Which Size is the Better Deal?

I am pretty new to couponing.  Before, I tracked flyer prices to stock up at rock bottom prices.  This worked great for us, but I knew I could do even better with coupons.

Before coupons I would look for the biggest sizes in the store because those usually had the lowest price per unit and therefore the greatest savings per unit for my family.

It was a shock to my system when I started couponing because the thought process is completely different.  Couponers look for the smallest size with the lowest price because then the coupon will take off a higher percentage of the cost and will likely result in a lower after coupon unit cost.

Suppose a store carries two sizes of toilet paper: single packs and 12-packs.  The single pack retails for $1.00 ($1.00/unit) and the 12 pack retails for $7.99 ($0.50/unit). 

Without coupons if you had the space and you had the cash, it would make sense to purchase the 12-pack.  If you go through one roll a week, It's like buying a single pack a week for 12 weeks at 50% off! 

If you had a $1 off coupon, however, things would change.  After coupon, the single pack would be free ($0.00/unit) and the 12 pack would be $6.99 ($0.58/unit).  The coupon has cut the per unit cost of the 12-pack by 42%.  However, the coupon was able to lower the unit cost on the single pack 100%!  It's now free!  Go get 12 coupons and buy 12 single packs!

Now for a real world example. 

This week when I was doing the coupon matchups I noticed something interesting.  Bounty paper towels are on sale at Shoppers Drug Mart at $4.99 for a 6-pack and at Rexall Pharma Plus at 2/$5 for a 2-pack.  I have a $1 off coupon and a $2.50 off when you buy 2 coupon.  Which is the better buy?

Here's some math:

Shoppers Drug Mart 6-Pack $4.99
$1 off coupon ~ $3.99/6 rolls = $0.67/roll
$2.50 off when you buy 2 ~ $7.48/12 rolls=$0.62/roll

Rexall Pharma Plus 2-Pack 2/$5
$1 off coupon ~ $1.50/2 rolls = $0.75/roll
$2.50 off when you buy 2 ~ $2.50/4 rolls = $0.62

In both cases it was better to use the $2.50 when you buy two than the $1.00 off.  This makes sense: $2.50 divided by 2 units is $1.25 off per unit versus $1 off per unit.

If the golden rule of couponing were to hold, the 2-pack should have a lower after coupon unit cost than the 12 pack.  The surprising thing here is that combining sale prices with coupons has pushed both options to be just $0.62 per roll!  There is no difference in per unit cost between the two sizes.

Looks like there isn't always a hard an fast rule you can use.

Although both have the same unit cost, I will be buying the larger size.  Why?  I like my coupons to give me the most bang for my buck.  With the 6-pack, one coupon would give me 12 discounted rolls.  With the 2-packs, each coupon would only discount 4.