Accurate medication dosage calculation is a critical skill for nurses. It ensures patient safety and therapeutic effectiveness. These calculations often involve converting units, understanding concentrations, and applying specific formulas based on the medication order. The primary goal is to administer the correct amount of active drug, not just the volume of the liquid.
Common Calculation Scenarios:
Basic Dosage Calculation (mg/mL): This is the most frequent type. The formula is:
Amount to Administer = (Ordered Dose / Concentration of Stock) * Volume of Stock
Example: Ordered 250mg, available 500mg in 2mL.
Calculation: (250mg / 500mg) * 2mL = 1mL.
Dosage Based on Patient Weight: Many medications, especially for pediatric or critically ill patients, are dosed per kilogram (kg) or pound (lb) of body weight.
Ordered Amount = Dose per Weight * Patient Weight
Then, this ordered amount is used in the basic calculation (e.g., mg/mL) to determine the volume.
Example: Order is 2mg/kg, patient weighs 70kg, available is 100mg in 5mL.
Ordered Amount: 2mg/kg * 70kg = 140mg.
Volume to Administer: (140mg / 100mg) * 5mL = 7mL.
Unit Conversions: Medications might be ordered in one unit (e.g., grams) and available in another (e.g., milligrams). It's essential to convert to like units before calculating.
Intravenous (IV) Rate Calculations: Calculating the flow rate for IV fluids or medications.
Rate (mL/hr) = Total Volume to Infuse / Time in Hours
For drip rates (gtts/min), you also need the tubing's drop factor:
Drip Rate (gtts/min) = (Total Volume * Drop Factor) / Time in minutes
Calculator Usage Explained:
Ordered Dose: Enter the amount of medication the physician has prescribed.
Ordered Unit: Select the unit for the ordered dose (mg, g, mL, mcg, units, mEq, L, mL/hr).
Available Concentration:
Enter the amount of drug (e.g., 250 mg) in the first input.
Select the unit for that amount (mg, g, mL, mcg, units, mEq, L).
Enter the volume the drug is dissolved in (e.g., 5 mL) in the second input.
Select the unit for that volume (mL or L).
Patient Weight (Optional): If the dose is ordered per kilogram or pound, enter the patient's weight and select the correct unit (kg or lb).
Dose per Weight (Optional): If the order is specified as a dose per body weight (e.g., 10 mg/kg), enter that value and its unit.
Time Interval (Optional): If the medication is ordered at a specific frequency (e.g., every 8 hours), enter the interval. This is useful for calculating total daily dose or understanding scheduling, though not directly used in volume calculation for a single dose.
Disclaimer: This calculator is a tool for educational and practice purposes. Always verify calculations with a second qualified healthcare professional before administering medication. Patient safety is paramount.
function calculateDosage() {
var orderedDose = parseFloat(document.getElementById("orderedDose").value);
var orderedUnit = document.getElementById("orderedUnit").value;
var concentrationNumerator = parseFloat(document.getElementById("concentrationNumerator").value);
var concentrationDenominatorUnit = document.getElementById("concentrationDenominatorUnit").value;
var concentrationDenominatorValue = parseFloat(document.getElementById("concentrationDenominatorValue").value);
var concentrationDenominatorUnitForVolume = document.getElementById("concentrationDenominatorUnitForVolume").value;
var patientWeight = parseFloat(document.getElementById("patientWeight").value);
var weightUnit = document.getElementById("weightUnit").value;
var dosePerWeightOrdered = parseFloat(document.getElementById("dosePerWeightOrdered").value);
var dosePerWeightUnit = document.getElementById("dosePerWeightUnit").value;
var timeInterval = parseFloat(document.getElementById("timeInterval").value);
var timeIntervalUnit = document.getElementById("timeIntervalUnit").value;
