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 40 mg of furosemide PO. The pharmacy stocks furosemide in 80 mg tablets.

Ordered dose40 mg
On hand80 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: 80 mg per tablet · Want: 40 mg
2. Set up dimensional analysis so mg cancels
40 mg × ( 1 tablet ÷ 80 mg )
3. Cancel mg and solve
= ( 40 × 1 ) ÷ 80 tablet
= 0.5 tablets

The prescriber orders 60 mg of furosemide PO. The pharmacy stocks furosemide in 20 mg tablets.

Ordered dose60 mg
On hand20 mg per tablet

How many tablets should the nurse give?

Answer: 3 tablets

Step-by-step solution

1. Identify what you have and what you want
Have: 20 mg per tablet · Want: 60 mg
2. Set up dimensional analysis so mg cancels
60 mg × ( 1 tablet ÷ 20 mg )
3. Cancel mg and solve
= ( 60 × 1 ) ÷ 20 tablet
= 3 tablets

The prescriber orders 1000 mg of metformin PO. The pharmacy stocks metformin in 500 mg tablets.

Ordered dose1000 mg
On hand500 mg per tablet

How many tablets should the nurse give?

Answer: 2 tablets

Step-by-step solution

1. Identify what you have and what you want
Have: 500 mg per tablet · Want: 1000 mg
2. Set up dimensional analysis so mg cancels
1000 mg × ( 1 tablet ÷ 500 mg )
3. Cancel mg and solve
= ( 1000 × 1 ) ÷ 500 tablet
= 2 tablets