Use the "Desired over Have" formula to calculate the liquid or tablet volume required.
Required Administration:
The patient should receive:
Understanding Dosage Calculations
Calculating the correct dose of medication is a critical skill for healthcare professionals and caregivers alike. The most common method used is the "Desired over Have" formula (also known as the formula method). This calculation ensures that the patient receives the exact amount of active ingredient prescribed by their doctor, regardless of how the medication is packaged.
The Dosage Formula
The math behind medication dosing is straightforward once you identify the three key components:
Desired Dose (D): The amount of medication ordered by the physician (e.g., 500 mg).
Stock Strength / Hand (H): The concentration of medication available (e.g., 250 mg).
Stock Volume / Quantity (Q): The form the stock comes in, such as 1 tablet or 5 mL of liquid.
The Formula: (D ÷ H) × Q = Amount to Administer
Practical Example:
A doctor prescribes 150mg of a liquid medication. The bottle you have says 75mg per 5mL.
Always ensure that the units of measurement for the "Desired Dose" and the "Stock Strength" match before you begin. For instance, if the order is in grams (g) but your stock is in milligrams (mg), you must convert one of the values so they are identical. 1 gram is equal to 1,000 milligrams.
When calculating tablets, if the result is 1.5, ensure the tablet is scored before attempting to split it. For liquid medications, always use a dedicated oral syringe or measuring cup rather than a household teaspoon for maximum precision.
function calculateDose() {
var desired = parseFloat(document.getElementById('desiredDose').value);
var strength = parseFloat(document.getElementById('stockStrength').value);
var volume = parseFloat(document.getElementById('stockVolume').value);
var resultDiv = document.getElementById('doseResult');
var displayDiv = document.getElementById('doseDisplay');
var breakdownDiv = document.getElementById('doseBreakdown');
if (isNaN(desired) || isNaN(strength) || isNaN(volume)) {
alert("Please enter valid numbers in all fields.");
return;
}
if (strength <= 0) {
alert("Stock strength must be greater than zero.");
return;
}
var dosage = (desired / strength) * volume;
// Formatting for display
var cleanDosage = Number.isInteger(dosage) ? dosage : dosage.toFixed(2);
resultDiv.style.display = "block";
displayDiv.innerHTML = cleanDosage + " (Units/mL/Tabs)";
breakdownDiv.innerHTML = "Formula calculation: (" + desired + " / " + strength + ") × " + volume + " = " + cleanDosage;
// Scroll to result
resultDiv.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}