The occupational effective dose limit is not a single annual number. It is 20 mSv per year averaged over five consecutive years, that is 100 mSv in any five-year block, with the additional constraint that no single year may exceed 50 mSv. The averaging exists so that a legitimately heavy year — a shutdown, a long pipeline campaign — does not by itself end a radiographer’s career, while the 50 mSv cap stops the whole five-year allowance being taken in one go. Both conditions apply at once, and the tighter of the two is the one that binds.

The consequence for a working radiographer is that the number to watch is not this year’s dose but the five-year running total. A technician at 8 mSv per year is comfortable on any single-year test yet is using 40 % of the five-year allowance every year, and a single 25 mSv shutdown on top of that puts the block over 100 mSv even though no individual year came close to 50. Employers therefore set investigation and dose-constraint levels well below the limits — commonly a few mSv per year — and ALARA obliges the reduction of dose below the limit wherever it is reasonably practicable, not merely up to it.

Effective dose is a whole-body quantity. Separate equivalent-dose limits apply to the lens of the eye and to skin and extremities, and they are not covered by the whole-body total: a radiographer handling a source guide tube can reach an extremity limit long before the whole-body figure looks interesting. Where source handling or hands-in-beam work is routine, extremity dosemeters are the only way to know.

This calculation is a planning and cross-check tool. The dose of record is the figure issued by the approved dosimetry service from the technician’s personal dosemeter, and it is that figure that goes on the statutory record. Use this to see where the five-year block stands before accepting the next job, not to replace the dose report.

Worked example

Dose, year 1 of the block	4.2 mSv
Dose, year 2	6.8 mSv
Dose, year 3	5.1 mSv
Dose, year 4	7.4 mSv
Dose, current year to date	3.5 mSv
Annual average limit	20 mSv
Single-year limit	50 mSv
Expected dose per remaining month	0.5 mSv
Months left in the current year	4
Five-year total	27 mSv
Five-year average	5.4 mSv
Highest single year	7.4 mSv
Five-year allowance	100 mSv
Remaining in the five-year block	73 mSv
Remaining under the single-year limit	46.5 mSv
Dose headroom available now	46.5 mSv
Five-year allowance used	27 %
Projected dose this year	5.5 mSv
Projected five-year total	29 mSv
Lens of the eye limit, annual	20 mSv
Skin and extremity limit, annual	500 mSv

Five-year total 4.2 + 6.8 + 5.1 + 7.4 + 3.5 = 27.0 mSv, average 27.0/5 = 5.40 mSv/y. Allowance 5 × 20 = 100 mSv, so 100 − 27.0 = 73.0 mSv remains in the block. The single-year constraint gives 50 − 3.5 = 46.5 mSv, which is the tighter of the two, so headroom is 46.5 mSv. Allowance used 27.0/100 = 27.0 %. With four months left at 0.5 mSv/month the year projects to 3.5 + 2.0 = 5.50 mSv and the block to 27.0 + 2.0 = 29.0 mSv.

Use at your own risk — verify before you act

These calculators support, and never replace, the judgement of qualified NDT and engineering personnel. Results are provided as is, without warranty of any kind, express or implied, and must be independently verified against the governing code edition named in your contract before being used in any inspection, acceptance, rejection, radiation-safety or fitness-for-service decision. By using them you accept full responsibility for how the results are applied; NDT Inspect, its owners and contributors accept no liability for any loss, damage, injury or death arising from their use or from reliance on them. If a result matters to safety, check it by hand and have it reviewed by a competent person.