When to Use Mental Math and When to Reach for a Calculator
Mental math is faster for estimates, calculators are safer for precision. Here is how I decide which to use for different tasks. I use mental math and calculators for different things, and confusing the two causes problems. Mental math is fast and approximate, good for estimates where being close is enough. Calculators are slow and precise, good for numbers that need to be exactly right. The mistake I see most often is using mental math for tasks that require precision, like medication doses and contract amounts, and reaching for a calculator for tasks where an estimate would do, like checking whether a grocery total is in the right ballpark. Here is how I split the work. Estimation Is a Skill Worth Practicing Estimation is the mental math skill I use most. When I see a restaurant bill of 87 dollars and want to leave 18 percent, I round to 90, take 10 percent to get 9, then add half of that for the extra 8 percent, landing near 16 dollars. The exact answer is 15.66, so my estimate is within a few cents. For a grocery cart total of 142 dollars, I round to 140 and add 10 percent tax to get 154, which tells me whether my budget will cover it before I reach the register. Estimation works because the goal is a decision, not a number.