Insulin Pens Per Month Calculator
Calculate how many insulin pens you need for a 30-day or 90-day supply based on your total daily dose and pen capacity.
All basal + bolus insulin combined per day.
Units lost to needle priming (typically 2 per new needle). Enter 0 to skip.
How to Use This Calculator
- Enter your total daily dose — all basal plus bolus units.
- Select the pen capacity (a standard pen holds 300 units).
- Set the priming units wasted per pen (about 2 is typical).
- Read the result: pens needed for 30 and 90 days, and how long one pen lasts.
Round up to whole pens and add a 1–2 pen buffer for corrections, sick days, travel and the occasional damaged pen.
Common Pen Capacities
| Pen | Capacity | TDD 20 u/d | TDD 35 u/d | TDD 50 u/d |
|---|---|---|---|---|
| Standard (FlexPen, KwikPen, SoloStar) | 300 units | ~15 days | ~8.6 days | ~6 days |
| Toujeo (glargine U-300) | 450 units (1.5 mL) | ~22 days | ~13 days | ~9 days |
Actual days per pen will be slightly less due to priming and any wasted partial pen at end of use.
Why You Should Always Round Up
Pens rarely divide evenly into a month. Two things eat into the usable insulin in each pen: priming — the 1–2 units you push out to clear air before each injection — and the partial pen left over when a month ends mid-pen. Both mean your real-world pens-per-month is a little higher than the raw division suggests.
Always round the result up to a whole pen and add a 1–2 pen buffer per month for corrections, sick days, travel, and the occasional dropped or damaged pen. Running out is far more disruptive than carrying a small surplus.
Insulin Pens: Types & In-Use Storage
Prefilled vs reusable pens
Most people use prefilled disposable pens — SoloStar (Lantus, Toujeo), KwikPen (Humalog, Basaglar), FlexPen / FlexTouch (NovoLog, Levemir, Tresiba) — which are thrown away when empty. Reusable (durable) pens such as NovoPen take a replaceable 3 mL cartridge, so only the cartridge is changed. Both attach a fresh pen needle for each injection.
How long an in-use pen lasts
The pen you're currently using is kept at room temperature, not in the fridge, and is discarded after its in-use limit — commonly 28 days, though some last longer (detemir ~42 days, degludec ~56 days). Spare, unopened pens stay refrigerated until you start them. Label the in-use pen with the date you opened it.
Frequently Asked Questions
How many insulin pens do I need per month?
Multiply your total daily dose by 30 and divide by the usable units per pen (a standard pen holds 300 units). For 40 units/day: (40 × 30) ÷ 300 = 4 pens. Round up and add a buffer.
How many units are in one insulin pen?
A standard insulin pen or cartridge holds 300 units (3 mL of U-100). Higher-concentration pens hold more total units in less volume — for example, Toujeo (U-300) holds 450 units in 1.5 mL.
How many pens are in a 90-day supply?
Multiply your monthly pen count by 3 and round up. If you use 4 pens a month, a 90-day prescription is 12 pens, plus any buffer your prescriber allows.
Why does my pen run out sooner than expected?
Priming before each dose wastes 1–2 units, and you usually can't use the last partial pen of the month for a full dose. Both reduce the effective units per pen, so plan for slightly more than the simple calculation.
Do I need to refrigerate the insulin pen I'm using?
No. Keep the pen you're currently using at room temperature (below about 30 °C / 86 °F) — room-temperature insulin injects more comfortably, and most manufacturers advise not refrigerating the in-use pen. Store your spare, unopened pens in the fridge until you start them, and discard the in-use pen after its labelled in-use period.
What's the difference between disposable and reusable insulin pens?
Disposable (prefilled) pens come filled with insulin and are thrown away when empty — most pens are this type. Reusable durable pens hold a replaceable 3 mL cartridge, so you change only the cartridge and keep the pen. Both deliver the same dose accurately; the choice comes down to cost, waste and personal preference.
Source
- Insulin pen package inserts (cartridge volume and priming guidance).
- American Diabetes Association. Standards of Care in Diabetes — 2024.
Last reviewed: June 2025