One of the things I always (well, okay, not always....I have 3 kids under 3...c'mon, but I sure wish it was always) have with me at the grocery store is a calculator.  I use during almost every shopping trip. 

I use it to help me calculate price / ea so I know when a larger size of something is actually cheaper.  When I tell people this, they often say to me: "Don't you know that the price / ea is already calculated out for you on the price tag?  Just look a little closer."  Yes, they are right, the price per each is often calculated for you.  But there are two situations where this still isn't enough information:

Different Brands Calculate Price / ea Differently
For example, one brand of mouthwash may say $0.53 / oz on the price tag while another may say $0.018 / mL.  Those two prices are the exact same ($8.99 for 16.9 oz or 500 mL), but because the two brands use a different form of measurement, it is difficult to make a fair comparison.

Coupons Can Change the Price / ea Significantly
Coupons often make smaller sizes a better deal, but it is hard to be sure unless you have a calculator.  For example, Let's say jumbo pack diapers are on sale for $8.99 ($0.25 ea for size 3) and mega packs are on sale for $13.99 ($0.23 ea for size 3).  Without a coupon, you can quickly glance at the price tags and see that the mega pack is the better deal.  But what if you have a $3 off coupon?  Now, your jumbo pack is $5.99 and your mega pack is $10.99.  You will need a calculator to figure the price / ea because this isn't written on the price tag.  The jumbo pack is now $0.166 / ea and the mega pack is $0.183 / ea.  The jumbo pack is the better deal, but without a calculator you may not have know that!