Insulin Pen Calculator
Find out how long a single pen will last, or how many units remain after using a pen for a set number of days.
If you use more than one pen (e.g. separate basal and bolus), enter the dose from this pen only.
If entered, shows units remaining after this many days of use.
Most open insulin pens expire within 28–56 days at room temperature regardless of units remaining. Check your pen's package insert for its specific room-temperature expiry window.
How to Use This Calculator
- Select the pen capacity (300 units is standard) or enter a custom amount.
- Enter the daily dose from this pen — if you use separate basal and bolus pens, count only this one.
- Optionally enter days already used to see units remaining and days left.
- Read how long the pen lasts, remembering the room-temperature discard limit below.
Discard an opened pen at its in-use limit even if units remain — degraded insulin causes unexplained high glucose.
Room Temperature Expiry by Pen Type
| Pen / Insulin | Room Temp Limit (once opened) |
|---|---|
| Glargine (Lantus, Basaglar) | 28 days |
| Glargine U-300 (Toujeo) | 56 days |
| Detemir (Levemir) | 42 days |
| Degludec (Tresiba) | 56 days |
| Rapid-acting (Humalog, NovoLog, Apidra) | 28 days |
Using an Insulin Pen: Priming & Dialing a Dose
Priming (the safety / air shot)
Before each injection, attach a fresh pen needle, dial 2 units, hold the pen needle-up and press the button until a stream of insulin appears at the tip. This clears air and confirms the pen is flowing — repeat if no insulin shows. The priming units are why a pen delivers slightly fewer doses than its raw capacity suggests.
Dialing and injecting the dose
Dial your dose on the counter — most pens click in 1-unit steps, while half-unit pens move in 0.5-unit steps. Insert the needle, press the plunger fully, and hold for about 6–10 seconds before withdrawing so the whole dose goes in. Remove and safely discard the needle after each injection.
Frequently Asked Questions
How long does one insulin pen last?
Divide the pen's usable units by your daily dose from that pen. A standard 300-unit pen at 20 units/day lasts about 15 days — but discard it at its room-temperature expiry (often 28 days) even if units remain.
How many days can an insulin pen stay out of the fridge?
Once in use, most pens are kept at room temperature for 28 days; some last longer — Levemir for 42 days, and Toujeo and Tresiba for 56 days. Always check your pen's package insert.
Can I keep using a pen that still has insulin after its expiry?
No. Discard an opened pen at its room-temperature limit even if units remain. Insulin loses potency over time, and degraded insulin causes unexplained high blood glucose. See our storage & expiration guide.
Should I refrigerate an in-use insulin pen?
No. Keep the pen you are currently using at room temperature; refrigerating an in-use pen can cause condensation and dosing problems. Store unopened spare pens in the fridge until first use.
How many units are in one click of an insulin pen?
Most insulin pens click in 1-unit steps, so one click is one unit. Half-unit pens — such as NovoPen Echo or the Humalog Junior KwikPen — move in 0.5-unit steps, which helps children and very insulin-sensitive adults dose precisely. Check your pen's dose counter to confirm.
What is the maximum single dose from one insulin pen?
Most pens dial up to a maximum of about 60–80 units in a single injection, depending on the model. If your prescribed dose is larger than the pen's limit, it has to be split into two injections — or a higher-concentration pen (such as U-200 or U-300) may let you give the same units in less volume. Confirm with your prescriber.
Source
- Insulin pen package inserts (in-use room-temperature limits by brand).
- American Diabetes Association. Standards of Care in Diabetes — 2024.
Last reviewed: June 2025