Quick Answer
The VA does not add disability percentages together. It combines them as fractions of what's left. Each condition is rated as a percentage of impairment, then applied to the remaining "healthy" portion of the veteran — 30% for the knee first, then 20% of what remains. That's why 30% + 20% comes back as 40%, not 50%. The final combined number is rounded to the nearest 10%, and conditions affecting both arms or both legs get an extra step (the bilateral factor) that usually works in the veteran's favor.
Why the Letter Never Matches the Math in Your Head
Almost every veteran knows exactly where their decision letter is, and almost every veteran misreads what the number means. You did the math the way school taught you: thirty for the knee, twenty for the back. That should be fifty. The letter says forty.
The first thought is never math. The first thought is "they got me." But it's not a trick — it's a different arithmetic, and once you see it, you can check the VA's own work.
The Actual Method: Percentages of What's Left
The VA treats each condition as a fraction of the whole person. The first condition takes its percentage of the whole. The second condition takes its percentage of what's left. Each subsequent rating applies to a smaller remainder.
Here's the standard example:
- Knee at 30%: you're now considered 30% impaired, 70% remaining.
- Back at 20%: 20% of the remaining 70% is 14%. Total impairment: 30 + 14 = 44%, 56% remaining.
44% rounds to the nearest 10% — and that's how a pair of ratings that "adds" to 50 comes back from the VA as a 40% combined rating. The ten-point gap between the letter and your arithmetic is the combined ratings table doing exactly what it's built to do.
The VA does this with a published table (the combined ratings table) rather than step-by-step subtraction, but the logic is exactly this. Every combination of ratings maps to a final combined number, and the table always rounds to the nearest 10% — a combined 34% becomes 30%, a combined 35% becomes 40%.
The Bilateral Factor
One more step can change the answer, and it usually helps: the bilateral factor.
If you have rated conditions affecting both arms, both legs, or paired bones (knees, ankles, wrists), the VA first combines those paired ratings together, then adds an extra 10% of that combination to account for the added disability of having both sides impaired. That adjusted figure then enters the main combined-ratings math.
It's one of the most commonly missed pieces of the math — and one of the reasons an independent check of the letter sometimes finds the VA itself under-calculated.
What a Rating Actually Measures
Worth saying plainly: a rating is not a score of how disabled you are as a person. It's the VA's estimate of how much your average earning capacity is affected compared to an unimpaired worker your age. That's why the functional evidence — what you can and can't do — drives the individual ratings before any math happens.
Checking Your Own Letter
You can verify the math yourself: list each condition and its percentage, combine them by the "percentages of what's left" method (or the VA's published table), apply the bilateral factor for paired limbs if applicable, and round to the nearest 10%. If your result differs from the letter's, that's a concrete, checkable question — and exactly the kind of thing an accredited VSO reviews for free. Our podcast episode "Combined Ratings and the Bilateral Factor" walks the whole calculation with real numbers, and the Free VA Claims Quick-Start Guide at standeasynetwork.com shows you how to pull the file the math came from.
This article is informational and educational only. It is not legal, medical, or mental health advice, and reading it does not create a representative relationship. Rating rules have exceptions; for claim-specific guidance, consult an accredited VSO or attorney.
(c) AI generated content. This article was produced with AI assistance and reviewed under Stand Easy Network editorial standards.
If you are a veteran in crisis, call the Veterans Crisis Line now: dial nine eight eight, then press one. Free, confidential, available 24/7. Prefer to text? Message 838255. Or chat live at veteranscrisisline.net. For immediate danger, call 911. You are not alone.