My son loves calculators and when Halloween was coming up a few years ago we told him he could be anything he wanted to be. Of course he wanted to be a calculator. I knew it would take a little work but it wasn’t so difficult.
We used a box and cut it down to size and fit. Then we painted the box gray and finished the edges with black duct tape. I picked up reflective numbers at Home Depot and used a silver sharpie on black construction paper for the display screen and function keys.
Everyone knew right away what he was and he was so very proud of his calculator costume. The only trouble he had was being able to put candy in his candy bag with one hand while holding the bag with other. He loved this costume so much we had to talk him out of being a calculator the following year.
Related costumes:
TI-89 Titanium Calculator Costume 
Coolest Homemade Halloween GPS Costume 
Great iPod Touch Homemade Halloween Costume for a Boy 
Original Last Minute Costume: A Guy in a TV with a SPAM Advertisement 
Cool Homemade Music Man Juke Box Costume 
Cute iPad Costume for a 4-Year-Old 
Coolest Homemade iPod Kids Costume Idea 
Coolest Old-Fashioned iPod Costume
That’s a riot – my son declared the same thing this year, so we made our own as well. We used black foam core for the front and back, and he’ll wear it like a sandwich board over his shoulders. We’ll see how it goes tonight!
I’m 13 and me and my friends chose to opt out of your scandalous average teen costumes, and we’re all being nerdy things. I’m going to be a calculator, my friend is going to be a book, and my other friends are being a nerd, and a Rubik cube! I will let you know how this goes :) I’m doing mine based off this!