BSA-Based Dosage Calculator
Compute a body-surface-area-based medication dose using the DuBois BSA formula multiplied by the per-m² dose. Standard approach for chemotherapy and a few other classes of drugs whose pharmacokinetics scale with BSA.
Last updated: May 2026
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About this calculator
The formula is Total Dose (mg) = BSA × dose_per_m², where BSA is computed via DuBois & DuBois (1916): BSA (m²) = 0.007184 × weight^0.425 × height^0.725, with weight in kg and height in cm. The two non-integer exponents reflect the non-linear scaling of body surface with linear dimensions. DuBois is the historical reference for chemotherapy dosing, though Mosteller (BSA = √(weight × height / 3600)) is now more common because it's mathematically simpler and agrees with DuBois within 2–5% for typical adult builds. The dose_per_m² input is the prescribed mg/m², set by the drug's clinical protocol (common chemotherapy doses range from 25 mg/m² for some agents to 1000+ mg/m² for others). Edge cases: BSA-based dosing is most appropriate for cytotoxic chemotherapy, where it improves dose consistency across patients with different body sizes and reduces over-dosing in small patients and under-dosing in large ones. For obese patients (BMI > 30), many oncology protocols cap or adjust body weight before computing BSA to avoid excessive doses — practices vary by institution and drug. BSA-dosing has been largely abandoned for many newer targeted therapies and monoclonal antibodies, which use fixed flat dosing because their pharmacokinetics are less BSA-dependent. The formula assumes the patient is in steady-state and not significantly edematous or amputated — significant deviations from typical body composition affect both the BSA estimate and the resulting dose.
How to use
Example 1 — typical adult chemotherapy dosing. Weight 70 kg, height 175 cm, dose_per_m² 100 mg/m² (a common moderate chemotherapy dose). Step 1: weight^0.425 = 70^0.425. Using ln(70) × 0.425 ≈ 1.804 then e^1.804 ≈ 6.078. Step 2: height^0.725 = 175^0.725. ln(175) × 0.725 ≈ 3.745 then e^3.745 ≈ 42.30. Step 3: BSA = 0.007184 × 6.078 × 42.30 ≈ 1.847 m². Step 4: total dose = 1.847 × 100 = 184.7 mg. Verify: average adult BSA falls between 1.7 and 1.9 m², and a 70 kg / 175 cm patient typically lands near 1.85 m² ✓; at 100 mg/m² the total dose of ~185 mg is consistent with standard practice and would be rounded to 185 mg or 180 mg per protocol. Example 2 — smaller patient, higher per-m² dose. Weight 50 kg, height 160 cm, dose_per_m² 250 mg/m². Step 1: 50^0.425 ≈ 5.18. Step 2: 160^0.725 ≈ 39.81. Step 3: BSA = 0.007184 × 5.18 × 39.81 ≈ 1.481 m². Step 4: total dose = 1.481 × 250 ≈ 370.3 mg. Verify: smaller patients have smaller BSA (1.48 m² for this 50 kg / 160 cm patient) and proportionally smaller absolute doses; at 250 mg/m² they receive ~370 mg total versus the larger patient's ~462 mg at the same per-m² dose — a 20% smaller dose, matching the ~20% smaller BSA ✓. This is exactly the point of BSA-dosing: it scales the dose to body size rather than giving every patient the same flat amount.
Frequently asked questions
Why use body surface area instead of body weight for chemotherapy dosing?
BSA correlates more closely with blood volume, organ size, and the renal and hepatic clearance pathways most cytotoxic drugs use than simple body weight does, helping maintain consistent drug plasma concentrations across patients of very different sizes. Using BSA reduces the risk of toxic overdose in smaller patients and sub-therapeutic dosing in larger ones, both of which can be life-threatening with narrow-therapeutic-index agents like methotrexate, doxorubicin, or cisplatin. The approach dates from work by Pinkel in the late 1950s and remains standard for most traditional chemotherapeutics. However, the BSA approach is being supplanted by fixed flat dosing for many newer targeted therapies and monoclonal antibodies because their pharmacokinetics are less BSA-dependent — pembrolizumab, nivolumab, and similar agents are now dosed at fixed mg amounts regardless of patient size. Always check the drug's specific dosing strategy: traditional cytotoxics use BSA; many newer agents do not.
