The 15% Rule in Radiography, Explained

Raising kVp by 15% increases exposure to the image receptor by about as much as doubling the mAs. Dropping kVp by 15% cuts it by about as much as halving the mAs. The arithmetic is just multiplication: ×1.15 to go up, ×0.85 to come down.

That's the whole rule, and most explanations stop there. The part they leave out is the part the registry actually leans on: how many kVp a 15% change works out to depends on where you already are. Fifteen percent of 60 is nine. Fifteen percent of 100 is fifteen. Those are very different adjustments, and treating the rule as one clean multiplier is where students get burned on the harder questions.

The arithmetic, worked

Start at 75 kVp.

• Up: 75 × 1.15 = 86 kVp. Moving from 75 to 86 does roughly the same thing to receptor exposure as leaving kVp alone and doubling your mAs.
• Down: 75 × 0.85 = 64 kVp. Moving from 75 to 64 does roughly what halving the mAs would do.

Two things about that pair are worth carrying into the exam.

First, up and down are not perfect mirrors. 1.15 × 0.85 = 0.98, not 1.00 — so a 15% increase followed by a 15% decrease doesn't land you exactly where you started. It's close enough to ignore clinically and precise enough that you shouldn't be surprised when the numbers don't reverse cleanly.

Second, the rule is most often tested as a paired move rather than a single change.

The compensation version (the one questions are built on)

The classic application: increase kVp by 15% and cut mAs in half. Receptor exposure comes out about where it started, and three other things change with it:

• Patient dose goes down. Fewer photons are being made, and the ones that are made penetrate better.
• Contrast goes down — a longer scale of grays, because higher-energy photons produce proportionally more scatter and less differential absorption.
• Penetration goes up, which is the point when you're dealing with a thicker part or a cast.

That dose result is worth pausing on, because it looks like a contradiction. On a factor-by-factor chart, "kVp up" sits next to "patient dose up" — and that's correct, because such a chart holds everything else constant. The 15% rule does not hold everything else constant: it halves the mAs in the same breath. That pairing is the one situation where reaching for more kVp lowers the dose you give the patient. If a question mentions raising kVp without mentioning mAs, you're on the chart. If it says "while maintaining receptor exposure," you're in the rule.

The caveat most guides skip: it's range-dependent

Both primary sources say plainly that the required change varies with your working range. Bontrager quantifies it in absolute kVp:

Fauber makes the same point from the other direction, and makes it twice: a beam already sitting at 90 kVp or above needs a larger jump than one down below 70 to get the same effect.

These two framings agree with each other, which is the reassuring part. Fifteen percent of 60 is 9 kVp — inside Bontrager's 8–10 band. Fifteen percent of 80 is 12 kVp — inside the 12–15 band. The percentage and the absolute anchors are describing the same physics; the percentage version just hides the range-dependence inside the multiplication.

Where this actually matters: untidy kVp values. The registry likes numbers such as 88, 90, and 96, where mental math on 1.15 is unpleasant and imprecise. Don't force the percentage. Ask instead how many of that range's doubling steps fit inside the gap:

Resist the urge to build yourself a tidier estimator than that ("5% chunks are about ×1.25 each," or similar). Those shortcuts land close numerically while teaching a uniformity the sources explicitly deny, and they fall apart exactly where the hard questions live — at the top and bottom of the range.

The trap: the 15% rule is not kVp²

This is the single most common mix-up on this topic, and the two rules genuinely do point in different directions.

• kVp² describes x-ray quantity leaving the tube, and it tracks patient dose. Doubling the beam intensity coming out of the tube by kVp alone takes roughly a 40% increase, not 15%.
• The 15% rule describes exposure reaching the receptor, after the patient has absorbed what it's going to absorb. Fifteen percent is enough there precisely because the higher-energy photons penetrate the patient better — more of what you made arrives at the detector.

Both get tested. The tell is what the stem is asking about: the tube and the patient, or the image. "Which change doubles the intensity of the beam?" is a kVp² question. "Which change maintains receptor exposure?" is a 15% rule question.

One more wrinkle: the books word it differently

Bontrager still frames the rule in terms of film density, while Fauber and Bushong describe exposure to the image receptor. They're describing the same phenomenon in two eras of vocabulary. If a classmate or an older review book says "15% more kVp doubles the density," they aren't wrong — they're just using the film-era wording. Answer the question in whatever terms the stem uses.

What to actually remember

• ×1.15 up, ×0.85 down. 75 → 86, 75 → 64.
• The paired move: +15% kVp with mAs halved keeps receptor exposure about the same, lowers patient dose, and lengthens the contrast scale.
• Low range needs less, high range needs more: 8–10 kVp down at 50–70, 12–15 kVp up at 80–100.
• For ugly numbers, count doubling steps in the relevant band instead of doing percentages in your head.
• kVp² is the tube and the patient. The 15% rule is the receptor. Read the stem for which one it wants.

The one-paragraph version

A 15% kVp increase does to receptor exposure what doubling the mAs does; a 15% decrease does what halving it does. Multiply by 1.15 or 0.85. Paired with halving the mAs, it's the one move that maintains your image while lowering patient dose — at the cost of contrast. But it isn't a uniform law: doubling takes only 8–10 kVp down in the 50–70 range and 12–15 kVp up in the 80–100 range, so for untidy values count doubling steps rather than percentages. And don't confuse it with kVp², which governs output at the tube and needs about 40% to double — that rule is about the patient, this one is about the image.

Sources: Fauber, 5th ed., Ch. 6; Bontrager & Lampignano, 10th ed., Ch. 1; Bushong, 12th ed., Ch. 8–9. Where the books diverge, both framings are given above rather than quietly picking one.

Keep going: kVp vs mAs — what each one actually controls covers the underlying division of labor this rule depends on, and the ARRT formula sheet has this rule alongside every other relationship worth memorizing.