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How to use the dosage calculator
The tool has four tabs. Each opens with a solved example and updates as you type.
- Dose: type the ordered dose (D), then the strength on hand (H) and what it comes in (Q), such as 125 mg in 5 mL or 250 mg in 1 tablet. A per-kg unit such as mg/kg adds the weight and per dose or per day.
- IV rates: choose mL/hr, gtt/min, infusion time, mcg/kg/min or units/hr.
- Practice: pick a problem type, answer, press Check, then read the solution worked three ways.
- Alligation: mix two strengths to a target, or dilute with C1V1 = C2V2.
Open Rounding rules to match your course. Units and mEq are never converted to grams: if the order and the label disagree, the tool says so instead of guessing.
Press Copy link to share the exact problem, or write a link by hand:
| Parameter | Meaning | Example |
|---|---|---|
tab | dose, iv, practice or alligation | tab=iv |
d du h hu q qu | order and its unit, strength on hand and its unit, quantity and form (mL, L, tablet, capsule) | d=0.5&du=g&h=250&hu=mg&q=1&qu=tablet |
wt wu per n | weight, kg or lb, per dose or per day, doses a day | du=mg/kg&wt=44&wu=lb&per=day&n=3 |
iv vol hrs min df rate start | IV mode (mlhr, gtt, time, dose, units), volume, time, drop factor, rate, start time | iv=gtt&vol=1000&hrs=8&df=15 |
dose unit amt au bag find | infusion dose and unit, drug in the bag and its unit, bag mL, find rate or dose | dose=5&unit=mcg/kg/min&amt=400&bag=250 |
type seed | practice problem type and number | type=weight&seed=42 |
hi lo target amt known | alligation strengths and the amount you know | hi=10&lo=2&target=5&amt=400 |
For example, /nursing-dosage-calculator/?tab=iv&vol=1000&hrs=8&df=15 opens 1,000 mL over 8 hours on 15 gtt/mL tubing.
Desired over have, ratio and proportion, dimensional analysis
All three methods give the same answer. Learn the one your course grades and use another to check it.
Desired over have (the formula method)
amount = D ÷ H × Q. D is the desired (ordered) dose, H is the dose you have on hand, and Q is the quantity H comes in. D and H must be in the same unit before you divide. It is the fastest method for a one-step tablet or liquid problem.
Ratio and proportion
Write what you have as a ratio and set it equal to what you need: H : Q = D : x. Cross-multiply, so H × x = Q × D, then divide by H. It follows the question "if 125 mg is in 5 mL, how many mL hold 250 mg?" and is easy to check by eye.
Dimensional analysis
Start with the unit you want, then chain fractions so every other unit cancels, top against bottom. Conversions such as 1 g = 1,000 mg, 1 kg = 2.2 lb and 1 hr = 60 min go straight into the chain, so multi-step problems (weight-based doses, mcg/kg/min, drops per minute) fit on one line. The tool strikes through each cancelled unit so you can see why the answer lands in mL.
Weight-based orders add two steps before the formula or ratio method: pounds ÷ 2.2 = kilograms, then dose per kg × kg. When the order is per day, divide by the number of doses.
Rounding rules nursing courses use
Keep full precision until the final answer, then round once. The defaults follow chapter 5 of the open textbook Open RN Nursing Skills, and the practice answers were checked against the answer keys in WisTech Open Nursing Dosage Calculations.
| Answer | Default rule | Example |
|---|---|---|
| Liquid, 1 mL or more | nearest tenth | 17.276 mL → 17.3 mL |
| Liquid, under 1 mL | nearest hundredth | 0.0467 mL → 0.05 mL |
| Scored tablets | nearest half | 2.5 tablets |
| Capsules | whole only | 2 capsules |
| Drops per minute | whole number | 31.25 → 31 gtt/min |
| Pump rate | tenths (switchable) | 12.2727 → 12.3 mL/hr |
Five rounds up, so 1.25 mL becomes 1.3 mL. Some pediatric courses keep hundredths for every liquid, and some round the weight to a tenth of a kg before dosing; both are options. If a tablet answer is not a whole or half, such as 2.25 tablets, the tool shows the exact number and warns you rather than quietly changing the dose.
IV flow rate and infusion formulas
- Pump rate:
mL/hr = volume (mL) ÷ time (hr). - Gravity drip:
gtt/min = volume (mL) × drop factor (gtt/mL) ÷ time (min). Macrodrip tubing is 10, 15 or 20 gtt/mL. Microdrip is 60 gtt/mL, so on microdrip gtt/min equals mL/hr: 75 mL/hr runs at 75 gtt/min. - Infusion time:
hours = volume ÷ mL/hr. Multiply the decimal part by 60 for minutes and add the total to the start time on the 24-hour clock. - Weight-based infusion:
mL/hr = dose (mcg/kg/min) × kg × 60 ÷ strength (mcg/mL), where the strength is the drug in the bag ÷ the bag volume. - Units per hour:
mL/hr = units/hr ÷ units per mL. With 25,000 units in 250 mL (100 units/mL), 1,000 units/hr runs at 10 mL/hr.
Worked examples
Example 1: a weight-based oral liquid
A child weighs 22 lb. The order is 15 mg/kg, and the liquid holds 160 mg in 5 mL.
- Weight: 22 ÷ 2.2 = 10 kg. Dose: 15 × 10 = 150 mg.
- Desired over have: 150 ÷ 160 × 5 = 4.6875 mL.
- Ratio and proportion: 160 mg : 5 mL = 150 mg : x, so 160x = 750 and x = 4.6875 mL.
