Formula of Dosage Calculation

Dosage Calculation Formula

Accurate medication dosage is critical in healthcare to ensure patient safety and therapeutic effectiveness. This calculator helps you determine the correct amount of medication to administer using the standard dosage calculation formula: Desired Dose (D) / On Hand Dose (H) × Quantity (Q) = Amount to Administer (X).

This formula is widely used by nurses, pharmacists, and other healthcare professionals to convert a physician's order into a measurable quantity for administration. It's essential to pay close attention to units to prevent medication errors.

Understanding the Formula Components:

• Desired Dose (D): This is the amount of medication prescribed by the physician or the target dose for the patient. It's what you want to give.
• On Hand Dose (H): This is the strength of the medication available in your stock. It's what you have on hand.
• Quantity (Q): This refers to the form or volume in which the on-hand dose is supplied. For example, if the on-hand dose is 10 mg/mL, the quantity is 1 mL. If it's 250 mg per tablet, the quantity is 1 tablet.
• Amount to Administer (X): This is the final calculated amount of medication you will give to the patient, expressed in a measurable unit like mL, tablets, or capsules.

Importance of Unit Consistency:

One of the most common sources of error in dosage calculation is inconsistent units. Always ensure that your Desired Dose (D) and On Hand Dose (H) are in the same unit before performing the calculation. This calculator includes unit conversion capabilities for common mass units (mg, mcg, g) to help prevent these errors. If 'Units' are selected, both desired and on-hand doses must be in 'Units'.

Examples:

Example 1: Liquid Medication

Order: Administer Amoxicillin 250 mg orally.

Available: Amoxicillin 125 mg / 5 mL suspension.

• Desired Dose (D): 250 mg
• On Hand Dose (H): 125 mg
• Quantity (Q): 5 mL

Calculation: (250 mg / 125 mg) × 5 mL = 2 × 5 mL = 10 mL

Example 2: Tablet Medication with Unit Conversion

Order: Administer Folic Acid 0.5 g orally.

Available: Folic Acid 250 mg tablets.

• Desired Dose (D): 0.5 g (which converts to 500 mg)
• On Hand Dose (H): 250 mg
• Quantity (Q): 1 tablet

Calculation: (500 mg / 250 mg) × 1 tablet = 2 × 1 tablet = 2 tablets

Example 3: Microgram Dosage

Order: Administer Levothyroxine 100 mcg orally.

Available: Levothyroxine 0.05 mg tablets.

• Desired Dose (D): 100 mcg
• On Hand Dose (H): 0.05 mg (which converts to 50 mcg)
• Quantity (Q): 1 tablet

Calculation: (100 mcg / 50 mcg) × 1 tablet = 2 × 1 tablet = 2 tablets

function convertToMg(value, unit) { if (unit === 'mg') { return value; } else if (unit === 'mcg') { return value / 1000; } else if (unit === 'g') { return value * 1000; } else if (unit === 'units') { // Units are treated as a separate category, no conversion to mg return value; } return NaN; // Should not happen with valid units } function calculateDosage() { var desiredDose = parseFloat(document.getElementById('desiredDose').value); var desiredDoseUnit = document.getElementById('desiredDoseUnit').value; var onHandDose = parseFloat(document.getElementById('onHandDose').value); var onHandDoseUnit = document.getElementById('onHandDoseUnit').value; var quantityAvailable = parseFloat(document.getElementById('quantityAvailable').value); var quantityAvailableUnit = document.getElementById('quantityAvailableUnit').value; var resultDiv = document.getElementById('result'); if (isNaN(desiredDose) || isNaN(onHandDose) || isNaN(quantityAvailable) || desiredDose <= 0 || onHandDose <= 0 || quantityAvailable <= 0) { resultDiv.innerHTML = 'Please enter valid positive numbers for all fields.'; return; } // Handle 'Units' as a special case where units must match exactly if (desiredDoseUnit === 'units' || onHandDoseUnit === 'units') { if (desiredDoseUnit !== onHandDoseUnit) { resultDiv.innerHTML = 'When using "Units", both Desired Dose and On Hand Dose units must be "Units".'; return; } // If both are 'units', no conversion needed, proceed directly var amountToAdminister = (desiredDose / onHandDose) * quantityAvailable; resultDiv.innerHTML = 'Amount to Administer: ' + amountToAdminister.toFixed(2) + ' ' + quantityAvailableUnit + ''; return; } // Convert desired and on-hand doses to a common base unit (mg) for mass units var convertedDesiredDose = convertToMg(desiredDose, desiredDoseUnit); var convertedOnHandDose = convertToMg(onHandDose, onHandDoseUnit); if (isNaN(convertedDesiredDose) || isNaN(convertedOnHandDose)) { resultDiv.innerHTML = 'Error during unit conversion. Please check units.'; return; } if (convertedOnHandDose === 0) { resultDiv.innerHTML = 'On Hand Dose cannot be zero.'; return; } var amountToAdminister = (convertedDesiredDose / convertedOnHandDose) * quantityAvailable; resultDiv.innerHTML = 'Amount to Administer: ' + amountToAdminister.toFixed(2) + ' ' + quantityAvailableUnit + ''; } // Initial calculation on page load for default values window.onload = calculateDosage;