Vial, Pen and Day-Supply Math

How many units are in what

Everything starts here, and the numbers are simpler than they look because almost all human insulin is U-100 — 100 units per millilitre.

Note the concentrated products. A Toujeo pen holds 450 units in a smaller barrel because it is U-300; a U-500 vial holds ten times a standard vial. Assuming 300 units per pen across the board is the first place day-supply maths goes wrong.

The basic calculation

Someone using 40 units a day from a box of five pens has 1,500 ÷ 40 = 37 days. For a 90-day supply at the same dose they need 90 × 40 = 3,600 units, which is 12 pens — so two boxes of five plus one more box, since pens are dispensed by the box.

Rounded up to whole containers, because you cannot dispense part of a pen. Our refill calculator and pens-per-month tool handle the rounding and the box sizes.

Two things that make the arithmetic wrong

Priming. Most pen instructions call for a 2-unit air shot before each injection to clear the needle and confirm flow. That insulin never reaches you, but it does leave the pen. Someone injecting four times a day loses 8 units daily to priming — nearly 20% of a 40-unit dose. Over 90 days that is 720 units, or two and a half extra pens that the simple calculation does not account for.

The 28-day rule. The larger problem. Most in-use insulins must be discarded 28 days after first use even if insulin remains — and for premixed pens the window is as short as 10 to 14 days. A patient on 15 units a day will not empty a 300-unit pen for 20 days, so time rather than volume governs. On a low dose, a meaningful share of every pen is discarded unused, and no arithmetic based on volume alone will predict how much you actually need. Our expiration date calculator shows which limit applies to your product.

A more realistic estimate

Combining both corrections: take your daily dose, add priming waste, then check whether the 28-day discard rule will cut a container short before you finish it. Whichever runs out first — the units or the days — determines the true consumption rate.

For a patient on 15 units a day with two injections: 15 + 4 priming = 19 units daily. A 300-unit pen would last 15 days on that basis, comfortably inside the 28-day window, so volume governs and they need about 6 pens per 90 days.

For a patient on 8 units a day with one injection: 8 + 2 = 10 units daily, so a pen would last 30 days on volume — but the 28-day rule discards it first. Their real consumption is one pen every 28 days regardless, meaning roughly 4 pens per 90 days rather than the 3 the volume calculation suggests.

Why the pharmacy may still say no

Calculated need and dispensed quantity are different things. Plans apply quantity limits per fill period, and those limits are usually set against a typical dose rather than yours. A high-dose patient can be entirely correct about needing 30 pens per 90 days and still hit a cap.

What usually resolves it is documentation rather than argument: a prescription written with the actual daily dose and injection frequency, so the pharmacist can see the arithmetic, and a prior authorisation if the plan requires one. Waste from priming and the 28-day rule is legitimate and worth stating explicitly, because it is invisible in a calculation based on units alone.

Limitations & Safety Notes

• These calculations assume a stable daily dose. Any titration changes consumption and therefore the supply you need.
• Priming losses are real but approximate — they depend on injection frequency and technique, and are not captured by unit-based arithmetic.
• The 28-day in-use rule (10–14 days for premixed pens) can govern consumption instead of volume, particularly on low doses.
• Concentrated products change the maths. U-300 and U-500 hold different unit counts in the same or smaller volume.
• Pharmacy quantity limits are set by plan rules, not by clinical need. Calculated requirement is a starting point for that conversation, not a guarantee.

Frequently Asked Questions