If you've ever opened a VA letter showing a 50% rating and a 30% rating and added them in your head to get 80%, you already know the punch line. The VA doesn't add ratings. It runs them through an iterative formula laid out in 38 CFR § 4.25, and the answer is almost always lower than what your calculator hands you.
This is the math. No mystique. No coaching. Just the rule and how to apply it.
Why ratings aren't additive
A disability rating describes how much a condition impairs your ability to function, expressed as a percentage of the whole person. A 50% rating means the condition takes you to 50% of "whole." A second condition can't take more than what's left.
Picture it as a glass. Your first 50% rating drinks down half the glass. The next rating doesn't get the full glass to work with, only the half that's still there. That's the entire mental model.
The regulation calls this the combined ratings table. The formula behind the table is what people mean when they say "VA math."
The rule in one sentence
Each disability is applied against what's left of "the whole person" after every prior disability has been subtracted.
The iterative efficiency formula
Step by step:
- Order your disability ratings from highest to lowest. Order matters for clarity, though the math comes out the same either way.
- Start at 100% whole.
- Subtract your highest rating from 100. That's your remaining efficiency.
- Take the next rating as a percentage of that remaining efficiency, not of 100.
- Subtract that result. That's your new remaining efficiency.
- Repeat for each rating.
- Subtract the final remaining efficiency from 100, that's your combined rating before rounding.
- Round to the nearest 10.
The cleanest way to do this in your head is to track efficiency (what's left) rather than disability (what's been spent).
A worked example, 50% + 30%
Two disabilities: a 50% rating and a 30% rating.
- Start: 100% efficient
- Apply 50%: 100 × (1 − 0.50) = 50% efficient
- Apply 30% to the remaining 50: 50 × (1 − 0.30) = 35% efficient
- Combined disability: 100 − 35 = 65%
- Round to nearest 10: 70%
Not 80%. The 30% rating only got to work on the half of you that wasn't already considered disabled.
A larger example, 70% + 50% + 30%
- Start: 100% efficient
- Apply 70%: 100 × 0.30 = 30% efficient
- Apply 50% to the remaining 30: 30 × 0.50 = 15% efficient
- Apply 30% to the remaining 15: 15 × 0.70 = 10.5% efficient
- Combined disability: 100 − 10.5 = 89.5%
- Round to nearest 10: 90%
A common eyeball estimate ("70 + 50 + 30 = 150, so capped at 100%") gives the wrong answer by 10 percentage points. Across the rating tiers, that's the difference between roughly $2,360/month and $3,940/month in 2026 base compensation for a single Veteran with no dependents. Real money.
2026 base monthly compensation, no dependents
- 60%: $1,435.02
- 70%: $1,808.45
- 80%: $2,102.15
- 90%: $2,362.30
- 100%: $3,938.58
Source: VA.gov disability compensation rates effective 2025-12-01
Run your own numbers against our disability calculator, it handles the combined-rating math and the dependent uplifts in one shot.
The rounding rule
38 CFR § 4.25 says to round to the nearest 10. The detail people miss: the regulation rounds up at 5. So a calculated combined rating of:
- 84.5% rounds to 80%
- 85.0% rounds to 90%
- 89.5% rounds to 90%
- 94.5% rounds to 90%
- 95.0% rounds to 100%
That single half-point at exactly 5 is the difference between an 80% and a 90% rating. If your calculated combined number lands at exactly 85.0, you cross the line. If it lands at 84.99, you don't.
This is why free calculators that "round half-to-even" or "round half-down" get the answer wrong. The VA rounds half up.
The bilateral factor
If you have paired disabilities affecting both arms, both legs, or both sides of paired skeletal muscles, 38 CFR § 4.26 adds a bilateral factor before combining with your other disabilities.
The mechanics: combine the bilateral disabilities first, add 10% of their combined value as a bonus, then combine that result with your other (non-bilateral) ratings.
Example: a 20% rating in your left knee and a 20% rating in your right knee.
- Combined: 20 + (80 × 0.20) = 36%
- Bilateral factor: 36 × 0.10 = 3.6%
- Bilateral-adjusted: 36 + 3.6 = 39.6%
Then 39.6 carries forward into the rest of your combined-rating math as if it were a single 39.6% rating. (Yes, for the bilateral piece you don't round until the very end.)
The bilateral factor is a v1.1 feature in our disability calculator. If you have paired-extremity conditions, your VA decision letter spells out the bilateral adjustment line by line, match it against the formula above.
Special Monthly Compensation (SMC), a flag, not a calculation
Beyond combined ratings, the VA pays Special Monthly Compensation for specific severe conditions, loss of use of a limb, loss of vision, requiring aid and attendance, certain combinations. SMC is paid in addition to your combined-rating compensation, often at fixed levels (SMC-K, SMC-L, SMC-M, SMC-N, etc.) rather than as a percentage.
If your VA letter mentions SMC, your total monthly amount is higher than what any combined-rating calculator alone will show. SMC determinations come directly from the VA, there's no formula a calculator can run from your rating list to predict SMC eligibility.
The honest framing: combined ratings tell you most of the story. SMC tells you the rest. Both numbers appear on your VA rating decision.
Mistakes most free calculators make
- Straight addition. "50 + 30 = 80." Wrong, see above.
- Rounding down at 5. Drops you a full tier when you should bump up.
- Treating bilateral conditions as separate ratings without the bilateral factor. Underestimates by a few points; sometimes that's the difference between two pay tiers.
- Ignoring dependent uplifts. The base 70% rate for a single Veteran is different from the 70% rate for a Veteran with a spouse and one child, by hundreds of dollars a month.
- Forgetting SMC. If your decision letter shows SMC, your total monthly check is higher than the base-rate table alone.
Our disability calculator handles all five. It also includes a what-if mode to compare scenarios, useful for understanding how an additional rating would affect your combined number before you have any VA decision letter to compare against.
The only number that matters
Calculators, ours included, produce estimates. The only binding number is what the VA puts on your rating decision letter. If your math disagrees with theirs, the math wins until they say otherwise.
If you believe your rating decision is wrong, contact a VA-accredited Veteran Service Organization like the DAV, VFW, or American Legion. They represent Veterans in claims and appeals at no cost. Benefitry doesn't represent claims, we calculate dollar values so you know what's on the table.
What to do next
- Run your ratings through the disability calculator and verify the result against your VA decision letter.
- If you live in Texas, Florida, or California, see the state benefits calculator, your combined rating drives state-level property tax and tuition benefits worth thousands a year.
- New to the VA system? See first 90 days after separation for the broader picture.
The math is the math. Once you can run it on a napkin, no one can hand you a rating estimate you don't understand.