Dose Per Body Surface Area Calculator
Dose Per Body Surface Area Calculator
Convert a protocol dose in mg/m² to milligrams and back, with body surface area computed by Mosteller — and with the capping question answered the way ASCO answers it: full weight-based dosing regardless of obesity status, on low-quality evidence and a moderate-strength recommendation.
mg/m2 to mg, with and without a BSA cap
Protocol dose to milligramsProtocol dose 60 mg/m2; height 170 cm, weight 125 kg; body surface area capped at 2.0 m2
The arithmetic, and the capping question
and backwards: dose per m² = total dose (mg) ÷ body surface area (m²)
Mosteller: BSA (m²) = √[ height (cm) × weight (kg) ÷ 3600 ]
With a cap, the body surface area used is the lesser of the computed value and the cap
- which weight
- actual body weight. ASCO’s 2021 update: “full weight-based dosing of cytotoxic chemotherapy should be offered regardless of obesity status”. Which weight to use for aminoglycosides, vancomycin or heparin is a separate question with its own disagreements
- the evidence grade, stated rather than hidden
- ASCO’s Recommendation 1 carries “evidence quality: low; strength of recommendation: moderate”. The case against capping is that obese patients given full doses have not shown increased toxicity while under-dosing is associated with inferior outcomes, and ASCO reports up to 40% receiving reduced doses
- capping is a practice, not a guideline
- a 2025 retrospective study of 130 patients above 2.0 m² treated for early breast cancer states that “there are no oncological guidelines that endorse this clinical practice” and that “current high-level evidence does not support routine capping in curative-intent treatment”. It found no effect on efficacy
- the agents this does not apply to
- carboplatin is dosed on kidney function, not on size — use the Calvert carboplatin dose calculator. Vincristine and bleomycin keep established fixed limits of their own, which ASCO’s guideline explicitly retains. Those are drug-specific limits and not obesity adjustments, and this page does not apply them
Worked example
Protocol dose 60 mg/m2; height 170 cm, weight 125 kg; body surface area capped at 2.0 m2
Mosteller: BSA = √(170 × 125 ÷ 3600) = √5.903 = 2.43 m²
Capped at 2.0: dose = 60 × 2.0 = 120.0 mg
Uncapped, full weight-based: 60 × 2.43 = 145.8 mg
The cap withholds 25.8 mg, which is 17.7% of the protocol dose. ASCO's recommendation is the uncapped figure
Cap at 2.2 m² instead and the dose is 60 × 2.2 = 132.0 mg — the same practice, a different number, and neither cap is in any guideline
Backwards: a flat 100 mg over the capped 2.0 m² is 50.0 mg/m² and over the real 2.43 m² is 41.2 mg/m² — one milligram figure, two intensities
For a patient of 165 cm and 60 kg the BSA is 1.66 m², below either cap, so both doses are 99.6 mg. Capping does nothing to most patients, which is why its effect on the few is easy to overlook
What a 2.0 m2 cap withholds, at a protocol dose of 60 mg/m2
| Height and weight | BSA by Mosteller (m2) | Full dose (mg) | Capped at 2.0 (mg) | Withheld |
|---|---|---|---|---|
| 165 cm, 60 kg | 1.66 | 99.5 | 99.5 | Nothing |
| 175 cm, 80 kg | 1.97 | 118.3 | 118.3 | Nothing |
| 175 cm, 95 kg | 2.15 | 128.9 | 120.0 | 8.9 mg, 6.9% |
| 170 cm, 125 kg | 2.43 | 145.8 | 120.0 | 25.8 mg, 17.7% |
| 185 cm, 150 kg | 2.78 | 166.5 | 120.0 | 46.5 mg, 27.9% |
What the published positions on capping actually say
| Source | Position |
|---|---|
| ASCO guideline update, 2021 | “Full weight-based dosing of cytotoxic chemotherapy should be offered regardless of obesity status.” Evidence quality: low. Strength of recommendation: moderate |
| ASCO guideline, 2012 | Actual body weight for choosing cytotoxic doses; full weight-based dosing for morbidly obese patients; dose reduction for side effects applied consistently for all patients. Established limits retained for vincristine, bleomycin and carboplatin |
| Retrospective study, 2025, 130 patients with BSA above 2.0 m² | “There are no oncological guidelines that endorse this clinical practice”; “current high-level evidence does not support routine capping in curative-intent treatment”. No effect of capping on efficacy found; prospective study called for |
| Practice, as reported | Capping at 2.0 m² described as a common clinical practice; 57.7% of that study’s patients had at least half of their treatment capped, and ASCO reports up to 40% of obese patients receiving reduced doses |
One multiplication, and a practice the evidence moved against
Almost every cytotoxic dose is written per square metre of body surface area, so the arithmetic on this page sits under nearly every chemotherapy prescription: multiply the protocol’s mg/m² by the patient’s body surface area. Body surface area itself is a proxy that has been criticised for decades — it correlates only loosely with the clearance of most cytotoxics — but it is what the protocols are written in.
