A filter form is a deliberately "heavy" form that asks a potential customer several questions. Its goal is the opposite of a simple form: not to increase lead count but to reduce it — passing only people with a real problem and budget to the sales team.
Every extra question is a filter. A curious or accidental clicker won't finish it; a real buyer will. As a result, cost per lead (CPL) rises, but lead quality rises sharply.
How many questions and what they ask
Usually 7-10 questions are enough. They check four things: is the problem present now, what's the budget level, who's the decision-maker, and when is the solution needed. At Parking24, a 10-question form pushed CPL to ~$10, yet 90% of the leads were sales-ready.