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Dosage Calculator
Table of Contents
**Updates**
We updated this page on 19th of November 2025, added interactive Dosage Calculator and explainer on 'need over have'. Added a detailed explanation of how the calculator works, and added related questions to the FAQs section.
What is the 'need over have' dose calculation formula and how does it work?
Volume Required = (SR × VS) ÷ SS, or 'need over have' is a universal medication dose formula.
Works for tablets, capsules, liquids, injections, and reconstituted drugs.
When used correctly it helps prevent common calculation errors.
Volume Required = (SR × VS) ÷ SS, or (SR × VS) the 'what you need part', over SS 'what you have'. The formula gives you the exact amount of medication to administer:
- SR (Strength Required) - the dose prescribed.
- SS (Stock Strength) - the strength of the medication you have.
- VS (Volume of Stock) - the amount the drug comes in.
VS (Volume of Stock) - For tablets/capsules: this is always 1, for liquids/injections: this is the mL amount supplied.
- Example (tablet) - this is always 1:
- Order = 250 mg
- Stock = 500 mg tablet
- VS = 1
- Volume Required = (SR × VS) ÷ SS
- (250 × 1) ÷ 500 = 0.5 tablets
- Example (liquid)- this is the mL amount supplied:
- Order = 25 mg
- Stock = 50 mg/mL
- VS = 1 mL
- Volume Required = (SR × VS) ÷ SS
- (25 × 1) ÷ 50 = 0.5 mL
Let's get to the heart of the formula so we inderstand it instead of just memorising it.
- What you need (SR × VS) is a combination of two numbers, here is why:
- Rx: Give 250mg of Amoxicillin. In stock you have Amoxicillin 250mg/5mL.
- SR (Strength Required) - the dose you need to give ie "Give 250mg of Amoxicillin".
- VS (Volume of Stock) - the volume of the 'what you have' medicine comes in, in this example it's "per 5mL".
- By multiplying SR × VS you are working out 'total drug amount' that your dose requires based on the container you have.
- What you have (SS) is the concentration of the medicine sitting on the shelf:
- SS (Stock Strength) - carrying on with our example, it will be "250mg", as Amoxicillin we have comes as 250mg/5mL.
- In this example where Amoxicillin 250mg is ordered we would give:
- (Strength Required x Volume of Stock) ÷ Stock Strength = Volume Required
- (250mg x 5mL) ÷ 250mg = 1mL
SR - Dose ordered (What you need).
SS - Strength on hand (What you have).
VS - Volume of what you have is supplied in (1 for tablets/capsules). ie 1ml for 25mg/ml vial.
Volume Required - The amount you should give.
How to use the dosage calculator
- Choose your calculation mode by selecting one of the 2 modes
- Need/have - what you need over what you have.
- mg/kg - calculate dose based on weight.
- Enter your numbers. Each field clearly tells you what to enter.
- Click "Calculate" to get the final result. The calculator will give the formula used and breakdown of the calculation steps.
- If you need to redo, click "Reset" to start fresh.
**All information is private, no data is saved or shared.**
Dosage Calculator
Disclaimer
The online clinical calculators provided on Aidstat.com are intended for informational purposes only. They are not a substitute for clinical reasoning, professional judgement, or expert advice from qualified healthcare practitioners.
While extensive effort has been made to ensure the accuracy and completeness of the information provided by these calculators, Aidstat.com cannot guarantee its accuracy or reliability. Users are advised to independently verify any results obtained through the use of these tools before making any medical or healthcare decisions.
Drug Dosage Calculator
Calculate drug doses using Need/Have or mg/kg dosing.
FAQs
Q1. Does need/have work for tablets?
- Yes. Set volume of stock (VS) to 1.
Q2. Does this calculator work for injections?
- Yes. Use mg to mL or Injection Volume mode.
References
1. Intravenous Drug and Fluid Administration Training Calculation Practice Formulae and Questions in Preparation for On Line Calculation Test. (n.d.). Available at: https://www.gloshospitals.nhs.uk/media/documents/IV_Drug_Calculation_Test_practice_papers.pdf [Accessed 19 Nov. 2025]. Link
2. Toney-Butler, T.J. and Wilcox, L. (2019). Dose Calculation (Desired Over Have or Formula). [online] Nih.gov. Available at: https://www.ncbi.nlm.nih.gov/books/NBK493162/ [Accessed 19 Nov. 2025]. Link
3. www.medicines.org.uk. (n.d.). Amoxicillin Sugar Free 250 mg/ 5 ml Powder for Oral Suspension - Summary of Product Characteristics (SmPC) - (emc). [online] Available at: https://www.medicines.org.uk/emc/product/10891/smpc#gref [Accessed 19 Nov. 2025]. Link