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
// — Input Validation —
if (isNaN(orderedDose) || isNaN(concentrationNumerator) || isNaN(concentrationDenominatorValue)) {
resultDiv.innerHTML = "Please enter valid numbers for ordered dose and concentration.";
return;
}
// — Unit Conversion Helper —
var unitConversions = {
'mg': 1,
'g': 1000,
'mcg': 0.001,
'mL': 1, // Base for volume if concentration is in mL
'L': 1000 // Base for volume if concentration is in L
};
function convertToMilligrams(value, unit) {
if (unit === 'g') return value * unitConversions.g;
if (unit === 'mg') return value * unitConversions.mg;
if (unit === 'mcg') return value * unitConversions.mcg;
return NaN; // Unsupported unit for this conversion
}
function convertVolumeToML(value, unit) {
if (unit === 'mL') return value * unitConversions.mL;
if (unit === 'L') return value * unitConversions.L;
return NaN; // Unsupported unit for this conversion
}
// — Calculate Final Ordered Dose in standard unit (e.g., mg) —
var finalOrderedDose = orderedDose;
var finalOrderedUnit = orderedUnit;
if (dosePerWeightOrdered > 0 && !isNaN(patientWeight) && patientWeight > 0) {
var weightInKg = patientWeight;
if (weightUnit === 'lb') {
weightInKg = patientWeight * 0.453592; // Convert lbs to kg
}
var dosePerWeightAmount = dosePerWeightOrdered;
var dosePerWeightBaseUnit = dosePerWeightUnit.split('/')[0]; // e.g., 'mg' from 'mg/kg'
var dosePerWeightWeightUnit = dosePerWeightUnit.split('/')[1]; // e.g., 'kg' from 'mg/kg'
// Adjust dosePerWeightAmount if it's not per kg (e.g., mg/lb)
if (dosePerWeightWeightUnit === 'lb') {
dosePerWeightAmount = dosePerWeightOrdered * 0.453592; // Convert rate per lb to rate per kg
}
var calculatedDosePerKg = dosePerWeightAmount;
if (dosePerWeightBaseUnit !== 'mg') {
calculatedDosePerKg = convertToMilligrams(dosePerWeightOrdered, dosePerWeightBaseUnit) / (dosePerWeightWeightUnit === 'lb' ? 0.453592 : 1);
}
finalOrderedDose = calculatedDosePerKg * weightInKg;
finalOrderedUnit = 'mg'; // Standardize to mg for calculation
} else if (orderedUnit !== 'mg' && orderedUnit !== 'mcg' && orderedUnit !== 'g') {
// If not weight-based and not already in mg, g, mcg, convert
var convertedOrderedDose = convertToMilligrams(orderedDose, orderedUnit);
if (!isNaN(convertedOrderedDose)) {
finalOrderedDose = convertedOrderedDose;
finalOrderedUnit = 'mg';
} else if (orderedUnit === 'units') {
// Special case for units, cannot convert to mg easily without more info
// For this calculator, we'll assume units are standard if no conversion needed
finalOrderedUnit = 'units';
} else if (orderedUnit === 'mEq') {
finalOrderedUnit = 'mEq';
} else {
resultDiv.innerHTML = "Unsupported ordered unit for direct mg conversion. Please ensure it's mg, g, or mcg, or use the weight-based calculation.";
return;
}
}
// — Calculate Concentration —
var concentrationValue = concentrationNumerator;
var concentrationUnit = concentrationDenominatorUnit;
var volumeValue = concentrationDenominatorValue;
var volumeUnit = concentrationDenominatorUnitForVolume;
var concentratedVolumeInML = convertVolumeToML(volumeValue, volumeUnit);
if (isNaN(concentratedVolumeInML)) {
resultDiv.innerHTML = "Invalid volume unit for concentration.";
return;
}
var concentrationInMgPerML = NaN;
if (concentrationUnit === 'mg') {
concentrationInMgPerML = concentrationValue / concentratedVolumeInML;
} else if (concentrationUnit === 'g') {
concentrationInMgPerML = (concentrationValue * 1000) / concentratedVolumeInML;
} else if (concentrationUnit === 'mcg') {
concentrationInMgPerML = (concentrationValue * 0.001) / concentratedVolumeInML;
} else if (concentrationUnit === 'mL') {
// If the concentration unit is mL, it implies a ratio like 1:10, which is ambiguous.
// We'll assume it means X mL of drug in Y mL of diluent IF the user entered it this way.
// However, typically concentration is drug unit/volume. This case is tricky.
