*Recently, I received a lengthy email from a reader who had a ton of basic personal finance questions contained within. I thought it might be interesting to start an irregular “personal finance 101″ series to answer and explain some of her questions.*

This question isn’t from the long email, but it is a great question concerning personal finance fundamentals that some of my readers might be interested in.

You keep talking about taking a calculator to the grocery store but I don’t know what to do with it or why it is useful. Shouldn’t I just buy the biggest one?

There are a lot of uses for a calculator in the grocery store. Here are three examples that shows how a calculator can be useful. Most of these examples are very basic math, but they show clearly how to use a calculator in a store to save a bit of money.

**Calculating price per unit.** Let’s say you want to buy Tide, but you want to get the best deal on it. One of the containers promises 52 loads for $5.99 and another one promises 96 loads for $10.99. Which one is cheaper?

This is a great time to break out your calculator. Type in 5.99, hit the divide button, type in 52, and hit the equal button. You’ll get a number that looks like 0.115192. That’s how many dollars you have to pay per load. In other words, with the 52 load container, it will cost you eleven and a half cents per load. Now, type in 10.99, hit the divide button, type in 96, and hit the equal button. You’ll get 0.114479. What does it mean? The larger one only saves you about a half a cent per load.

You’ll be surprised to find that sometimes the bulk offering is a great deal, other times it’s not much of a deal at all, and on occasion *the smaller size is actually cheaper* per unit.

**Converting incompatible sizes.** You’re checking out the toilet paper. One offers 9 rolls of paper with 150 sheets on each. The other one offers 16 rolls of paper with 88 sheets on each. Which one gives you more sheets?

Whip out the calculator and see how many sheets you get in the first package: enter 9, hit the multiply button, enter 150, and hit the equal button. You’ll see that that package gives you 1,350 sheets. How about the second one? Enter 16, hit the multiply button, enter 88, and hit the equal button. 1,408 sheets! The one with 16 rolls, even though the rolls are much smaller, gives you more sheets. You might want to compare the price per sheet to see which one is really the better deal, using the tip above.

**Coupon math.** I have a coupon for $1 off any package of Pampers Cruisers. There’s a 108 count box for $26 and a 144 count box for $34.50. I see that the larger box is cheaper without the coupon, and I can get another coupon for no problem. Which one is the better deal *with* a coupon?

Let’s figure up the first one. Enter 26, hit the minus button, enter 1, and hit the equal button. It costs $25 after the coupon. Divide that by 108 to see the cost per diaper: $0.231481. How about the other box? Enter 34.50, hit the minus button, enter 1, and hit the equal button. It costs $33.50 after the coupon. Divide that by 144 to see the cost per diaper: $0.232639. *It’s actually cheaper to get the smaller package with the coupon!*

I generally think savings below 0.5 cents per unit is not worth messing with. Especially with regularly purchased consumables. Obsessing over one tenth of a cent (as in the diaper example) is a sure sign that more sensible budget cuts could be made elsewhere.

Trent these are good tips! I have a suggestion that is sort of related to your Tide example that can help you save additional money. I would suggest that instead of using the number of loads promised on the box as your divider, use the weight of the package contents when dealing with laundry soap.

Why? First off, you don’t know for certain if the 52 loads in the first box and the 96 loads in the second box are using the same amount of soap, or same size scoop. Presumably they are, but you should check.

Secondly, and more importantly, if you REALLY want to save money on laundry soap, disregard the recommended amount of soap to be used in a single load. The soap companies have an interest in getting you to spend more money buying their product as often as possible, so the amount of soap they suggest you use per load is unnecessary. I use about half the recommended amount of soap and have never had a problem – my clothes come out clean and smelling fresh every time! In fact using too much soap can actually do more harm than good. If you use too much, sometimes not all the soap will wash out of your clothes in the rinse and it can be itchy for sensitive skin to wear dry clothes with soap residue in them. Soap residue can also build up in your washing machine and cause it to need repairs more frequently than necessary if you use too much soap.

So use the weight of the contents for your calculation for a more accurate result, and use less soap and you’ll only need to buy soap half as often.

By the way this tip also works for fabric softener, dryer sheets (cut them in half), dishwashing liquid, etc.

In Canada, where our grocery stores seem to have everything in French but can not decide if the metric or imperial weights (or both) should be used on price tags or product packages, a calculater is also useful for establishing like units.

I tend to convert most things to metric (Litres, Kilograms) and sometimes have to convert an adverisement, package, price tag, flyer or coupon into metric to compare the unit cost of one $2.49 5lb bag of apples to the $1.50/kg sign in the produce bin. Unit price Bag = $.92, saving .57 per kilo.

A calculator, handy to have if you are not rain-man

“I generally think savings below 0.5 cents per unit is not worth messing with.”

I’m not so sure. If you take that 0.5 cents and invest it at 10%, in 30 years it will be worth an entire dime — not something to be taken lightly.

Seriously, though, I do use the number of loads printed on the detergent, so I can compare liquid soap versus powerded soap. Instead of worrying about ounces versus fluid ounces, I find it easiest to simply use the number of loads.

You may not need a calculator to find the price per unit. Where I live, all of the large chain grocery stores print the price per unit on the display tag underneath the actual product price. The print is usually so small that I’m sure most consumers don’t even realize that it’s there. Take a closer look.

Hey .5 cents is still .5 cents. Think how many items you buy at the grocery store. That .5 cents per item starts to add up after a while.

In Sweden, all retailers are required to post “comparison prices” on all goods. This means that everything lists two prices: the price for the unit and price per kilo, litre or whatever unit that is most logical. And yes, that goes for detergent (prize per wash), tablets for your dishwashing machine (per wash again), toothpaste (per gram), milk (per litre), toilet paper (per meter) etc. This is to make it easier for the customer to compare prices between brands AND stores. Very nice. Your need for a calculator would go away if you had this system in the states.

Which is why I said anything less than 0.5 cents is more of a bother than anything. His example had a savings of 1/10 of a cent. There is obsessive, and then there is just crazy.

“I’m not so sure. If you take that 0.5 cents and invest it at 10%, in 30 years it will be worth an entire dime — not something to be taken lightly.”

Your right. And I am all for a healthy return on investment. My point was, in my opinion, my energy is best served cutting expenses elsewhere than trying to save 1/10 of a cent per unit at the grocery store.

Don’t forget – most (if not all?) mobile/cell phones have a built-in calculator, so you don’t even have to carry an extra one.

If you’re not using the calculator on your mobile, check it out!

One thing to be careful of is when dealing with similar items that measure by weight. Some packagers include the weight of the packaging material into the total weight. When you compare smaller items (like individually wrapped items) to bulk foods, you need to be wary. A good example can be seen by comparing hallowen candy to bulk candy of the same type. It may be cheaper per ounce with the individually wrapped items, but you’re getting more ounces of packaging. (In this case you want that packaging though… most cases you don’t.)

I got on your site to discussed the difference of pay wages in different stores in long island. This page was reffered to me from my manager who asked us tolook up the difference in pay