It's happened to almost all DIYers: You're working on a project and you drop a screw down a hole and it needs to be replaced. Finding the right screw for the project requires measuring the screw. Not only do you need the length of the screw to prevent damaging the material you're screwing in to, you will likely need the diameter and the thread size. Measuring your screws prevents costly mistakes from using improperly sized threads and screw sizes.
Measure from the surface of the material to the point of the screw. For countersunk screws, measure from the top of the screw to the end. Raised and round or domed screws are measured from the flat bottom of the head to the point.
Measure the shank length and write it down.
Measure the thread length and write it down. Count how many threads per inch are there.
Measure across the head of the screw to get the diameter.
Use the head diameter and length to find the gauge of the screw on a screw chart, available at most hardware stores.