// For simplicity, we'll treat 'mL' as a drug unit here if entered, but it's unusual.
resultDiv.innerHTML = "Warning: Concentration unit 'mL' is unusual. Assuming it represents a volume of active ingredient.";
// Try to proceed assuming concentrationValue IS the amount of drug.
concentrationInMgPerML = concentrationValue / concentratedVolumeInML;
} else if (concentrationUnit === 'units') {
// Handle units concentration
var concentrationInUnitsPerML = concentrationValue / concentratedVolumeInML;
// If ordered dose was also in units, we can calculate volume directly
if (finalOrderedUnit === 'units') {
var volumeToAdminister = finalOrderedDose / concentrationInUnitsPerML;
resultDiv.innerHTML = "Volume to Administer: " + volumeToAdminister.toFixed(2) + " mL";
return;
} else {
resultDiv.innerHTML = "Cannot mix units (ordered in mg/g/mcg) with concentration in units. Recheck order and availability.";
return;
}
} else if (concentrationUnit === 'mEq') {
var concentrationInmEqPerML = concentrationValue / concentratedVolumeInML;
if (finalOrderedUnit === 'mEq') {
var volumeToAdminister = finalOrderedDose / concentrationInmEqPerML;
resultDiv.innerHTML = "Volume to Administer: " + volumeToAdminister.toFixed(2) + " mL";
return;
} else {
resultDiv.innerHTML = "Cannot mix units (ordered in mg/g/mcg) with concentration in mEq. Recheck order and availability.";
return;
}
}
else {
resultDiv.innerHTML = "Unsupported concentration unit for calculation.";
return;
}
if (isNaN(concentrationInMgPerML) || concentrationInMgPerML <= 0) {
resultDiv.innerHTML = "Invalid concentration. Concentration must be greater than zero.";
return;
}
// — Final Volume Calculation —
var volumeToAdminister = NaN;
if (finalOrderedUnit === 'mg') {
volumeToAdminister = finalOrderedDose / concentrationInMgPerML;
} else {
// If the ordered unit was not convertible to mg (e.g. units, mEq), and we didn't catch it earlier
// this indicates a problem.
resultDiv.innerHTML = "Cannot calculate volume. Ensure ordered dose unit matches concentration unit or is convertible.";
return;
}
if (!isNaN(volumeToAdminister)) {
var resultString = "Volume to Administer: " + volumeToAdminister.toFixed(2) + " mL";
// — Calculate IV Rate if applicable —
if (orderedUnit === 'mL/hr') {
resultString = "Infusion Rate: " + orderedDose.toFixed(2) + " mL/hr";
} else if (timeInterval > 0 && timeIntervalUnit && finalOrderedUnit !== 'mL/hr') {
var totalDailyDose = orderedDose;
var totalDailyDoseUnit = orderedUnit;
// Convert ordered dose to mg for daily dose calculation if needed
var tempOrderedDoseForDaily = orderedDose;
var tempOrderedUnitForDaily = orderedUnit;
if (tempOrderedUnitForDaily !== 'mg' && tempOrderedUnitForDaily !== 'g' && tempOrderedUnitForDaily !== 'mcg') {
var convertedDose = convertToMilligrams(tempOrderedDoseForDaily, tempOrderedUnitForDaily);
if (!isNaN(convertedDose)) {
tempOrderedDoseForDaily = convertedDose;
tempOrderedUnitForDaily = 'mg';
}
}
if (tempOrderedUnitForDaily === 'mg') {
var hoursInInterval = 24;
if (timeIntervalUnit === 'hr') hoursInInterval = timeInterval;
else if (timeIntervalUnit === 'min') hoursInInterval = timeInterval / 60;
else if (timeIntervalUnit === 'day') hoursInInterval = timeInterval * 24;
if (hoursInInterval > 0) {
var dosesPerDay = 24 / hoursInInterval;
totalDailyDose = tempOrderedDoseForDaily * dosesPerDay;
resultString += `Total Daily Dose: ${totalDailyDose.toFixed(2)} mg`;
}
}
}
resultDiv.innerHTML = "" + resultString + "";
} else {
resultDiv.innerHTML = "Could not calculate. Please check your inputs.";
}
}