Dosage Calculation Calculator
Practice the math of dosage calculation: work out how many tablets or how many mL an order needs with desired ÷ have × quantity, scale a dose by body weight in kg or lb, and convert between mg, g, mcg, mL, tsp and more — with every step and the unit cancellation shown, plus invented practice problems. It is for learning, not for patient care. Free, no sign-up; nothing leaves your browser.
Practice calculator — not for patient care. This page is for learning dosage-calculation math. It has no drug information, checks no safe dose range and knows nothing about any real order or patient. Real doses come from the prescriber's order and a drug reference, with an independent double-check.
For 250 mg per tablet enter 1 tablet; for 25 mg per 5 mL enter 5 mL.
Enter an order and a supply to see the working.
Practice problem 1 of 8
A practice order is 500 mg. The supply is 250 mg per 1 tablet. How much do you give?
All practice orders are invented for the exercise and name no drug.
For learning, not for patient care
This calculator teaches the arithmetic of dosage calculation using numbers you type or invented practice orders. It contains no drug information, checks no safe dose range and knows nothing about any real order or patient. Real doses come from the prescriber's order and a current drug reference, with an independent double-check.
How to Use the Dosage Calculation Calculator
- 1. Choose the kind of problem. Amount to give from an order and a supply, a weight-based dose, or a plain unit conversion.
- 2. Enter the order and the supply with their units. If the units differ within one family — grams against milligrams — the calculator converts first and shows the factor.
- 3. Read the working. Each step is written out with units, and the units that cancel are named, so you can compare it with your own paper working.
- 4. Practice. Try an invented problem on paper, then reveal the working.
Desired Over Have
The commonest nursing-math formula compares the dose ordered (desired, D) with the dose in stock (have, H) and scales the quantity (Q) it comes in: amount to give = D ÷ H × Q. If tablets are 250 mg each and 500 mg is ordered, 500 ÷ 250 × 1 tablet = 2 tablets. If a liquid is 25 mg per 5 mL and 75 mg is ordered, 75 ÷ 25 × 5 mL = 15 mL. The units on D and H must match before you divide, which is why a 0.25 g order against 125 mg tablets is first converted: 0.25 g × 1,000 mg/g = 250 mg, and 250 ÷ 125 × 1 tablet = 2 tablets. Once D and H share a unit, that unit cancels and the answer carries the unit of Q.
Weight-Based Doses
Some orders are written per kilogram of body weight: dose = ordered dose per kg × weight in kg. Body weight in pounds is first divided by 2.2: a child of 22 lb weighs 10 kg. An order of 15 mg/kg per dose is then 15 mg/kg × 10 kg = 150 mg (the kg cancel); on a liquid of 100 mg per 5 mL that is 150 ÷ 100 × 5 mL = 7.5 mL. An order per day is divided by the number of doses a day: 2 mg/kg per day for a 30 kg child is 60 mg a day, or 30 mg for each of two doses. This calculator never checks whether a dose is safe — that needs a drug reference.
Conversion Factors
| Family | Factors the calculator uses |
|---|---|
| Mass | 1 kg = 1,000 g; 1 g = 1,000 mg; 1 mg = 1,000 mcg |
| Body weight | 1 kg = 2.2 lb |
| Volume | 1 L = 1,000 mL; 1 tsp = 5 mL; 1 tbsp = 15 mL; 1 fl oz = 30 mL; 1 cup = 240 mL |
The metric factors are exact. The household ones, and 2.2 lb per kg, are the teaching approximations used in nursing-math texts such as Open RN's Nursing Skills; precise values differ slightly (a fluid ounce is about 29.57 mL), so use whichever your course specifies.
Rounding and Common Slips
Round once, at the end, and by the rule your course gives: tablets are often rounded to the nearest half, liquids to a volume you can measure. The calculator shows the unrounded value in its working and lets you choose how many decimals to round to.
- Dividing without matching units. Convert grams and milligrams, or micrograms and milligrams, first.
- Forgetting to convert pounds to kilograms before multiplying by an order per kg.
- Treating a per-day order as a per-dose order. Divide by the doses per day.
- Dropping the quantity. 25 mg per 5 mL means Q is 5 mL, not 1.
Frequently Asked Questions
- What is the desired over have formula?
- It is amount to give = (desired dose ÷ dose on hand) × quantity. The desired dose is what was ordered, the dose on hand is what each tablet or each measured volume contains, and the quantity is the tablet count or volume that the dose on hand is in. Both doses must be in the same unit first.
- How do you calculate a dose by weight?
- Convert the body weight to kilograms (pounds ÷ 2.2), then multiply by the dose ordered per kilogram. If the order is per day, divide the result by the number of doses a day. The units work out as mg/kg × kg = mg.
- How do I convert mg to g and mcg to mg?
- Multiply or divide by 1,000 each step. To go from grams to milligrams, multiply by 1,000; from milligrams to micrograms, multiply by 1,000; the reverse divides. Writing the units as a fraction, such as 0.25 g × 1,000 mg/g, shows that the grams cancel.
- Why do the units matter so much?
- Because a wrong factor of 10 or 1,000 is the commonest source of dosage errors in coursework, and writing the units and canceling them catches it. If the units do not cancel to leave what you want, a step is wrong.
- How should I round a dosage calculation?
- Keep full precision through the working and round only the final answer, following your course or facility rule — for example whole drops, halves of a tablet, or a measurable number of mL. Check the rule before an exam; this calculator lets you pick the decimals.
- Can I use this calculator for real patients?
- No. It is a practice tool for learning the arithmetic. It has no drug data, does not check safe dose ranges and cannot know the real order, the patient's circumstances or the product supplied. Real doses are calculated and independently checked under your facility's policy.