Tablet Dosage Calculator (Desired over Have)
Calculate how many tablets or capsules to give with the desired over have formula, including g, mg and mcg conversions.
Formula
Tablets = desired dose ÷ dose on hand × quantity (1 tablet)
D ÷ H × Q
How to calculate it step by step
- Write the desired dose (D) from the order and the dose on hand (H) from the label.
- Make sure both are in the same unit. Convert if needed: 1 g = 1,000 mg; 1 mg = 1,000 mcg.
- Divide D by H and multiply by the quantity (Q), which is 1 tablet or 1 capsule.
- Check that the answer makes sense: usually ½ to 3 tablets.
Worked examples
Example 1. Order: hydrochlorothiazide 12.5 mg PO. Available: 25 mg scored tablets. How many tablets will you give?
- Desired ÷ have × quantity = 12.5 mg ÷ 25 mg × 1 = 0.5 tablets.
- The tablet is scored, so a half tablet can be given.
Answer: 0.5 tablets
Example 2. Order: furosemide 20 mg PO. Available: 40 mg scored tablets. How many tablets will you give?
- Desired ÷ have × quantity = 20 mg ÷ 40 mg × 1 = 0.5 tablets.
- The tablet is scored, so a half tablet can be given.
Answer: 0.5 tablets
Rounding
Tablets are given whole or, if scored, as halves. Capsules, extended-release and enteric-coated tablets are never split or crushed unless the label or the pharmacist allows it.
Common mistakes
- Mixing units: 0.1 mg and 50 mcg are not comparable until both are in mcg (0.1 mg = 100 mcg, so 2 tablets).
- Moving the decimal point the wrong way: from a larger unit to a smaller one, multiply by 1,000.
- Giving a fraction of a capsule or of an unscored tablet.
Nursing safety points
- If the answer is more than 3 tablets or a strange fraction, recheck the order and the calculation and ask the pharmacist about another strength.
- Use a pill cutter for scored tablets and give the other half at the next dose or discard it per policy.
- Check the rights of medication administration before every dose: patient, drug, dose, route, time and documentation.
Frequently asked questions
What is the desired over have formula?
Desired dose ÷ dose on hand × quantity. For an order of 650 mg acetaminophen with 325 mg tablets: 650 ÷ 325 × 1 = 2 tablets.
What if the order and the tablet are in different units?
Convert one of them first. For levothyroxine 0.1 mg with 50 mcg tablets: 0.1 mg = 100 mcg, then 100 ÷ 50 = 2 tablets.
Can you split any tablet?
No. Only scored tablets should be split. Extended-release, delayed-release, enteric-coated tablets and capsules must not be split, crushed or opened unless the label or the pharmacist says it is safe.