VA Math Isn’t Regular Math.

You open the letter. You flip past the boilerplate to the page with the numbers. You see your conditions listed. PTSD: 50%. Low back: 20%. Left knee: 10%. Tinnitus: 10%. Right knee: 10%. You add them up in your head. That’s 100%.

Then you read the combined rating at the bottom: 70%.

You read it again. You check the individual ratings. You add them again. Still 100%. The letter still says 70%. And the first thing you feel isn’t confusion. It’s betrayal. How does the VA take five separate disabilities, each one documented and rated, and tell you the total is thirty percentage points lower than what basic arithmetic produces?

The answer is 38 CFR § 4.25, the Combined Ratings Table. It’s the single most misunderstood regulation in the VA disability system, and it determines how much money hits your bank account every month. This post explains how it works, why it works that way, and what you can do about it once you understand the math.

The Whole Person Theory

The VA doesn’t add your ratings together. It applies them sequentially to what remains of a theoretical “whole person.”

The logic goes like this: you start at 100%, meaning fully able-bodied. Your first disability reduces that efficiency by its percentage. Your second disability reduces what’s left. Your third reduces what’s left after that. And so on. Each rating applies to the remaining capacity, not the original 100%.

The VA’s reasoning is that you can’t be more than 100% disabled, so each additional disability is reducing a smaller pool of remaining function. Whether this feels fair is a different question from how it works. Right now, we’re focused on how it works.

Walking Through the Math

Let me use the example from the letter above. Five disabilities: 50%, 20%, 10%, 10%, 10%.

The VA always starts with the highest rating and works down.

You begin with 100% efficiency. The 50% rating takes half of that. You’re left with 50% efficiency.

Now the 20% applies to what remains (50%), not to the original 100%. Twenty percent of 50 is 10. Your remaining efficiency drops to 40%.

The first 10% applies to 40%. Ten percent of 40 is 4. Remaining efficiency: 36%.

The second 10% applies to 36%. Ten percent of 36 is 3.6. Remaining efficiency: 32.4%.

The third 10% applies to 32.4%. Ten percent of 32.4 is 3.24. Remaining efficiency: 29.16%.

Your combined disability is 100% minus what remains. 100% minus 29.16% equals 70.84%.

The VA rounds to the nearest 10%. Values ending in 5 or higher round up. 70.84% rounds to 70%.

With straight addition: 100%. With VA math: 70%. That’s the gap, and now you can see exactly where it comes from. Each disability after the first one is applied to a shrinking base.

Why Every Decimal Matters: The Rounding Cliff

The VA only rounds once, at the very end, and it rounds to the nearest 10%. This creates cliffs that can cost or gain you significant money.

A combined value of 74.5% rounds to 70%. A combined value of 75.0% rounds to 80%. The difference between those two numbers, half a percentage point, can mean hundreds of dollars a month in compensation. At certain thresholds it means even more: 100% pays roughly three times what 90% pays, and it opens the door to additional benefits like Dependents’ Educational Assistance under Chapter 35.

This is why seemingly small ratings matter. A 0% service-connected condition might not put money in your pocket directly, but it establishes a connection that can later be increased or that can support a secondary claim. And a 10% rating on a condition you might have dismissed as minor can be the difference between rounding down to 80% and rounding up to 90%.

A Realistic Veteran’s Profile

Let me walk through a more complete example that shows how this plays out across a real set of conditions. This isn’t any specific veteran. It’s a composite that represents what I see when veterans talk about their claims.

The veteran has PTSD rated at 70%, a lumbar spine condition at 20%, left knee limitation of flexion at 10%, left knee instability at 10%, right knee pain (§ 4.59 minimum compensable) at 10%, tinnitus at 10%, and migraines at 10%.

Starting from 100%, the math works like this:

70% applied to 100% leaves 30%.

20% applied to 30% (that’s 6) leaves 24%.

10% applied to 24% (that’s 2.4) leaves 21.6%.

10% applied to 21.6% (that’s 2.16) leaves 19.44%.

10% applied to 19.44% (that’s 1.944) leaves 17.496%.

10% applied to 17.496% (that’s 1.7496) leaves 15.7464%.

10% applied to 15.7464% (that’s 1.57464) leaves 14.17176%.

Combined disability: 100% minus 14.17% equals 85.83%. Rounds to 90%.

Straight addition of 70 + 20 + 10 + 10 + 10 + 10 + 10 gives 140%. VA math gives 90%. The gap gets wider with more conditions, but each additional condition still moves the needle. That last 10% for migraines was the difference between 84% (rounds to 80%) and 85.83% (rounds to 90%). Ten percentage points of compensation hinged on a single additional claim.

The Bilateral Factor: Free Percentage Most Veterans Miss

Under 38 CFR § 4.26, if you have service-connected disabilities affecting both legs, both arms, or paired organs (kidneys, eyes, ears), the VA applies a bilateral factor before combining.

The bilateral disabilities are combined together using the same sequential method, and then 10% of that combined value is added as a bonus before those disabilities get combined with your non-bilateral conditions.

Using the veteran above, both knees are involved. Left knee flexion at 10% and left knee instability at 10% combine to 19%. Right knee at 10% combines with that 19% to give 27.1%. The bilateral factor adds 10% of 27.1%, which is 2.71 percentage points, giving an adjusted bilateral value of 29.81% (rounded to 30% for the bilateral group).

That 30% then gets combined with the remaining non-bilateral conditions using standard VA math. The bilateral factor can bump a combined rating by a few percentage points, and when you’re sitting at 74% and need to reach 75% to round up to 80%, those few points matter.

Most veterans don’t know the bilateral factor exists. If you have conditions affecting paired limbs or organs, make sure they’re documented and that the bilateral factor is being applied in your rating calculation. If you’re not sure, ask your VSO to review the math on your rating decision letter.

