"Need to find a nut to fit a particular screw, but you're not sure what size? 

You can do it the hard way by measuring the diameter of the screw then counting how many threads the screw has per inch. But a screw thread gauge (figure A) helps take the guesswork out of picking the right nut.
The gauge offers an assortment of measurements to choose from. Try flipping through the different sizes and holding onto the screw until you find the one that fits like a glove (figure B). "

                    Figure A                                              Figure B

  Additional background information: