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Days' supply from an insulin vial

A 10 mL vial labelled 100 units/mL holds 1,000 units, and the days' supply is those units divided by the units used in a day.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

Insulin is the days'-supply question that gets asked most and rejected most, because the vial's label talks about millilitres and units per millilitre while the order talks about units a day. The two never meet until somebody multiplies.

How to think about it

Multiply the volume by the strength to get the units in the vial. Add up every dose in the day to get the units a day. Divide. Round DOWN to whole days.

Worked example

10 mL x 100 units/mL = 1,000 units in the vial
Volume times concentration. Every vial of that size and strength holds the same.
Units a day = 18 units + 12 units = 30 units
Whatever the order says, added up across the day.
1,000 / 30 = 33.3, so 33 days
Round DOWN: part of a day is not a day.
Two vials at 30 units a day = 2,000 / 30 = 66 days
More vials, the same division.

Your turn

A vial holds 1,000 units and the order uses 50 units a day. Write the days' supply.

1,000 / 50 =  days

The trap

Reading 100 units/mL as the size of the vial. That is the strength of every millilitre in it; the vial holds ten of those millilitres.

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