The combined-ratings table
A 50% rating combined with a 30% rating does not equal 80%. The VA combined-ratings table treats each disability as taking a percentage of the remaining "efficient" person, so a 50% rating leaves 50% of the person, and the next 30% rating applies to that remaining 50%, contributing 15 percentage points for a combined 65% (rounded to the nearest 10% only at the final step).
VBN implements 38 CFR § 4.25 as a pure-TypeScript function in src/lib/rating-math/. The function operates on integer percentages, sorts inputs from highest to lowest, and applies the formula iteratively. It returns both an unrounded value (for transparency) and the final rounded value (for display).
The bilateral factor
When a veteran has service-connected disabilities affecting both arms, both legs, or paired skeletal muscles, 38 CFR § 4.26 adds 10% of the bilateral combined value as a bilateral factor. This is added before the result is combined with any non-bilateral disabilities.
VBN’s tool prompts for which conditions affect paired extremities and applies the bilateral factor only when at least two paired-extremity conditions are present. It does not apply the bilateral factor to single-side conditions, even when those conditions are severe.
Rounding and 38 CFR § 4.26(d) safety check
VA rounds to the nearest 10% in the final step. Rounding earlier would produce a different (and incorrect) answer. The tool keeps full precision through every intermediate step and rounds only on display.
A safety check enforces 38 CFR § 4.26(d): the combined rating cannot exceed 100%. The tool caps results at 100% rather than displaying 110% or 120%, which would be invalid.
What the tool does not do
The combined-rating tool does not assign individual disability ratings, evaluate medical evidence, or determine whether a condition is service-connected. Those determinations are made by VA based on the full record. The tool computes the math given a set of already-assigned (or hypothetical) ratings.