A lag screw, which is more commonly known as a lag bolt, definitely is a screw. There is confusion in the name because these screws tend to be quite large. They were originally used for barrels. Here is how to determine the length of a lag screw.
Step 1 - How to Measure the Length
All too often people don’t understand how to measure the length of lag screws. Some feel it should be from the top of the head to the tip of the screw. This is wrong. The correct measurement for the length of the lag screw is from the base of the head to the tip of the screw.
Step 2 - Measuring
Use a steel tape measure, hold the screw along the tape measure with the end of the tape against the base of the lag screw. Be sure that the tape measure is straight. From here all you need to do is read off the measurement on the tape measure to obtain the correct length of the screw.
If you’re going to buy lag screws of a certain length from a hardware store, take the tape measure with you and check the sizes before you buy. As there are so many lengths available, you can save yourself an extra trip by making sure you buy the right length the first time.