The builder Margin differs from my IB margin

What you see. The Margin column in the Trade builder differs from the margin IB requires or holds.

Why. By design. The builder’s Margin is the structural worst case from the legs, the most the row can lose by its front expiration, not your broker’s requirement.

What to do. For IB’s own figure after an order, read “Excess after” in the Commit dialog. See Margin and Lots and limit price.