Need help navigating life with diabetes? You’ve come to the right place! Ask D’Mine is our weekly advice column, hosted by veteran type 1 and diabetes author Wil Dubois.

This week, Wil’s talking expiration dates for some key diabetes supplies. Read on…

Tammy, type 2 from New York, writes: I know this should be simple math, but I’m confused about how many days my insulin pen should last. Can you tell me how to figure this out? Also, when you remove the cap does the countdown on its useful life start, even if you don’t use it? One last thing: I usually end up with some leftover towards the end of the pen, but not enough to make up my next dose. I have been just tossing it and opening another pen, but I hate wasting it. Any thoughts?

Wil@Ask D’Mine answers: Happy to help. It can be confusing with all the different strengths of insulin nowadays, and all the various pens out there on the market. That said, you can easily work out how many days a pen will last you with some super-simple math and by knowing just three things: The strength of the juice in the pen, the size of the pen in question, and your daily dose.

Strength is the number of units of insulin per millimeter of fluid, expressed in “U/mL” on the pen’s label, box, and paperwork. In today’s world we have insulin that’s 100 U/mL, 200 U/mL, 300 U/mL, and 500 U/mL. Oh, and veterinary insulin for cats and dogs that’s 40 U/mL. To make matters worse, some brand names of insulin come in more than one strength. For instance, Tresiba comes in both 100 U/mL and 200 U/mL variations!

As you can see, your insulin may vary. So check your pen carefully.

Next, you need to know the size of the pen, in terms of the volume of liquid it holds. Again, refer to the pen’s label, box, or paperwork. Most pens hold 3 mL, but not all. Toujeo is one exception with its standard SoloStar pen holding only 1.5 mL, further confused by the fact that there’s also a 3 mL Toujeo pen called the Max SoloStar. There are probably other oddball-sized pens that slipped out there too; I thought I’d read somewhere about a 2 mL pen, but now I can’t find it—but it doesn’t really matter. All that matters is that you read the label to know the size of the pen you are using.

Then, all ya gotta do is multiply the strength of your insulin by the volume of your pen and divide by your dose. It sounds worse than it is. Believe me. Everybody agrees.

For instance, if the insulin was a traditional 100 U/mL (a.k.a. U-100), and the pen a typical 3 mL, then you just take 100 and multiply by 3 to get 300 units in the pen. This is the total volume of the pen, in units. If you divide your daily dose into the 300, you’ll find out how many days your pen will last you. Naturally, if you take two doses a day, you need to add those two together to get the total dose for the day before you divide that into the total number of units in the pen.

Of course, if you use small doses, a pen might “go bad” before you can use it up. But this too, varies a lot. Most modern insulins last between a month and six weeks at room temperature, once in use, depending on the brand and type. Once again, I have to send you to the paperwork for the insulin you use. And be verrrrrrry careful about trying to stretch it longer. Insulin is a protein, like meat, and you wouldn’t eat a steak that was left out on the counter for overly long, would you? Oh, and don’t worry about pulling the cap off. The clock doesn’t start running on an insulin pen until you breach the rubber seal at the pen’s tip with the first needle.

Meanwhile, I agree with you that—like a mind—insulin is a terrible thing to waste. Both because it’s frickin’ expensive and because every time you hold a pen in your hand you know that somewhere in the world someone is actually dying because he or she doesn’t have enough insulin — both in the Third World and right here at home. You can avoid wastage, assuming the insulin isn’t beyond its useful life, by taking a split shot to use up every drop of insulin. Here’s the beautiful thing: An insulin pen is designed to not let you dial up any more than what’s left in the cartridge.

When you get down to the end, note how much is left, shoot it up, then take whatever number of units you were short using the next pen. Heck, you can even move the needle over to the new pen and re-use it one more time.

This is not a medical advice column. We are PWDs freely and openly sharing the wisdom of our collected experiences — our been-there-done-that knowledge from the trenches. Bottom Line: You still need the guidance and care of a licensed medical professional.