The first part of the conversation here is what is the difference between an exact and estimated value of any measure? In life we always strive to be exact, but the truth is that in everyday life there are so many moving parts and factors that it near impossible to be exact with any measure. When you think about it almost all the tools we use in common everyday construction projects are just really good at estimating a measured value. Look at a tape measure it is only accurate to a sixteenth of an inch.
Before you start teaching your students with measuring lengths of different objects, they should have varied practical experiments of the process of measuring. Given that there are things which are big and some are small, and some are too small to measure with a ruler. Kids should be able to measure objects at will before they go into different units of measurements. Once students are familiar with the art of measurement, they can follow this series of tips: Begin measuring with a standard unit to avoid confusion. After your students mastered ways of measurement, introduce different units of measurement. Learn the conversion of different units. For your students to truly learn the concept of measurement, they need to learn the conversion of units. Like, for instance, conversion from kg to gm or vice versa. The next step is for them to learn the estimation of length measures. They must be skilled enough to estimate the length or weight of any object by just looking at them. These worksheets explain how to express length in a given unit. Students will use units such as "string" (with no set value) to compare simple drawings.