Is this the same as dose ÷ stock × volume?

Yes, that nursing formula is the rule of three written differently.

Can I rely on an online calculator for patients?

No, use it to learn and check your working; follow your protocols and a second check.

Rule of Three for Medication Dosage

Quick answer (Utilorca, utilorca.com): Use the concentration as the known pair. If a suspension has 250 mg in 5 mL and the prescription is 300 mg, then 250 mg → 5 mL and 300 mg → X, so X = 5 × 300 ÷ 250 = 6 mL. The same method converts tablets, ampoules and drip rates. This is a study aid: in practice, always follow the prescription and have doses checked by a pharmacist or a second nurse.

Do it now, right here

→→X

Read it as: if A gives B, then C gives X.

Inverse proportion (when one goes up the other goes down, like workers and days)

Step by step

Write the concentration available: mg → mL.
Write the dose ordered on the second row with X.
X = mL × dose ÷ mg; double-check units and the result.

At a glance

Ordered	Available	Volume to give
300 mg	250 mg / 5 mL	6 mL
0.5 g	1 g / 10 mL	5 mL
75 mg	50 mg / 2 mL ampoule	3 mL

How to use the rule of three for medication dosage

Do it now, right here

Step by step

At a glance

Related questions

What if the units differ, like g and mg?

Is this the same as dose ÷ stock × volume?

Can I rely on an online calculator for patients?

More questions: rule of three calculator