How the calculator works
The calculator uses the desired over have formula, written D/H × Q:
- D is the dose ordered.
- H is the dose on hand, the amount of drug in each tablet or each Q of liquid.
- Q is the quantity that dose comes in, such as 1 tablet, 1 mL or 5 mL.
Open RN’s Nursing Skills textbook sets these problems up by dimensional analysis, and the arithmetic comes out the same. The calculator converts both doses to milligrams first, using 1 g = 1,000 mg and 1 mg = 1,000 mcg, so you can mix units the way a real order and label often do.
Weight-based (mg/kg) orders
Some orders are written per kilogram of body weight, especially for children. Convert pounds to kilograms first (1 kg = 2.2 lb), then multiply by the mg/kg in the order to get D. For example, a 22 lb child is 10 kg, so 15 mg/kg is 150 mg. Choose “per kg of body weight” in the calculator to do both steps.
To check a safe dose range, multiply the weight by the lowest and highest mg/kg. For a 7 kg infant and a range of 10 to 15 mg/kg per dose, the safe range is 70 to 105 mg.
Liquid doses
For a liquid, put the label’s mg in H and its mL in Q. For example, 150 mg from a liquid with 160 mg per 5 mL is (150 ÷ 160) × 5 mL = 4.6875 mL. If you only need to convert mg to mL from a label, the mg to mL calculator does just that.
Worked examples you can try
| Order | On hand | Answer |
|---|---|---|
| 25 mg | 6.25 mg tablets | 4 tablets |
| 12.5 mg | 25 mg per 1 mL | 0.5 mL |
| 250 mg | 125 mg per 5 mL | 10 mL |
| 1 g | 500 mg capsules | 2 capsules |
| 0.25 mg | 50 mcg per mL | 5 mL |
| 2 mg/kg for a 45 lb child | 10 mg per mL | 4.1 mL |
These come from Open RN’s Nursing Skills and WisTech Open’s Nursing Dosage Calculations, and the calculator’s tests check every one of them.
Rounding
Open RN says to follow your agency’s policy and not to round until the final answer. Its general rules are:
- Liquid doses of 1 mL or more round to the tenth, so 17.276 mL becomes 17.3 mL.
- Doses under 1 mL round to the hundredth, so 0.276 mL becomes 0.28 mL.
- The book’s worked examples for children round to the hundredth even above 1 mL, so 4.6875 mL becomes 4.69 mL.
- Always write a leading zero (0.5 mL) and never a trailing zero (5 mL, not 5.0 mL).
The calculator shows the rounded answer and the full working, so you can check each step against your own.
Sources
- Open RN (WisTech Open), CC BY 4.0, Nursing Skills 2e, 5.8 Tablet dosage. The tablet example (25 mg from 6.25 mg tablets is 4 tablets). Checked October 16, 2026.
- Open RN (WisTech Open), CC BY 4.0, Nursing Skills 2e, 5.9 Liquid concentrations. The liquid example (12.5 mg from 25 mg/mL is 0.5 mL). Checked October 16, 2026.
- Open RN (WisTech Open), CC BY 4.0, Nursing Skills 2e, 5.4 Equivalencies. 1 g is 1,000 mg and 1 mg is 1,000 mcg. Checked October 16, 2026.
- Open RN (WisTech Open), CC BY 4.0, Nursing Skills 2e, 5.5 Rounding. Rounding to the tenth or hundredth, leading and trailing zeros, follow agency policy. Checked October 16, 2026.
- Open RN (WisTech Open), CC BY 4.0, Nursing Skills 2e, 5.11 Weight-based calculations. Converting pounds to kilograms and the weight-based example (15 mg/kg for 22 lb, 4.69 mL). Checked October 16, 2026.
- Open RN (WisTech Open), CC BY 4.0, Nursing Skills 2e, 5.12 Safe dosage range. The safe dose range example (10 to 15 mg/kg for 7 kg is 70 to 105 mg). Checked October 16, 2026.
- WisTech Open, CC BY 4.0, Nursing Dosage Calculations, oral medication practice and answer key. The practice answers the calculator is tested against, including g to mg and mcg to mg conversions. Checked October 16, 2026.