Why Secondary Conditions Are Your Most Powerful Tool

Post 4 touched on secondary conditions in the context of mental health claims. In the math, they’re even more important.

A secondary condition is a disability that was caused or aggravated by an already service-connected condition. You don’t need to prove it happened during service. You need to prove that your existing service-connected condition caused or worsened it. That’s a different and often easier standard of evidence.

Common secondary chains that veterans miss: chronic pain from a back injury causes depression (secondary mental health claim). PTSD causes sleep disturbance, which leads to a sleep apnea diagnosis (secondary respiratory claim). Long-term NSAID use for a service-connected musculoskeletal condition causes GERD (secondary digestive claim). A knee condition alters your gait, which causes or worsens a hip or ankle condition (secondary musculoskeletal claim). Erectile dysfunction secondary to PTSD medication or chronic pain (secondary genitourinary claim, also qualifies for SMC-K, covered in Post 6).

Each of those secondary conditions gets its own rating, and each one feeds into the VA math calculation. A 10% rating for GERD doesn’t sound like much, but when it pushes your combined rating from 84% to 86% and that rounds to 90% instead of 80%, it’s worth over $400 a month. A 50% rating for sleep apnea (CPAP use) is even more significant. Filed as secondary to PTSD, it can be the single claim that changes your combined rating and your family’s financial picture.

The strategic question isn’t just “what conditions do I have?” It’s “what conditions do I have that are connected to conditions I’m already rated for?” Every link in the chain adds another rating to the calculation.

When the Math Won’t Get You There: TDIU

Some veterans run the numbers and realize that even with every legitimate condition claimed and rated, VA math won’t get them to 100%. Their combined rating lands at 80% or 90%, but they can’t work. Their conditions, taken together, make reliable full-time employment impossible.

This is where Total Disability based on Individual Unemployability (TDIU) comes in.

TDIU pays at the 100% rate even if your combined rating is less than 100%. There are two pathways.

Schedular TDIU (38 CFR § 4.16a) requires either one disability rated at 60% or more, or a combined rating of 70% or more with at least one condition at 40% or more. You must also demonstrate that your service-connected conditions prevent you from securing and maintaining substantially gainful employment.

Extraschedular TDIU (38 CFR § 4.16b) exists for veterans who don’t meet the schedular thresholds but whose service-connected conditions still prevent them from working. This pathway requires referral to the VA Director of Compensation Service and is less common, but it’s available.

The key phrase in both pathways is “substantially gainful employment.” This doesn’t mean you can’t do anything. It means you can’t reliably perform the type of work that would sustain you financially, given your education, training, and work history. A veteran with a 70% PTSD rating who can technically bag groceries but whose occupational history is in leadership and management, and whose conditions prevent them from functioning in that capacity, may qualify for TDIU.

TDIU is not a consolation prize. For many veterans, it’s the correct outcome. If your service-connected conditions prevent you from working, the VA owes you the 100% rate regardless of what the combined math produces. Talk to your VSO about whether TDIU applies to your situation.

Running Your Own Numbers

You don’t have to do this math by hand. The VA publishes the Combined Ratings Table in 38 CFR § 4.25. You can also find free VA combined ratings calculators online that let you plug in your individual ratings and see the combined result immediately.

But understanding the math, not just the output, is what lets you think strategically about your claim. When you see that your combined rating is sitting at 73% and you know that one more 10% condition would push you to 76% and round up to 80%, you start looking at your body differently. That GERD you’ve been managing with antacids. The radiculopathy shooting down your leg that you assumed was just part of the back injury. The scars from surgery. The sleep issues. Each one is a potential claim. Each claim is a potential rating. Each rating shifts the math.

This isn’t about inventing conditions. It’s about recognizing and claiming what’s already there. Most veterans are underrated not because the VA is out to get them, but because they didn’t claim everything they were entitled to claim. They filed for the three conditions that bothered them most and left ten others on the table because they didn’t know secondary connections existed, didn’t think “small” conditions were worth filing, or assumed their problems were just normal aging.

If it’s connected to your service, claim it. Let the math do the rest.

The Number Isn’t the End

VA math is frustrating. Seeing 50% + 30% + 10% + 10% equal 74% instead of 100% feels wrong, and I won’t pretend the system is perfectly fair. But the math is knowable. The rules are published. And once you understand them, you can work within them instead of being blindsided by them.

Claim every condition that’s connected to your service. Document every secondary connection. Know about the bilateral factor. Understand the rounding cliffs. And if the math won’t get you to where your actual impairment lives, TDIU exists for exactly that reason.

The regulation that governs combined ratings starts with an assumption that you’re a whole person. Every rating you earn acknowledges that you gave a piece of that wholeness in service to this country. Getting the math right is how you make sure the compensation reflects what you actually gave.

Next in the series: “The Money You’re Leaving on the Table: Special Monthly Compensation and the Conditions Most Veterans Never Claim”

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Adam Bishop is a medically retired Navy SEAL Lieutenant and the founder of XOPS360 LLC, a veteran-owned technical consultancy. He served 17 years on active duty including multiple combat deployments. He writes about the intersection of military experience and civilian systems at xops360.com.

This article provides educational information based on published federal regulations. It is not legal advice. For personalized assistance with your VA disability claim, contact an accredited Veterans Service Organization.

Free Resources:

• DAV (Disabled American Veterans): Free claims assistance
• VFW (Veterans of Foreign Wars): Accredited representatives
• American Legion: Benefits assistance
• Wounded Warrior Project: Peer support and benefits guidance
• VA.gov Claims Portal: File or check your claim online
• VA Combined Ratings Table: The official math