Ask any group of veterans about "VA math" and someone groans. A 50 percent rating and a 30 percent rating don't make 80. They make 70. That isn't an error on your letter, and it isn't the VA shortchanging arithmetic. It is a published formula, and once you see the idea behind it, every number on your decision letter starts making sense.
The idea: ratings apply to what's left
The formula treats you as starting at 100 percent efficiency, in the regulation's language. Each rating then applies to the efficiency that remains, not to the original 100. So your ratings are combined one at a time, starting with the largest, and each one takes its percentage out of what is left after the ratings before it.
Worked example: 50 and 30
- Start at 100. A 50 percent rating takes half of it. Combined value so far: 50. Remaining: 50.
- The 30 percent rating now applies to the remaining 50. Thirty percent of 50 is 15.
- Add that to the running total: 50 + 15 = 65.
- Final step, round to the nearest 10: 65 rounds up to 70 percent.
Add a third rating: 50, 30, and 20
- From above, 50 and 30 combine to 65, with 35 remaining.
- The 20 percent rating applies to that 35. Twenty percent of 35 is 7.
- 65 + 7 = 72, which rounds down to 70 percent.
Notice what happened: the third rating moved the raw number only 7 points, and after rounding it didn't move the final rating at all. That is the nature of the formula. The more that is already rated, the less room each new rating has to work with. It is why combined ratings feel like they crawl as they get higher.
The rounding rule
The combined value always rounds to the nearest 10, and 5 rounds up. A raw 65 becomes 70. A raw 44 becomes 40. So a 30 and a 20 combine to 44 and land at 40, while a 50 and a 30 combine to 65 and land at 70. Two veterans can have raw values 21 points apart and official ratings 30 apart. The rounding is part of the published rule, not a mistake.
Paired arms and legs get one extra step
When both arms or both legs carry ratings, the regulation adds a small extra amount, 10 percent of those ratings' combined value, before everything is combined with the rest. It is called the bilateral factor. The details live in the regulation, but the plain version is: matching limbs count a little extra.
Want to run your own numbers? The free combined rating calculator on this site walks the same steps and shows its work.
Where this comes from
The formula and its lookup table are published in federal regulation, 38 CFR 4.25, with the bilateral factor in 4.26. None of it is secret and none of it is discretionary. Your decision letter's combined rating is that table applied to your individual ratings.
The important caveat
Your official combined rating is the one on your VA decision letter, period. This guide exists so the letter makes sense when you read it, not to predict anything. And if you use MusterVault, this is exactly what its benefits summary does: you type in the ratings from your own letters, and it runs the published formula so you can see the math. It never estimates a rating from your symptoms, because nothing honest can.