Tablet Dosage Calculator

The D/H × Q formula

Tablets to give = (Desired ÷ Have) × Quantity. Desired = the dose ordered (e.g., 500 mg). Have = the strength of each tablet on hand (e.g., 250 mg/tablet). Quantity = the number of tablets per the 'Have' (almost always 1). Result: how many tablets to administer. Pediatric and capsule formulations follow the same formula with mL or capsule units.

Worked example

Order: amoxicillin 500 mg PO. On hand: 250 mg tablets. Calculation: (500 ÷ 250) × 1 = 2 tablets. Always double-check unit consistency — if the order is in grams and the tablet is in mg, convert first (1 g = 1,000 mg).

When the answer is a fraction

If the calculation produces a half tablet (e.g., 1.5 tablets), confirm the tablet is scored — only scored tablets can be split safely without altering the dose. Capsules CANNOT be split. Extended-release (ER, XR, SR) tablets CANNOT be split because splitting destroys the controlled-release mechanism. Enteric-coated (EC) tablets CANNOT be split. If in doubt, call pharmacy.

What to do when the math doesn't yield a whole or half

If the calculation returns 1.7 tablets or 0.3 tablets, something is off — the strength on hand likely doesn't match the order's intended formulation. Verify with pharmacy or the prescribing provider. Common cause: order was written for an oral suspension or different formulation than what's stocked. Don't approximate — call.

The five (or six, or eight) rights revisited

Every nursing program teaches the "five rights" of medication administration — right patient, right drug, right dose, right route, right time — and most curricula now include three more: right documentation, right reason, and right response. The dosage calculation sits at the center of "right dose," but tablet-splitting and capsule administration introduce a sixth and seventh layer: right formulation and right preparation method. Splitting an extended-release tablet to "save half for tomorrow" can cause dose-dumping and toxicity. Crushing an enteric-coated proton-pump inhibitor destroys the protective coating and the patient gets only a fraction of the active drug. Sublingual nitroglycerin held in the buccal pocket too long degrades; given as a swallowed pill, it has near-zero bioavailability. The math is necessary but not sufficient.

Before you administer any oral medication that requires a calculated split or a sub-tablet dose, ask three questions: Is this tablet scored for splitting? Is the formulation extended-release, enteric-coated, or otherwise modified? Is there a more appropriate dose form available? If the answer to the third question is "yes" — for example, a 250 mg tablet exists when the order is for 250 mg and the floor stock is only 500 mg — call pharmacy and request the correct strength. Splitting tablets routinely is poor practice when an exact-strength formulation exists.

High-alert oral medications and the role of double-checks

Several oral medications are classified as high-alert by the Institute for Safe Medication Practices, including warfarin, methotrexate, oral chemotherapy agents, opioids, and oral hypoglycemics. For these drugs, an independent double-check is required at the point of preparation, not just verification at the bedside. The second nurse must independently recalculate the dose, not just confirm the calculation already done. This is the difference between a real safety check and a perfunctory one. Document the second nurse's name and the time of the verification in the eMAR.

Methotrexate is a particular trap: oral methotrexate for rheumatoid arthritis is dosed weekly, not daily, and the most catastrophic dosing errors occur when a patient or nurse interprets the order as daily. A "Once weekly" instruction that looks like "Once daily" on a generic eMAR view has caused fatalities. Always confirm dosing frequency on high-alert oral agents with the patient and the prescriber.

Pediatric oral dosing and weight-based math

Pediatric oral dosing introduces weight-based math on top of the basic D/H × Q formula. The order may read "amoxicillin 50 mg/kg/day divided BID" — for a 14 kg child, that is 700 mg/day, or 350 mg per dose, given as 14 mL of 125 mg/5 mL suspension. Three things to verify on every pediatric oral medication: weight in kg (never lbs in clinical math), the resulting daily dose against the safe-dose range from a pediatric reference (Lexi-Comp, Harriet Lane, or the institution's pediatric formulary), and the volume of suspension to draw up using an oral syringe — never an IV syringe and never a household teaspoon. Pediatric dosing errors disproportionately involve volume measurement; a 5 mL household teaspoon can be anywhere from 3 to 7 mL, and unit confusion between household and metric measurements has killed children. Always supply a calibrated oral syringe with discharge teaching and demonstrate use to the caregiver.