DoseDrill

Lessons · Oral tablets

Dosage calculation

Oral tablet dosage calculation

Oral tablet problems ask how many tablets deliver the ordered dose. They use the desired-over-have rule, D/H × Q, where D is the ordered (desired) dose, H is the strength on hand, and Q is the number of tablets that strength comes in — usually 1.

Practice this nowMore worked examples

The formula

Tablets = (Desired dose ÷ dose On hand) × Quantity per tablet

How to solve it

  1. Write down the desired dose (what the prescriber ordered) and the dose on hand (the tablet strength).
  2. Set up the ratio as dimensional analysis so milligrams cancel: ordered mg × (1 tablet ÷ strength mg).
  3. Cancel the mg units and divide to get the number of tablets.
  4. Sanity-check: tablets are normally whole or half units, and more than 2–3 tablets for one dose is a flag to re-verify.

Common mistakes

  • Inverting the fraction (dividing have by desired) — always put the unit you want on top.
  • Forgetting to convert to matching units first (e.g. grams ordered vs milligrams on hand).
  • Reporting an unsafe partial tablet that cannot actually be split.
Safety check: A tablet answer that is not a whole or half tablet usually means a unit-conversion or setup error. Re-check before giving.

Worked examples

See it solved

The prescriber orders 500 mg of atenolol PO. The pharmacy stocks atenolol in 500 mg tablets.

Ordered dose500 mg
On hand500 mg per tablet

How many tablets should the nurse give?

Answer: 1 tablet

Step-by-step solution

1. Identify what you have and what you want
Have: 500 mg per tablet · Want: 500 mg
2. Set up dimensional analysis so mg cancels
500 mg × ( 1 tablet ÷ 500 mg )
3. Cancel mg and solve
= ( 500 × 1 ) ÷ 500 tablet
= 1 tablet

The prescriber orders 5 mg of atenolol PO. The pharmacy stocks atenolol in 10 mg tablets.

Ordered dose5 mg
On hand10 mg per tablet

How many tablets should the nurse give?

Answer: 0.5 tablets

Step-by-step solution

1. Identify what you have and what you want
Have: 10 mg per tablet · Want: 5 mg
2. Set up dimensional analysis so mg cancels
5 mg × ( 1 tablet ÷ 10 mg )
3. Cancel mg and solve
= ( 5 × 1 ) ÷ 10 tablet
= 0.5 tablets

The prescriber orders 50 mg of levothyroxine PO. The pharmacy stocks levothyroxine in 50 mg tablets.

Ordered dose50 mg
On hand50 mg per tablet

How many tablets should the nurse give?

Answer: 1 tablet

Step-by-step solution

1. Identify what you have and what you want
Have: 50 mg per tablet · Want: 50 mg
2. Set up dimensional analysis so mg cancels
50 mg × ( 1 tablet ÷ 50 mg )
3. Cancel mg and solve
= ( 50 × 1 ) ÷ 50 tablet
= 1 tablet