unreasonable answers.
Establish personal benchmarks (width of thumb ≈ 1 inch; arm span ≈ 1 meter) early and refer to them repeatedly. Play 'estimate then measure' with classroom objects. Discuss what makes an estimate good — not that it is exactly right, but that it is reasonable.
You already know how to measure lengths with a ruler — that's your prerequisite skill. Estimating is different from measuring: instead of getting an exact number, you are making a confident, informed guess before you even pick up a ruler. The goal is not to be exactly right; the goal is to be close enough that your estimate is useful and to avoid obviously wrong answers.
The foundation of good estimation is benchmark references — objects or body parts with known lengths that you can visualize instantly. A fingernail is about 1 centimeter wide. An adult's thumb is about 1 inch. A standard classroom door is about 2 meters tall. Your arm span roughly equals your height. Once you own a few of these benchmarks, you can estimate almost anything by mentally comparing it to the references you know.
The process works like this: look at the object, find the best benchmark to compare it to, and then scale up or down. If a book is about as wide as three of your hands, and your hand is about 4 inches across, the book is about 12 inches wide. You're using your measurement skills but skipping the exact step. "Estimate then measure" is the best practice routine: commit to a guess first, then measure to check. When your guess is off, ask why. Did you compare to the wrong benchmark? Did you misjudge the scale?
The deeper purpose of estimation is to catch errors and build a feel for magnitude. If a problem says a classroom is 30 meters long and your gut says that's way too long for a room, you've caught a likely error. Good estimators develop a mental number line for each unit — they just know that a meter is roughly a large step, that a centimeter is about a fingernail, that an inch is about a thumb width. These intuitions are built through repeated comparison, not through formulas.