Weight-based and surface-area dosing rarely lands on a manufactured tablet or vial strength. A calculation might say 47 mg when the formulary stocks 25 mg and 50 mg. This tool rounds the calculated value to a practical strength and tells you how far that rounding moved the dose, so you can confirm it stays within an acceptable margin.
How it works
You enter the target dose and the strengths your pharmacy stocks. The calculator then either selects the single nearest strength or snaps the dose to the nearest whole multiple of a chosen unit:
nearest = strength from list with smallest |strength - target|
multiples = round(target / unit) x unit
deviation% = (rounded - target) / target x 100The deviation is compared against the tolerance you set. If the absolute deviation is within tolerance the result is flagged as acceptable; otherwise it is flagged for review.
Tips and notes
Use multiples mode when a dose can be built from several identical tablets, and nearest-strength mode when only discrete vials or fixed-strength products are available. Tighten the tolerance for narrow-therapeutic-index medicines, and remember that modified-release or enteric-coated products generally must not be split to reach an exact dose. The arithmetic here supports your decision; it does not replace the clinical check.
Worked example
A child weighing 23 kg needs a weight-based dose of 2 mg/kg. The calculated target dose is 46 mg. Your formulary stocks 25 mg, 50 mg, and 100 mg tablets.
- Nearest strength mode: the closest single tablet is 50 mg.
- Deviation: (50 − 46) / 46 × 100 = +8.7%
- With a 10% tolerance, this is within range and flagged acceptable.
Alternatively in multiples mode with a 25 mg unit:
- Nearest whole multiple: round(46 / 25) × 25 = 2 × 25 = 50 mg.
- Same result, same deviation.
If the formulary instead stocked only 25 mg:
- Nearest whole multiple: 2 × 25 mg = 50 mg (same conclusion).
- Or the pharmacist could consider 1.5 tablets at 25 mg = 37.5 mg (deviation −18.5%), which falls outside a 10% tolerance and would flag for clinical review.
Tolerance selection in clinical practice
The right tolerance depends on the drug’s therapeutic index and the clinical context. General guidance (not a substitute for individual clinical judgement):
| Drug category | Typical rounding tolerance |
|---|---|
| Most oral medicines with a wide therapeutic index | 10% |
| Anticoagulants (warfarin, heparin), immunosuppressants | 5% or tighter |
| Anticonvulsants (phenytoin, valproate) — narrow therapeutic index | 5% or tighter |
| Oncology (cytotoxic agents) | Follows protocol-specific rules |
| Paediatric doses where small absolute deviations matter more | Consider absolute mg, not just percentage |
For narrow-therapeutic-index drugs, note that the percentage deviation alone can be misleading — a 10% deviation on a small paediatric dose is a very small absolute milligram change and may be clinically irrelevant, while the same percentage on a large adult dose could push the patient outside a safe range. Always apply clinical context.
What this calculator does not cover
This tool performs rounding arithmetic and deviation checking only. It does not verify whether the rounded dose is within the licensed dose range for the indication, check against renal or hepatic dose adjustments, account for formulation-specific restrictions (such as modified-release products that must not be crushed or split), or replace pharmacist clinical verification. Use it as one step in a larger checking process, not as a standalone decision tool.