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15% kVp rule
Raise kVp 15% and receptor exposure about doubles; lower it 15% and exposure about halves.
The rule
Increasing kVp by 15% roughly doubles receptor exposure, the same effect as doubling mAs. Decreasing kVp by 15% roughly halves it. The rule lets you trade kVp against mAs while keeping exposure steady: raise kVp 15% and cut mAs in half, or lower kVp 15% and double mAs. It is an approximation that works best in the middle of the diagnostic range.