# PSA calculation skills: doses, rates and units Source: PassthePSA, PSA revision notes URL: https://passthepsa.com/psa-revision/calculations/ Updated: 2026-10-01 Topic: UK Prescribing Safety Assessment (PSA) revision Short answer: PSA calculation items reward a fixed routine: read the units, identify the target (one dose, daily dose, volume or rate), calculate in the preparation's units, convert, then check the magnitude. Learn the ladder (1 g = 1000 mg = 1,000,000 micrograms), that 1% is 10 mg/mL and 1 in 1000 is 1 mg/mL, and that rate equals volume divided by time. ## What is the unit conversion ladder? Each step down the ladder multiplies by 1000 and each step up divides by 1000. Most thousandfold errors come from moving the wrong way, or from skipping a step. Unit | Equals | Example --- | --- | --- 1 g | 1000 mg | 0.5 g = 500 mg 1 mg | 1000 micrograms | 0.25 mg = 250 micrograms 1 microgram | 1000 nanograms | 62.5 micrograms = 62,500 nanograms 1 litre | 1000 mL | 0.25 litre = 250 mL Write micrograms, nanograms and units in full on a prescription. Abbreviations such as mcg, ng and U are easily misread and are discouraged in the BNF. ## How do percentage and ratio strengths convert to mg per mL? A percentage strength means grams per 100 mL, so multiply the percentage by 10 to get mg per mL. Strength | Meaning | Concentration --- | --- | --- 1% | 1 g in 100 mL | 10 mg/mL 2% | 2 g in 100 mL | 20 mg/mL 1 in 1000 | 1 g in 1000 mL | 1 mg/mL 1 in 10 000 | 1 g in 10 000 mL | 0.1 mg/mL (100 micrograms/mL) Units (insulin, heparin) are a measure of biological activity, not mass, and cannot be converted to milligrams. Read the strength on the label, for example units per mL, and work from that. ## Which weight do I use for mg/kg doses? Use the weight given in the question unless the BNF or the item says otherwise. - Adults: actual body weight is the default. Some drugs and local protocols use ideal or adjusted weight in obesity, and many have a maximum dose. Check the BNF. - Children: use current weight, and do not exceed the adult dose. Some doses are by age band or body surface area (mg/m2) instead. - Per dose or per day? A dose per kg per day must be divided by the number of doses before you convert to a volume. - Dose divided by concentration gives volume. ## How do I calculate infusion rates and drops per minute? Rate in mL per hour is volume in mL divided by time in hours. - Drops per minute = volume (mL) x drop factor (drops per mL) / time (minutes). The giving set's drop factor will be stated in the question. - Syringe pumps: concentration is drug per mL, so rate (mL/hour) = prescribed dose per hour / concentration per mL. - Make sure the drug amount and the dose rate are in the same unit before dividing. ## How do I calculate BMI and creatinine clearance? BMI is weight in kg divided by height in metres squared. Cockcroft-Gault creatinine clearance in mL/min = (140 - age in years) x weight in kg x constant, divided by serum creatinine in micromol/L. The constant is 1.23 for men and 1.04 for women. - eGFR is reported by the laboratory in mL/min/1.73 m2 and suits most adult dosing decisions. - The BNF advises creatinine clearance for older people, extremes of body weight and drugs with a narrow margin of safety that are renally cleared. DOAC dosing is based on creatinine clearance. - In obesity, malnutrition or oedema the weight you enter changes the answer. Check the BNF. ## Worked examples All figures below are invented for practice. 1. Adrenaline 1 in 1000, 500 micrograms IM. 1 in 1000 is 1 mg/mL. 500 micrograms = 0.5 mg. Volume = 0.5 mg / 1 mg per mL = 0.5 mL. 2. How many mg are in 5 mL of a 2% solution? 