An order written in mg/m² gives a medication amount for each square metre of body surface area. To calculate the BSA-based amount in milligrams, use calculated amount (mg) = ordered amount (mg/m²) × BSA (m²). There is no universal mg result from an mg/m² value without the patient's BSA.
The short version
Calculated amount (mg) = ordered amount (mg/m²) × BSA (m²)
This is also the answer to “how do you convert mg/m² to mg?”: multiply by the verified BSA. The m² units cancel, leaving mg.
What does mg/m² actually mean?
If an order says 100 mg/m², read it as: 100 milligrams for every square metre of the patient's calculated body surface area.
It does not automatically mean 100 mg total. The final milligram dose depends on the patient's BSA.
BSA: 1.0 m²
Fictional order: 100 mg/m²
Calculated dose: 100 mg
BSA: 1.8 m²
Fictional order: 100 mg/m²
Calculated dose: 180 mg
There are two separate calculations
Students often try to put height, weight and the medication order into one big equation. It is much easier to understand if you keep the jobs separate.
OpenStax teaches BSA dosing in the same sequence: calculate the patient's BSA, multiply that patient size by the ordered dose, then work out the amount to administer if another conversion is needed.
Need the patient's BSA first? Use the BSA Calculator →Why does multiplying m² by mg/m² give mg?
The units show you what is happening:
Written as units, that is m² × mg/m². The m² appears once on top and once on the bottom, so it cancels. The unit left behind is mg.
Worked example: mg/m² to mg
Patient BSA: 1.62 m²
Fictional order: 80 mg/m²
Multiply the BSA by the ordered dose:
If the question asks only for the patient's calculated dose, the result is 129.6 mg before applying any supplied rounding rule, dose cap or protocol modification.
Need to round the final result? Review medication-calculation rounding →Do not use height and weight directly in the medication-dose step
Height and weight are used to estimate BSA. Once you have the BSA result, the medication-dose step uses that m² value.
Keeping those two stages separate makes it much easier to spot where an error has happened.
Use current measurements and the current calculation source
Keep the calculation in one source state. Use the verified BSA, or the verified current height and weight plus the BSA method required for the active treatment calculation. Do not combine a current order with a previous-cycle BSA, stale weight, conflicting height/weight, or a BSA produced by a different method unless the active source explicitly tells you to.
Current eviQ anti-cancer administration guidance includes height and weight in baseline assessment, ongoing weight assessment, BSA recalculation where required, and checking BSA using the same calculation method against the treatment protocol.
Do not automatically cap BSA or substitute another weight because the patient is large
A large BSA or obesity is not, by itself, an instruction to cap BSA, replace measured/current weight with ideal or adjusted body weight, or reduce the calculated amount. Any BSA cap, alternate weight descriptor, dose reduction or other adjustment must come from the active medicine, regimen, protocol or another authorised source.
For obese adults receiving cytotoxic chemotherapy, current ASCO guidance recommends full weight-based dosing regardless of obesity status rather than empirical reduction based on obesity alone. That recommendation does not replace medicine- or regimen-specific dose rules.
Which BSA formula should you use?
BSA is estimated from height and weight rather than directly measured. Different published equations can produce slightly different BSA results. Current eviQ resources include both Mosteller and Du Bois & Du Bois formulas, and eviQ education material stresses that the method should be standardised according to the applicable clinical practice or protocol.
If the treatment protocol, medication reference or local policy specifies a BSA formula, follow that source rather than switching formulas because another one produces a preferred result.
Learn why BSA formulas differ and how to choose between them →mg/m² is not the same as mg/kg
Both methods adjust a dose for body size, but they use different measurements.
| Dose format | What it is based on |
|---|---|
| mg/kg | Body weight in kilograms. |
| mg/m² | Calculated body surface area in square metres. |
Do not swap mg/kg and mg/m² unless an authoritative medicine reference explicitly provides the required method. They are not interchangeable units.
Need the mg/kg pathway instead? Review weight-based medication dosing →Why is BSA used for some medicines?
BSA-based dosing has long been used for many anticancer medicines and is also taught in some paediatric and other specialised medication calculations. The aim is to individualise the starting dose using an estimate of body size rather than giving every patient the same fixed amount.
That does not mean BSA is the correct dosing method for every medicine. A protocol may instead use a fixed dose, mg/kg, renal-function adjustment, AUC-based dosing or another method.
eviQ's current anti-cancer administration guidance requires doses to be checked against the dosing method required by the treatment protocol, including BSA, AUC or mg/kg where applicable.
Once you have the mg dose, you may still have another calculation
A BSA calculation usually gives you the required medication amount. It does not automatically tell you the volume or number of tablets to administer.
