It should not be so hard to compare pizza prices. My family has pizza on Friday nights - it is a tradition, and it prevents us from ordering desperation pizza on random other nights. Being cheap frugal, I like to be sure that I am getting the best pizza deals available. Unfortunately, comparing the prices of different sized pizzas isn't very straightforward.
First, I check out the three major chains in my town. I use the websites because I often find the best deals online, and also because the employees at the pizza stores don't always seem motivated to help me find the best deals. Also, I am registered with each of the three sites, so I get emails that often have even better offers. Now comes the hard part: comparing prices between different pizza sizes.
We have six people, so we usually order two large or three medium pizzas. I prefer to order three medium because the "halves" are about the right portion size for our various topping requests. (If I order a half pepperoni of a large, we have too much pepperoni pizza.) However, I'm willing to go with large pizzas if they are significantly cheaper than the mediums; heck, I'll even get two extra-larges if they are the right price. But how do I know what is "significantly" cheaper?
My formula works if your pizza places use 12 inch pizzas for mediums, 14 inch pizzas for larges, and 16 inch pizzas for extra larges. If you aren't sure, check their nutritional information. The sizes are usually listed there. I'll put the actual math at the bottom in case you need to make adjustments for different sizes. There are a couple of different ways to tackle this problem, but this is the way that works for me.
If you think of a large pizza as your base price, you can compare prices by knowing how the other sizes measure in comparison. If a large pizza is one unit, a medium pizza is about 3/4 of a large pizza (73%, to be exact) and an extra- large pizza is about 1 1/3 of a large pizza (131%).
medium pizza = 3/4 large pizza
extra large pizza = 1 1/3 large pizza
Next, you compare the prices and see if the price is lower or higher than the percentages. Fortunately for my comparisons, the best deal at our local store is any large pizza for $10. It is very easy to compare against $10 and it does get a little harder when you are working with other prices, but not TOO hard. The store we were comparing was offering medium pizzas for $7 each, or 70% the cost of a large. If you order the medium, you are paying 70% the price and getting 73% as much pizza - a better deal on a per bite basis. Not a significantly better deal in this example, but still a better deal.
Of course, you also have to consider your needs. In this equation, three medium pizzas will cost $21, which is $1 more than two large pizzas. Of course, we are getting a little bit more more pizza with three medium. This works for my family but it might not be the right choice for your family. Making the less expensive choice isn't a good deal if you end up throwing it away.
I don't know if this is confusing or helpful, but it has helped me to think a little bit more clearly.
Now here's the math if you are wondering where I came up with this. (Feel free to check my math!) The formula for the area of a circle is Pi (3.14) times the radius squared. Pizzas are measured by diameter, so you'll have to take the diameter and divide it by two to get the radius. I'm throwing the Pi out because I'm comparing a bunch of the same equations and it cancels itself out, and therefore I'm comparing the radius squared of one size versus the radius squared of the other size. For a medium, the radius squared is 36, for a large, it is 49. By dividing, you get that the medium is 73% as big as the large. Compare the ratio of the price to the ratio of the size to figure out which is better.