Desired / Have calculator
The desired over have calculator answers the most common dosage question in nursing: given the strength you have on hand, how much do you give to deliver the dose that was ordered? It applies the classic (desired / have) x quantity formula and returns tablets or millilitres.
The formula
Amount to give = (desired dose / available strength) x quantity, where quantity is the volume or number of units that the available strength comes in — 1 for a tablet, or the millilitres per that strength for a liquid.
Two worked examples
Tablets: 500 mg ordered, 250 mg tablets on hand, quantity 1. (500 / 250) x 1 = 2 tablets.
Liquid: 125 mg ordered, stock labelled 250 mg per 5 mL. (125 / 250) x 5 = 2.5 mL.
Reading the label correctly
For liquids, the quantity is the volume that contains the stated strength — the 5 in "250 mg / 5 mL" — not the total bottle volume. Using the bottle size instead is the classic error this formula produces, and it produces it silently.
What this does not do
This does not know the medication, so it cannot warn you that an answer of half a tablet is inappropriate for a modified-release form, that a result should be rounded to a measurable volume, or that the dose itself is unsafe. Every result is a check on your arithmetic, not a clinical decision. Confirm the order, the concentration on the label and your own policy before administering anything.
Frequently asked questions
What do I enter for quantity?
The amount that contains the available strength: 1 for a tablet, or the millilitres per that strength for a liquid, such as 5 for 250 mg per 5 mL.
What if the answer is not a whole tablet?
Check whether the form can be split at all. Modified-release, enteric-coated and capsule forms usually cannot, and a different strength may be needed.
Do the units have to match?
Yes. Convert the desired dose and the available strength to the same unit before entering them — mixing milligrams and micrograms is a factor-of-1000 error.
By The Nursana team · Last updated · Canonical: https://nursana.aoneahsan.com/tools/desired-over-have
Nursana provides reference tools only and does not give medical, legal or tax advice.