Halloween is a time when you can express your penchants, hobbies and interests through your costume. Math geeks don't often get to display their love of numbers through their clothing choices, but there are some math-related Halloween costumes that will let everyone know how much you love your scientific calculator. You won't have much luck with shop-bought costumes unless you want to go the route of general nerdiness and add some math accessories. Home-made math costumes are even more fun.
Paint the number 3 onto a shirt with fabric paint. Then mess up your hair, wear mismatched socks, and do any other strange things to your attire and make-up. Voilá -- you're an odd number. Of course, any odd number would work if you'd rather wear 7, 15, etc.
Pie Chart/Bar Graph
Attach two large pieces of card stock together so they can be worn like a sandwich board over your shoulders. Print and cut out pictures of different pies or draw them on in bold markers. Label the pies and tell people you are a pie chart. Similarly, but for adults only, make the sandwich board into a "bar" graph and graph how much time you spend at different bars or rate how much you like different bars.
Use a large cardboard box from an appliance, large speakers etc. Cut a hole in the top front like the display panel in a calculator and cut off the bottom flaps for your feet to stick out. Then draw on all the buttons for the keypad. You can make your calculator a complicated graphing calculator or a simple desk calculator.
Acquire any sort of pumpkin costume, or just buy an orange sweatsuit or a T-shirt. Write out in fabric paint as much of the number pi as you can fit on the costume or orange shirt. Tell people you are "pumpkin pi."
Group Costume: Pi or Number Line
If you are part of a math department or a group of math-loving friends, you can dress up together as the number pi. Simply make T-shirts with the digits of pi and have one person wear the 3, another wear the decimal point, another wear the 1, etc. You can continue this costume for as many colleagues or friends as you would like. Similarly, a group of people can wear numbers and be a number line when they stand in a row.