For people living with type 1 diabetes, insulin math can become part of every day. How many carbs are in a meal? What is your current blood sugar? What correction do you need? What insulin have you already taken? For Drew Mendelow, those questions became the starting point for an idea that would eventually become T1D1, a free insulin calculator app.
Drew was diagnosed with type 1 diabetes at age 13 in 2020. Like many people who are newly diagnosed, he found himself trying to learn a whole new language while also keeping track of insulin doses, blood sugar numbers, and settings. He started tracking everything in an Excel spreadsheet. Then he had a simple question: was there an app that could help him calculate insulin? When he couldn’t find one that worked for him, he decided to build it himself.
Building T1D1 From Scratch
Drew already knew some basic coding. Before his diagnosis, he had taught himself how to make video games. Building a diabetes app, however, was a completely different challenge. He taught himself what he needed to know and worked with doctors, nurses, and educators at Children’s National in Washington, D.C. He also recruited people with type 1 diabetes to test early versions of the app and share their feedback.
That feedback helped shape three ideas that became central to T1D1: simplicity, reliability, and accessibility. Drew wanted the app to be easy to understand, dependable, and available to as many people as possible.
What Does the T1D1 Insulin Calculator Do?
T1D1 is designed to help people using multiple daily injections, or MDI, calculate and track insulin. Users enter their blood sugar and carbohydrate information. The app uses personalized settings from their healthcare team, such as their target blood glucose, correction factor, and carbohydrate ratio, to calculate an insulin dose.
The app also includes tools for everyday tracking:
- Insulin log: Users can save calculations, view previous doses and settings, edit entries, and share their information with their doctor.
- Reminders: If someone returns to the app within three hours of logging insulin, T1D1 reminds them that insulin was recently taken.
- Food library: Users can save foods and homemade recipes they regularly eat instead of searching through a large database.
- Multiple profiles: Different settings can be saved for situations such as breakfast, dinner, exercise, or nighttime.
- Offline access: The app can be used without an internet connection and sync later.
The Road to FDA Clearance
T1D1 launched as a free app, and it grew quickly. Six months after launch, Drew said it had 45,000 downloads and users in 74 countries. However, shortly thereafter, the app was suddenly removed from the app stores. Drew learned that T1D1 needed FDA clearance because it was calculating insulin doses. What followed was several years of work that included rebuilding the app, preparing extensive documentation, conducting a human factors study, and working with experts who could help guide the process.
A major turning point came when Diabetes Center Berne, a Swiss organization that had encountered Drew through an innovation competition, offered to fund the FDA process. Drew also worked with Comerge, a Swiss software development company, and eventually established T1D1 as a nonprofit organization with an advisory board.
After about five years of work, T1D1 received FDA clearance. Drew says T1D1 is the first FDA-cleared over-the-counter bolus calculator in the United States.
Who Is T1D1 For?
Drew originally built T1D1 with people who were newly diagnosed and using multiple daily injections in mind. Over time, he saw people using it in other ways, too. Some pump users have turned to T1D1 as a backup when their pump breaks or when they take a break from their pumps. Drew also sees potential for people who have limited access to healthcare, people in remote areas, and people who may find diabetes math or technology challenging.
One example he shared was a six-year-old named Lila who lived in a rural area with limited Wi-Fi. Her parents were able to use T1D1 to share diabetes information between them, and a school nurse could access the same log. Another user, James, was 78 and newly diagnosed with type 1 diabetes. After having trouble with his insulin pump, he switched to injections and found T1D1’s simple design and reminders helpful.
Built With the Diabetes Community
From the beginning, Drew has relied on people with diabetes to help shape T1D1. Users have sent him emails with questions, feature requests, and stories about how they use the app. That feedback continues to influence what comes next.
At the time of the conversation, Drew said T1D1 had more than 70,000 downloads and about 3,500 monthly active users. For a project that started with a teenager wondering if he could make insulin math a little easier, T1D1 has come a long way. It is more than an insulin calculator; it’s a valuable resource that helps people with type 1 diabetes manage their daily lives with greater confidence and ease.
A note from Blue Circle Health: This conversation is for general education and support, not medical advice or a specific product recommendation. Talk with your healthcare team about your individual insulin doses, settings, and treatment decisions. To learn more about Blue Circle Health and its free clinical care, education, and support program for adults with type 1 diabetes, visit www.bluecirclehealth.org.
