Estimating a product means rounding factors to convenient numbers before multiplying, giving a quick approximate answer. For example, 4×38 ≈ 4×40 = 160. Estimation checks whether an exact answer is reasonable and allows quick mental calculations. Students should be able to tell if a computed answer is in the right ballpark.
After computing an exact product, have students estimate to check it. Present word problems and ask for estimates before exact solutions. Emphasize that estimates are not wrong — they serve a different purpose than exact answers.
You already know two things that make this skill possible: how to round numbers to the nearest ten, and how to multiply single-digit numbers from your multiplication facts. Estimating products combines both skills. The idea is to replace messy factors with round, easy ones, multiply quickly, and get an answer that's close enough to be useful.
Here's the process: look at the factors, round each one to a convenient number, then multiply the rounded numbers. For 4 × 38, round 38 to 40. Now the problem is 4 × 40 = 160 — a fact you can do instantly. The actual answer is 152, so your estimate of 160 is close. That closeness is the whole point: you're not trying to be exact, you're trying to be in the right neighborhood.
Why bother if you're going to compute the exact answer anyway? Because estimation gives you a reasonableness check. Suppose you multiply 4 × 38 on paper and get 272. Your estimate says the answer should be around 160 — so 272 is clearly wrong. You'd catch the error before moving on. In real life, estimation is often all you need: "Will 4 boxes of 38 pencils be enough for 150 students?" A quick mental estimate of ~160 tells you yes, without needing the exact count.
The difference between estimation and guessing is the use of a deliberate rule — rounding — rather than an arbitrary number pulled from thin air. An estimate is a structured approximation that you can defend. You chose to round 38 to 40 because multiples of 10 are easy to multiply; that's a mathematical reason. When you're comfortable estimating, you start to develop number sense — a feel for what answers should look like before you compute them. That intuition is one of the most valuable skills in all of mathematics.