Radiation Dose Calculator

This calculator estimates external gamma radiation exposure using two well-established physics relationships. First, the inverse square law: dose rate at a distance = (source activity × isotope-specific gamma constant) / distance², so doubling your distance from a source cuts the dose rate to one-quarter, not one-half — a fact people frequently underestimate. The gamma constant (Γ) varies significantly by isotope (the tool notes Cs-137 ≈ 85, Co-60 ≈ 305, Ir-192 ≈ 130 µSv·m²/(GBq·hr)), so picking the right constant for your actual source matters far more than most other inputs.

Second, shielding is modeled through half-value layers (HVLs): each HVL of shielding material you enter multiplies the dose rate by exactly 0.5, following true exponential attenuation, so 3 HVLs of any material reduces dose to 12.5% regardless of what the material actually is (the calculator doesn't distinguish lead from concrete — you must convert your shielding thickness to HVLs for the specific material and photon energy elsewhere first). From the shielded dose rate, it multiplies by your daily exposure hours and workdays per year to project an annual dose in millisieverts, then expresses that as a percentage of both the OSHA occupational limit under 29 CFR 1910.1096 (a whole-body limit of 1.25 rem per calendar quarter — 5 rem, or 50 mSv, per year under the standard limit) and the more conservative ICRP recommendation from ICRP Publication 103 (20 mSv/year averaged over defined five-year periods, with no single year exceeding 50 mSv) — flagging that meeting the legal OSHA limit does not necessarily satisfy modern ALARA (As Low As Reasonably Achievable) practice. It also back-solves the inverse square equation for two reference boundaries: the distance at which dose rate falls to 2 µSv/hr (a common controlled-area threshold) and to 0.25 µSv/hr (a stricter ALARA/unrestricted-area target), both before any additional shielding beyond what you've specified.