So, how much does that savings actually amount to? To answer that question, I started checking through my past year's spending records to see how many trips I'd made to different grocery stores. Here's what I found:
The store we shop at the most often is our local Stop & Shop, less than a mile down the road. Since I generally take a walk in the middle of the day anyhow, I frequently stop by there along the way to pick up just one or two items that I happen to need. Thus, on a typical Stop & Shop trip I use only one bag (generally the little Chico Bag, which I carry with me in my purse so I can never be caught without one). Stop & Shop gives a nickel discount for each bag, so that's 5 cents off per trip—and in the past year, I've made 113 trips to the Stop & Shop. So that's $5.65 right there.
The store that gives the most generous bag discount of all those we visit regularly is the Whole Earth Center, a natural foods store in Princeton. They give a dime off for each container you bring from home—not just the grocery bag itself, but also all the bags you use for your produce and all the containers you use to carry items from the bulk bins. (This produces the interesting result that if you buy a very small amount of something—for example, a spice—the amount you save on the bag may actually exceed what you spend on the product. I don't know whether anyone has ever made a serious attempt to exploit this loophole.) I estimate that on a typical trip to the Whole Earth Center, we use around five containers, including the shopping bag. So at 10 cents each, that's 50 cents per trip. That means our 31 trips to the Whole Earth Center in the past year have earned us $15.50.
Another store we visit frequently is the Shop-Rite a few miles away. It doesn't have the best everyday prices, but it tends to have the best sales in our area, so we stop in once or twice a month to stock up on sale items. Usually, we take only one bag on these trips (not the Chico Bag but the big string bag I bought years ago from the Whole Earth Center, which can expand by a larger amount than you'd believe possible). This one bag gets us a nickel off on every trip, so on our 21 trips in the past year, we've saved $1.05.
We also stop by the Aldi once or twice a month to stock up on staples that are cheapest there (such as cereal and orange juice). Unlike most stores, Aldi actually charges for bags—ten cents each, according to this post on mommysavers.com. When we shop at Aldi, we usually load up the handy folding plastic crate that lives in the trunk of our car; if we didn't have it, we'd probably have to use two shopping bags at a cost of 20 cents per trip. So over the course of 16 trips per year, we save $3.20.
At the bottom of the list, savings-wise, is the Pathmark, which we visit a few times a year for sale items, using an average of one bag per trip. Pathmark is less generous with its bag discount than other retailers—only 2 cents off per bag—so in 6 trips a year, we save a measly 12 cents.
The other grocery store where we shop often is Trader Joe's, which we visit every couple of months to stock up on the items that are cheapest there (such as raisins, 100-percent-postconsumer-recycled toilet paper, and cruelty-free toothpaste). However, Trader Joe's doesn't give you a discount for bringing your own bags; instead, they offer you an entry into a drawing to win a gift card. Since I've never won it, I assume that bringing my own bag there 9 times in the past year has gained me nothing but personal satisfaction.
So, adding it all up, I find that bringing our own bags has saved us $25.52 over the course of the past year. It's not a big number, but look at it this way: bringing our own bags all year long gets us the equivalent of one bag of groceries for free. Or, to put it another way: a $7 Chico Bag could potentially pay for itself more than three times over in its first year of use.