Insulin Pen Calculator
Medically reviewed by Dr. Chhaya Makhija, MD, DipABLM — October 2026
Find out how long a single pen will last, or how many units remain after using a pen for a set number of days. To count how many pens to order for a month or a 90-day refill, use the pens per month calculator.
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 the Insulin Pen 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, discontinued in the US at the end of 2024) | 42 days |
| Degludec (Tresiba) | 56 days |
| Rapid-acting (Humalog, NovoLog, Apidra, Fiasp, Lyumjev) | 28 days |
| NPH (Humulin N KwikPen) | 14 days |
| NovoLog Mix 70/30 FlexPen | 14 days |
| Humalog Mix 75/25 & 50/50, Humulin 70/30 KwikPen | 10 days |
Which insulin pens are used in type 2 diabetes
Most people starting insulin for type 2 diabetes begin with a long-acting basal pen, because the first target is fasting glucose overnight rather than meals. Mealtime pens are added later, and often not at all.
| Role | Class | Pens you may be prescribed |
|---|---|---|
| Usually first | Long-acting basal | Lantus, Basaglar, Semglee, Toujeo, Tresiba (Levemir was discontinued in the US at the end of 2024) |
| Added later if meals need cover | Rapid-acting | Humalog, Lyumjev, NovoLog, Fiasp, Apidra, Admelog |
| Sometimes used instead of two pens | Premixed | Humalog Mix, NovoLog Mix, Humulin 70/30, Novolin 70/30 |
Insurance formulary coverage often influences which brand within a class is prescribed, which is one reason two people with similar diabetes can be given different basal pens. The pens within a class are not identical, but they are closer to each other than to a pen from another class.
Pen capacity differs by product, so switching brands changes how long a pen lasts even when the daily dose is unchanged. The calculator above handles that; check the units per pen for the specific product you are given.
Using an Insulin Pen: Priming & Dialing a Dose
Priming (the safety / air shot)
Before each injection, attach a fresh pen needle and dial the safety dose your pen's instructions give: 2 units on most pens, 3 units on Toujeo SoloStar and 4 units on Toujeo Max SoloStar. 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.
How much priming shortens a 300-unit pen
Priming insulin never reaches you, so it shortens the pen. With three injections a day and a 2-unit safety test before each, priming uses 6 units a day:
| Dose from this pen | Days, no priming counted | Days with 3 × 2-unit primes |
|---|---|---|
| 10 u/day | 30.0 days | 18.8 days |
| 20 u/day | 15.0 days | 11.5 days |
| 30 u/day | 10.0 days | 8.3 days |
| 40 u/day | 7.5 days | 6.5 days |
At low doses the 28-day in-use limit usually ends the pen first: at 10 units a day the insulin would last about 30 days, but the pen is discarded at day 28.
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. Vials, refills and dose maths are covered by our vial and refill calculators.
Frequently Asked Questions: Insulin Pen Calculator
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?
The limit depends on the pen: 60 units for the Humalog KwikPen and NovoLog FlexPen, 80 units for the Lantus and Toujeo SoloStar, and 160 units for the Toujeo Max SoloStar and Tresiba U-200 FlexTouch. The Humalog Junior KwikPen stops at 30 units. 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.
How many units does priming an insulin pen waste?
Each safety test (prime) pushes out 2 units on most pens, 3 units on Toujeo SoloStar and 4 units on Toujeo Max SoloStar. At three injections a day with a 2-unit prime, that is 6 units a day, or 60 units out of a 300-unit pen over 10 days.
How long does a Toujeo pen last?
A Toujeo SoloStar holds 450 units and a Toujeo Max SoloStar holds 900 units. Divide by your daily dose plus the safety-test units (3 or 4 per injection). Either pen must be discarded 56 days after first use, even if insulin remains.
Source
- Insulin pen package inserts (in-use room-temperature limits by brand).
- Sanofi. Toujeo (insulin glargine) prescribing information and Instructions for Use, revised 05/2025. Link
- ADCES Danatech. Humulin N KwikPen product detail. Link
- American Diabetes Association. Standards of Care in Diabetes — 2026. Link
Written by Ryan Mitchell · Last updated: October 2026