2% = 20 mg/mL. 5 x 20 = 100 mg. 3. A child weighs 12 kg and is prescribed 30 mg/kg/day in 3 divided doses, using a suspension of 250 mg in 5 mL. Daily dose = 12 x 30 = 360 mg. Each dose = 360 / 3 = 120 mg. Strength = 250 / 5 = 50 mg/mL. Volume = 120 / 50 = 2.4 mL. 4. 1000 mL over 8 hours. Rate = 1000 / 8 = 125 mL/hour. With a 20 drops/mL set: 125 x 20 = 2500 drops per hour; 2500 / 60 = 41.7, so about 42 drops per minute. 5. Heparin 25,000 units in 50 mL = 500 units/mL. Prescribed 1000 units/hour: 1000 / 500 = 2 mL/hour. 6. Dilution: 5 mL of 10 mg/mL contains 50 mg. Made up to 50 mL, concentration = 50 / 50 = 1 mg/mL. 7. BMI for 85 kg and 1.70 m. Height squared = 2.89. 85 / 2.89 = 29.4 kg/m2. 8. Creatinine clearance, man aged 70, 80 kg, creatinine 100 micromol/L. (140 - 70) = 70. 70 x 80 = 5600. 5600 x 1.23 = 6888. 6888 / 100 = 68.9, about 69 mL/min. For a woman with the same values: 5600 x 1.04 = 5824; 5824 / 100 = 58 mL/min. Note: Confirm in the BNF: these examples use invented numbers to teach arithmetic. Check every dose, maximum and preparation strength in the BNF, which is available throughout the exam. ## What checking routine catches calculation errors? Check the answer against what a real patient would receive before you type it. 1. Write down the units asked for in the answer box. 2. Confirm the target: one dose, daily dose, volume or rate. 3. Carry full figures through and round once, at the end. 4. Ask whether the answer is plausible: would a nurse draw up that volume, and would that rate empty the bag in a sensible time? 5. If the answer is ten or a hundred times too big or small, recheck the decimal point and the unit ladder. ## Check yourself - How many micrograms are in 0.75 mg? - What concentration in mg/mL is a 0.5% solution? - What does 1 in 10 000 mean in mg/mL? - A daily dose is given in 4 divided doses. What is the single dose? - How do you turn a dose in mg and a strength in mg/mL into a volume? - What is the rate in mL/hour for 500 mL over 5 hours? - Which constant is used for women in Cockcroft-Gault? - Why can units of insulin not be converted to mg? ## Questions people ask Q: Should I write mcg or micrograms? A: Write micrograms in full, and nanograms and units in full too. Abbreviations are easily misread, and a micrograms-to-milligrams slip is a thousandfold error. Q: How do I convert 1 in 1000 to mg per mL? A: 1 in 1000 means 1 g in 1000 mL, which is 1000 mg in 1000 mL, so 1 mg/mL. A 1 in 10 000 solution is 0.1 mg/mL. Q: What is the Cockcroft-Gault formula? A: Creatinine clearance in mL/min = (140 - age) x weight in kg x 1.23 for men or 1.04 for women, divided by serum creatinine in micromol/L. Q: Is eGFR the same as creatinine clearance? A: No. eGFR is standardised to 1.73 m2 body surface area, while Cockcroft-Gault uses the patient's own weight. The BNF indicates when creatinine clearance is preferred. ## Related notes - How to write a safe prescription: https://passthepsa.com/psa-revision/prescribing-safely/ - Renal dosing for the PSA: https://passthepsa.com/psa-revision/renal-dosing/ - Emergency prescribing for the PSA: https://passthepsa.com/psa-revision/emergency-prescribing/ - Drug monitoring for the PSA: https://passthepsa.com/psa-revision/drug-monitoring/ Sources: BNF; BNF for Children; NICE guidance on obesity and BMI. Exam-revision notes, not patient-specific advice; the BNF is the authority. More: https://passthepsa.com/psa-revision/ (all notes), https://passthepsa.com/psa/ (exam guide), https://passthepsa.com/ (practice questions).