Under 38 CFR § 4.26, when two or more service-connected disabilities affect paired body parts — both arms, both legs, paired skeletal muscles — the VA combines those bilateral disabilities first, adds 10% of that combined number back, and only then combines that bundle with the rest of your disabilities.
Translated to plain English: two 20% knees do not become 36%; they become roughly 39.6%. Over a lifetime that small difference is the gap between 70% and 80% for many veterans.
Worked example — both knees and both ankles
| Disability | Rating | Bilateral? |
|---|---|---|
| Right knee strain | 20% | Yes |
| Left knee strain | 20% | Yes |
| Right ankle | 10% | Yes |
| Left ankle | 10% | Yes |
| Tinnitus | 10% | No |
Combine the four bilateral disabilities first using the standard VA formula: 20 → 36 → 42.4 → 48.16. Add 10% of 48.16 (which is 4.82) to get 52.98. Now combine 52.98 with tinnitus (10%): 1 - (1 - 0.5298) × (1 - 0.10) = 57.68. Round to nearest 10 = 60%.
Without the bilateral factor that same veteran would round to 50%. The 10 percentage points represent thousands of dollars per year for the rest of the veteran's life.
To run your own numbers, open the main combined-rating calculator and tick the bilateral checkbox on each paired-extremity row. The factor is applied automatically, and the result panel shows exactly how much was added.