The interesting part is what happens when the answer is large. Capping body surface area at 2.0 m² for obese patients was once routine, and the reasoning was intuitive: area is a proxy for clearance, adipose tissue is relatively poorly perfused, and dosing a very large patient on their full calculated area felt like an extrapolation beyond the trials. The evidence then went the other way. Obese patients given full weight-based doses did not show increased short- or long-term toxicity, myelosuppression was the same or less pronounced than in patients of healthy weight, and retrospective data linked under-dosing to inferior outcomes. ASCO’s guideline, published in 2012 and updated in 2021, recommends full weight-based dosing of cytotoxic chemotherapy regardless of obesity status.
Two things about that recommendation deserve to be said plainly. The first is its grade: evidence quality low, strength of recommendation moderate. This is a guideline recommendation against a widespread practice, not a settled fact, and a 2025 retrospective study of 130 patients with a body surface area above 2.0 m² made the state of play explicit — no oncological guideline endorses capping, current high-level evidence does not support it in curative-intent treatment, and prospective study is still needed. The second is the size of the effect. A cap is not a rounding convention: at 2.0 m², a patient of 2.43 m² loses 17.7% of the protocol dose and one of 2.78 m² loses 27.9%, and ASCO reports up to 40% of obese patients receiving reduced doses. The practice does nothing to most patients and takes a quarter of the dose from the largest. Three drugs sit outside this, and ASCO keeps them outside deliberately: carboplatin is dosed on kidney function rather than size — the Calvert carboplatin dose calculator is the page for that — while vincristine and bleomycin retain established fixed limits of their own. Those are drug-specific ceilings, not obesity adjustments, and this page does not apply them: a protocol’s own limit governs. A figure from a cohort is not this patient’s outcome and a response category is not a diagnosis: a stratum in which 42 per cent were alive at fifteen years tells you about that stratum, not which 42 per cent. This page computes a published quantity and states the criteria behind it. It renders no dose, no prescription and no treatment decision — that is the treating team’s. Every coefficient, conversion factor and threshold here is attributed to the source it was read in and, where it is a prognostic figure, to its derivation cohort; where the treating protocol differs, the protocol takes precedence.
Frequently asked questions
How do I convert mg/m2 to a total dose in mg?
Multiply the protocol dose in mg/m² by the patient’s body surface area in m². For 60 mg/m² in a patient of 2.43 m² that is 145.8 mg. To go the other way, divide the total dose in milligrams by the body surface area — and be clear which area you are dividing by, because a capped and an uncapped area give different intensities for the same milligram figure.
Should body surface area be capped at 2.0 m2 for chemotherapy?
No oncology guideline endorses capping. ASCO recommends full weight-based dosing of cytotoxic chemotherapy regardless of obesity status, with evidence quality graded low and strength of recommendation moderate, and a 2025 study of 130 patients with a body surface area above 2.0 m² reported that current high-level evidence does not support routine capping in curative-intent treatment. Capping is a practice with an evidence position against it, not a rule.
How much dose does a 2.0 m2 cap actually withhold?
It rises steeply with the patient. One of 1.97 m² loses nothing; 2.15 m² loses 6.9%; 2.43 m² loses 17.7%; 2.78 m² loses 27.9%. The practice has no effect on most patients and removes a quarter or more of the intended dose from the largest.
Does this apply to carboplatin?
No. Carboplatin is dosed to a target exposure from kidney function by the Calvert formula and is calculated in milligrams rather than mg/m², which its label states explicitly. ASCO’s obesity guideline keeps it, along with vincristine and bleomycin, among the agents with established dosing approaches of their own rather than recommending full weight-based dosing for it.
Related calculators
References
- Griggs JJ, Bohlke K, Balaban EP, et al. Appropriate systemic therapy dosing for obese adult patients with cancer: ASCO guideline update. J Clin Oncol. 2021;39(18):2037–48.
- ASCO issues new guideline on chemotherapy dosing for obese patients. The ASCO Post, 15 May 2012.
- Influence of capping chemotherapy prescriptions on efficacy and tolerability in medium and high-risk early-stage breast cancer. Sci Rep. 2025;15:14279.
- Mosteller RD. Simplified calculation of body-surface area. N Engl J Med. 1987;317(17):1098.
- Dosing for obesity: new chemotherapy guidelines linked to body weight. OncLive.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/