My Easy Science
Pharmacy Calculations Pack
Seven calculations a pharmacist does by hand, in one place. Pick one, enter the numbers, check the answer.
Inputs
Worked example
Loads a hand-worked test case from the spec, then shows the expected answer.
Dilution (C1V1 = C2V2)
Tap Show the volume to use.
Percent strength (% w/v)
Tap Show the percent strength.
mg to mEq
| Salt | MW (g/mol) | Valence |
|---|---|---|
| NaCl | 58.44 | 1 |
| KCl | 74.55 | 1 |
| CaCl2 | 110.98 | 2 |
| NaHCO3 | 84.01 | 1 |
| MgSO4 | 120.37 | 2 |
Tap Show milliequivalents.
Alligation (mixing two strengths)
Tap Show the parts to mix.
Isotonicity (NaCl to add)
Tap Show the salt to add.
IV drip rate
Tap Show the drip rate.
Dose by weight
Tap Show the dose.
Temperature and units
Tap Show the converted temperature.
Calculation diagram
Model assumptions: ideal mixing with volumes that add, a constant drop factor through the set, the stated NaCl equivalent (E) value, and the dose and volume units as labelled. Doses here are learning examples only.
| # | Detail |
|---|---|
| Run a calculator to see the numbers. | |
Answer
— answer
Run a calculator to see the answer here.
Checked against hand-worked examples for all eight calculators (see the page source comments).
Each calculation rounds its results as shown; use the Rounding box next to a calculation to show more or fewer places.
Pick a calculator, then tap its button.
For learning only. Not for patient care decisions. Always check with a qualified professional and the product label.
How it works
These eight calculations come up again and again in pharmacy practice and exams. Dilution keeps the amount of drug the same while the volume grows: C1V1 = C2V2. Percent strength is grams in 100 mL. Milliequivalents count charge, not mass, so you need the molecular weight and the valence. Alligation mixes a strong and a weak preparation to land on a strength in between, and the parts come from the differences on a cross. Isotonicity uses the NaCl equivalent, E, to work out how much salt to add so an eye drop or injection matches body fluids. The drip rate turns a volume and a time into drops per minute through the set's drop factor. And a paediatric dose is simply weight times mg per kg, with a maximum single dose as a safety check.
Worked examples, all checked: make 50 mL of 10% from a 70% stock and you need 7.14 mL of stock plus 42.86 mL of diluent. 5% w/v is 5 g in 100 mL, which is 50 mg/mL. 1 g of NaCl, molecular weight 58.44 and valence 1, is 17.1 mEq. Mix 70% and 20% to reach 50% and you need 3 parts of the 70% to 2 parts of the 20%. 1000 mL over 8 hours through a 15 gtt/mL set runs at 125 mL/h, 31.25 drops a minute, rounded to 31. A 12 kg child at 15 mg/kg/day in 3 divided doses gets 180 mg a day, 60 mg per dose. 1 g of a drug with E 0.18 in 100 mL needs 0.72 g of NaCl to be isotonic. And 100 °C is 212 °F.