What's the difference between DuBois, Mosteller, and Haycock BSA formulas?
DuBois & DuBois (1916) is the historical reference: BSA = 0.007184 × weight^0.425 × height^0.725, derived from just 9 subjects but widely validated since. Mosteller (1987) is the most popular modern alternative: BSA = √(weight × height / 3600), much simpler to compute mentally and agreeing with DuBois within 2–5% for adults of typical build. Haycock (1978) was derived in pediatric populations and is preferred under about 10 kg: BSA = 0.024265 × weight^0.5378 × height^0.3964. For most adults, the three agree within 5%, which is clinically acceptable. DuBois tends to underestimate BSA in obese patients by 5–8% compared to direct measurement (3D body scanning), while Mosteller is more accurate in obese populations. For chemotherapy, use whichever formula your hospital's protocol specifies and stick with it consistently across cycles — switching between formulas mid-treatment can cause small dose variations that affect drug exposure and toxicity tracking.
How should BSA be calculated for obese patients?
Standard formulas use actual body weight, which over-estimates BSA in obese patients because fat tissue contributes to body surface differently than lean tissue. For oncology dosing, practices vary: some institutions use actual body weight regardless (simplest, may overdose); some cap BSA at 2.0 or 2.2 m² (avoids extreme doses but may underdose true clinical need); some use Adjusted Body Weight = IBW + 0.4 × (actual − IBW) before computing BSA; others use ideal body weight directly. The ASCO 2012 guideline (updated 2021) recommends using full actual body weight for most chemotherapy in obese patients, with selective exceptions for drugs with high toxicity profiles or established alternative practice. The American Society of Clinical Oncology guideline shifted thinking toward not capping BSA because retrospective data showed that capped dosing in obese patients was associated with worse outcomes without reducing toxicity. Always defer to your institution's protocol — variability in practice is large, and changing approaches mid-treatment risks both under- and overdosing.
What are the common mistakes when using BSA-based dosing?
The biggest mistake is mixing units — DuBois requires weight in kg and height in cm; using pounds or inches without conversion gives wildly wrong BSA. The second is using BSA-dosing for drugs that don't use it — many newer targeted therapies, monoclonal antibodies, and PD-1 inhibitors use fixed flat dosing, and applying a BSA formula to those produces inappropriate doses. The third is switching BSA formulas mid-treatment, producing small but trackable dose drift across cycles. People also forget the role of cap or adjusted weight in obese patients, where uncritical use of actual weight can produce dangerous doses. For chemotherapy with significant toxicity (e.g., methotrexate, doxorubicin), failing to recompute BSA after significant weight change (>5–10% from baseline) means subsequent doses are calibrated for a patient who no longer exists. Finally, BSA-dosing is only as accurate as the formula and the height/weight measurements — typing 165 instead of 175 cm changes BSA by 5% and shifts dose by 5%, which over multiple cycles affects cumulative exposure.
When should I not use this calculator?
Do not use it for drugs that use fixed flat dosing (most modern monoclonal antibodies: pembrolizumab, nivolumab, durvalumab, daratumumab; many immune-checkpoint inhibitors) — applying a BSA formula gives an inappropriate dose. It is not appropriate for pediatric patients under 10 kg, where Haycock is preferred over DuBois. Do not use it for AKI, edema, ascites, or significantly amputated patients where measured weight does not reflect true body composition; the resulting BSA will be wrong and the dose dangerous. It is unsuitable for non-cancer applications where mg/kg dosing is standard (most antibiotics, anticoagulants, antihypertensives) — use weight-based formulas instead. For obese patients, follow your institution's protocol on capping or adjusting body weight before computing BSA; uncritical use of actual weight can produce excessive doses. Finally, this calculator does not include drug-specific adjustments for renal or hepatic function, drug-drug interactions, or genetic factors (CYP variants, DPD deficiency) that affect real clinical dosing; always cross-check against the drug's specific dosing reference and clinical pharmacy.