BSA: 1.60 m²
Fictional order: 75 mg/m²
Calculated patient dose: 1.60 × 75 = 120 mg
At this point, 120 mg is the medication amount required. If the medicine is supplied as a liquid, you still need the concentration before you can work out mL.
mg/m²/dose and mg/m²/day are different instructions
Read what comes after the m². 50 mg/m²/dose refers to each individual dose. 50 mg/m²/day refers to the total amount across the day.
If a daily amount must be divided into several doses, that instruction needs to come from the active protocol, medicine reference or order. Keep per dose, per day, per cycle, frequency and cycle-day wording separate. Do not multiply or divide by a frequency you have invented.
Review how to spot per-dose, per-day and multi-step wording →A BSA cap and a final dose maximum are different operations
A BSA cap changes the m² value used before the medication multiplication. A final dose maximum limits the calculated medication amount after the core mg/m² × m² step. They are not interchangeable.
Some protocols contain one type of limit, some contain another, and many contain neither. Never transfer a cap from another medicine, regimen, protocol or previous cycle.
Arithmetic precision is not the same as clinical rounding policy
In MedMaths worked exercises, keeping the unrounded BSA through the downstream multiplication is used to prevent premature-rounding error unless the exercise states another rule. That is an education convention, not a universal oncology rounding policy.
In clinical care, use the calculation and rounding convention required by the active protocol, product information, approved prescribing/calculation system or local policy. Do not silently replace a source-specified convention with the MedMaths practice rule.
A calculated amount does not by itself authorise administration
The equation establishes the calculated BSA-based amount from the supplied values. It does not independently confirm that the regimen is appropriate, required investigations are complete, patient-specific modifications have been applied, or the medicine should be administered.
The 2024 ASCO-ONS antineoplastic therapy administration safety standards place dose calculation inside a broader system of ordering, preparation, verification, monitoring and administration safeguards.
BSA is useful, but it is still an estimate
BSA-based dosing standardises some medicines according to body size, but body size does not explain every difference in how a medicine is handled by an individual patient. Reviews of anticancer dosing have found that BSA alone does not reliably remove interpatient pharmacokinetic variability for many drugs.
Depending on the medicine and protocol, other factors such as kidney function, liver function, age, previous toxicity, treatment history or protocol-specific dose modifications may also matter.
The practical lesson is simple: use BSA arithmetic to follow a BSA-based order or protocol, not as an independent prescribing rule.
Full worked chain: BSA → mg → mL
Patient BSA: 1.84 m²
Fictional order: 60 mg/m²
Fictional concentration: 40 mg/mL
Step 1 — Calculate the patient's medication dose:
Step 2 — Use the concentration to find the volume:
If the exercise then tells you to round to the nearest tenth, the final volume would be 2.8 mL. The important sequence is:
Common mistakes
Multiply the ordered mg/m² by the patient's BSA.
mg/m² requires an m² value, not kilograms.
They are different dosing methods.
You need concentration to connect medication amount to liquid volume.
Read the full dose unit before calculating.
Use a cap only when the relevant protocol or reference supplies it.
Keep the working precision unless the exercise states otherwise; clinical rounding follows the active protocol/system/policy.
Keep the current instruction, BSA or current height/weight, and required method in the same calculation state.
Use those changes only when the active source specifically requires them.
Remember
1. Calculate BSA in m².
2. Multiply BSA × ordered mg/m².
3. The m² cancels, leaving the patient dose in mg.
4. If you need tablets or mL, that is another calculation.
BSA tells you the patient's calculated size. mg/m² tells you how much medicine is ordered for each m².
Where to go next
Sources and references
- DailyMed. Caspofungin acetate for injection — prescribing information. Current label example specifying BSA-based pediatric dosing, Mosteller provenance and medicine-specific limits. DailyMed.
- ASCO. Appropriate Systemic Therapy Dosing for Obese Adult Patients With Cancer: Guideline Update. J Clin Oncol. 2021;39:2037-2048. ASCO.
- Siegel RD, LeFebvre KB, Temin S, et al. Antineoplastic Therapy Administration Safety Standards for Adult and Pediatric Oncology: ASCO-ONS Standards. JCO Oncol Pract. 2024;20:1314-1330. ASCO-ONS.
- Mosteller RD. Simplified calculation of body-surface area. N Engl J Med. 1987;317(17):1098. PubMed.
- Cancer Institute NSW, eviQ. Body Surface Area Calculator. eviQ.
- Cancer Institute NSW, eviQ. Safe administration of anti-cancer drugs. eviQ.
- OpenStax. Pharmacology for Nurses — 2.4 Dosage Calculations. OpenStax.
- Kaestner SA, Sewell GJ. Chemotherapy dosing part I: scientific basis for current practice and use of body surface area. Clin Oncol (R Coll Radiol). 2007;19(1):23-37. PubMed.