- Dimensional analysis: (5 mL / 160 mg) × (15 mg / 1 kg) × (1 kg / 2.2 lb) × (22 lb / 1) = 1,650 ÷ 352 = 4.6875 mL.
Rounded to tenths that is 4.7 mL. A course that keeps hundredths for children gets 4.69 mL, the answer in the Open RN worked example.
Example 2: drops per minute and the finish time
Infuse 1,000 mL over 8 hours. On a pump that is 1,000 ÷ 8 = 125 mL/hr. By gravity with 15 gtt/mL tubing, 8 hours is 480 minutes, and 1,000 × 15 ÷ 480 = 31.25, so you count 31 gtt/min. Started at 0800, the bag finishes at 1600.
Example 3: mcg/kg/min to mL/hr, and back
The order is 5 mcg/kg/min for an 80 kg patient, and the bag holds 400 mg in 250 mL. Dose each hour: 5 × 80 × 60 = 24,000 mcg, which is 24 mg. Bag strength: 400 ÷ 250 = 1.6 mg/mL. Rate: 24 ÷ 1.6 = 15 mL/hr. Working backwards, the same bag running at 15 mL/hr for a 72 kg patient delivers 5.5556 mcg/kg/min, or 5.6 rounded.
Alligation and dilution for pharmacy technicians
Alligation alternate answers "how much of each product do I mix?" Draw the grid: the stronger strength top left, the weaker bottom left, the wanted strength in the middle. Subtract along the diagonals. Parts of the stronger = wanted − weaker; parts of the weaker = stronger − wanted.
To make 400 g of 5% ointment from 10% and 2%, the stronger gets 5 − 2 = 3 parts and the weaker gets 10 − 5 = 5 parts, 8 parts in all. Use 400 × 3/8 = 150 g of 10% and 400 × 5/8 = 250 g of 2%. Check: (150 × 10 + 250 × 2) ÷ 400 = 5%. Type 0% for a diluent such as water or a plain base. If you know the amount of one product instead of the total, pick it in the Amount menu.
For a simple dilution use C1 × V1 = C2 × V2. 500 mL of 10% from a 50% stock needs 10 × 500 ÷ 50 = 100 mL of stock plus 400 mL of diluent.
Common mistakes and how to read the answer
- Dividing the wrong way. D goes on top. With 250 mg ordered and 125 mg in 5 mL, H ÷ D × Q gives 2.5 mL instead of 10 mL. The practice tab names this slip when it sees it.
- Skipping a conversion. An order in mcg against a vial in mg is off by a factor of 1,000. Convert first, or let the dimensional analysis chain carry it.
- Minutes and hours. mL/hr needs hours and gtt/min needs minutes. 100 mL over 30 minutes is 200 mL/hr, not 3.3.
- Multiplying pounds by 2.2. Divide pounds by 2.2 to get kilograms.
- Rounding in the middle. In a per-day order split into 3 doses, keep 266.6667 mg until the last step, not 267 mg.
- Trailing zeros. Write 5 mg, never 5.0 mg. A missed decimal point turns 5.0 into 50, a tenfold overdose.
- Missing leading zeros. Write 0.5 mg, never .5 mg, which is easily read as 5 mg.
- Using the diluent volume. In reconstitution, use the strength after mixing (such as 250 mg per mL), not the millilitres of diluent you added.
The large number is the answer rounded by your rule; the grey line shows the exact value and why it was rounded. Yellow notes flag answers worth a second look, such as more than 4 tablets for one dose.
Use these results to learn and to check practice problems. They are not for clinical decisions: follow your facility's protocols and confirm doses with a pharmacist or prescriber.
Embed the Nursing Dosage Calculator on your website
Teachers, bloggers and course sites can add this tool to any web page for free. Paste the code below into an HTML or embed block (WordPress, Google Sites, Canvas, Moodle, Blogger, Wix). It loads the same tool you see above, without our menus or ads. Please keep the credit line under the frame. Sizes, a live preview and step-by-step guides for each platform are on the calculator widget page.
Frequently asked questions
What is the formula for dosage calculation?
The basic formula is desired over have times quantity: D ÷ H × Q. For 250 mg ordered and 125 mg in 5 mL, 250 ÷ 125 × 5 = 10 mL. Convert D and H to the same unit before you divide.
Which method is best: desired over have, ratio and proportion, or dimensional analysis?
Use the one your course grades. Desired over have is fastest for one-step problems, ratio and proportion is easy to check by eye, and dimensional analysis handles weight-based doses and drip rates in one line. They always give the same answer, which the tool shows side by side.
How do you round in nursing dosage calculations?
Round only the final answer. Most courses round liquids to tenths at 1 mL or more and to hundredths under 1 mL, scored tablets to the nearest half, and drops per minute to whole numbers. Five rounds up, so 0.75 mL to tenths is 0.8 mL.
How do you calculate IV drops per minute?
Multiply the volume by the drop factor and divide by the time in minutes. 1,000 mL over 8 hours on 15 gtt/mL tubing is 1,000 × 15 ÷ 480 = 31.25, so 31 gtt/min.
How do I convert mcg/kg/min to mL/hr?
Multiply the dose by the weight in kg and by 60 to get mcg per hour, convert to the unit on the bag, then divide by the bag strength. For 5 mcg/kg/min, 80 kg and 400 mg in 250 mL: 24 mg/hr ÷ 1.6 mg/mL = 15 mL/hr.
Can I use this calculator for real patient doses?
No. It is built for learning and for checking practice and exam problems, and it holds no drug information. In practice, follow your facility's protocols and have doses confirmed by a pharmacist or prescriber.