Radiography technique calculator
Work out mAs, a new technique for a new SID or grid, the inverse square law and the 15 percent rule. Everything updates as you type.
New mAs
32.4 mAs
10 x (180 / 100)² = 10 x 3.24
Technique drill
The exam asks these as short word problems. Practice one fresh question at a time.
Work it out first, as on exam day, then check.
The five formulas
| Rule | Formula | Example |
|---|---|---|
| mAs | mA x exposure time in seconds | 200 mA x 0.05 s = 10 mAs |
| New SID | New mAs = old mAs x (new SID / old SID)² | 10 mAs at 100 cm becomes 32.4 mAs at 180 cm |
| Inverse square law | New intensity = old intensity x (old distance / new distance)² | 8 mGy at 1 m is 2 mGy at 2 m |
| Grid change | New mAs = old mAs x new grid factor / old grid factor | 8 mAs with an 8:1 grid (4) is 2 mAs with no grid (1) |
| 15% rule | kVp up 15 percent with half the mAs keeps receptor exposure about the same | 70 kVp at 20 mAs becomes 80.5 kVp at 10 mAs |
Then the whole exam
The free diagnostic has 15 questions across all eight areas and takes 15 minutes. No account. You get your score in each area and a short lesson on the one to fix first.
Questions people ask
What is the exposure maintenance formula?
New mAs = old mAs x (new SID / old SID)². It keeps receptor exposure the same when the distance changes, because intensity falls with the square of the distance.
How do you use the inverse square law?
Multiply the old intensity by the square of the old distance over the new distance. Twice the distance gives one quarter of the intensity.
What are the grid conversion factors?
The common textbook factors are No grid 1, 5:1 2, 6:1 3, 8:1 4, 12:1 5, 16:1 6. Multiply the old mAs by the new factor and divide by the old one.
What is the 15 percent rule?
Raising kVp by 15 percent roughly doubles receptor exposure, so halving the mAs keeps it about the same. Image contrast is lower at the higher kVp.
Can you use a calculator on the radiography registry exam?
Yes. Basic and scientific calculators are built into the exam, and the test center will give you a four-function calculator if you ask. You may not bring your own.