Desired Over Have (D ÷ H × Q)
Desired Over Have is a short formula for finding how many tablets, capsules or milliliters match an ordered dose.
In this lesson
- What D, H, Q and X stand for
- Why D and H must share a unit
- Tablet, liquid and unit-conversion examples
- Mistakes that change the answer
What you're solving for
X: the amount of the product (tablets, capsules or mL) that contains the ordered dose.
The formula
D divided by H, times Q, equals X
- D
- Desired dose — the ordered amount, such as 50 mg.
- H
- Have — the dose available in one unit of the product, from the label.
- Q
- Quantity — the amount of product that contains H: 1 tablet, 1 capsule, or a volume such as 5 mL.
- X
- The calculated amount, in the same unit as Q.
D and H must use the same unit before you divide. Dividing mg by mg cancels the unit and leaves a plain number, which is then multiplied by Q. If D is in g and H is in mg, the units don't cancel and the answer will be off by a factor of 1,000.
Worked examples
Example 1: Tablets
Ordered: 50 mg. On hand: 25 mg tablets. How many tablets?
Step 1: Calculation
50 mg ÷ 25 mg × 1 tablet
= 2 tablets
Step 2: Formula
Desired ÷ Have × Quantity = Amount
Step 3: Check your units
mg cancels with mg, leaving tablets.
Answer: 2 tablets
Example 2: Liquid medication
Ordered: 375 mg. On hand: 125 mg / 5 mL. How many mL?
Q is 5 mL here, not 1 mL, because the label says 125 mg is in 5 mL.
Step 1: Calculation
375 mg ÷ 125 mg × 5 mL
= 15 mL
Step 2: Formula
Desired ÷ Have × Quantity = Amount
Step 3: Check your units
mg cancels with mg, leaving mL.
Answer: 15 mL
Example 3: A unit conversion first
Ordered: 0.5 g. On hand: 250 mg tablets. How many tablets?
D is in g and H is in mg, so convert first (1 g = 1,000 mg).
Step 1: Unit conversion
0.5 g = 500 mg
Step 2: Calculation
500 mg ÷ 250 mg × 1 tablet
= 2 tablets
Step 3: Formula
Desired ÷ Have × Quantity = Amount
Step 4: Check your units
mg cancels with mg, leaving tablets.
Answer: 2 tablets
Common mistakes
Dividing H by D
- What goes wrong
- 25 mg ÷ 50 mg × 1 tablet = 0.5 tablet
- Why it fails
- The formula is Desired over Have. Flipping it gives the reciprocal.
- Correct approach
- 50 mg ÷ 25 mg × 1 tablet = 2 tablets
Leaving out Q
- What goes wrong
- 375 mg ÷ 125 mg = 3, written as 3 mL
- Why it fails
- 3 is how many 5 mL portions you need, not how many mL.
- Correct approach
- 375 mg ÷ 125 mg × 5 mL = 15 mL
Mixing g and mg
- What goes wrong
- 0.5 g ÷ 250 mg × 1 tablet = 0.002 tablet
- Why it fails
- g and mg don't cancel, so the result is meaningless.
- Correct approach
- Convert 0.5 g to 500 mg, then 500 mg ÷ 250 mg × 1 tablet = 2 tablets
Quick self-check
Ordered 150 mg; on hand 50 mg / 2 mL. Set up D ÷ H × Q.
Show the answer
150 mg ÷ 50 mg × 2 mL = 6 mL
Compare methods
The same problems can be solved with dimensional analysis or ratio and proportion. All three give the same answer.
Calculate and practice
Related lessons
- Dimensional Analysis for Dosage Calculations — Chain conversion factors so unwanted units cancel.
- Ratio and Proportion for Dosage Calculations — Compare the known relationship to the one you need, then cross multiply.
- Tablet and Capsule Calculations — Solid dosage forms: whole tablets, scored tablets and capsules.
- Liquid Medication Calculations — Read a concentration correctly and calculate the mL.
Sources
- Dose Calculation Desired Over Have Formula Method — NCBI StatPearls
- Pharmacology for Nurses — 2.4 Dosage Calculations — OpenStax
- Nursing Skills — Math Calculations — Open RN / NCBI Bookshelf
Explanations are original Dosage Calcs wording. See all sources and the methodology.
Educational Use & Safety
Educational content only. These lessons teach medication math using fictional values and do not determine an appropriate medication, dose, infusion rate, or treatment for an actual patient.
Educational Use & Safety