X-Ray Technique (mAs, kVp & Distance)
Adjust X-ray mAs for a change in distance or kVp using the inverse square law and the 15% rule, free online.
About this calculator
Exposure varies with the inverse square of distance, so mAs₂ = mAs₁ × (d₂ ÷ d₁)². The 15% rule says raising kVp by 15% doubles the exposure (so mAs can be halved), and lowering kVp by 15% halves it. Both are combined here; this is a teaching aid, not a clinical protocol.