Estimate the right length for a formal letter.
A formal letter has a fixed frame and a variable middle. The frame is the opening statement of purpose and the closing request for specific action, which together run around 160 words and should not be compressed — an unclear ask is the most common failure in formal correspondence. The middle scales with your supporting points, and complaints and appeals need more words per point than requests because each point must be evidenced rather than asserted. Attachments need a referencing line each. Keeping the whole thing inside two pages matters: beyond that, detail belongs in a numbered annex the reader can consult rather than must wade through.
Target length
Words = frame + supporting points x words per point + attachments x 18
Page estimate
Pages = target words / words per page at your formatting
Because formal letters are read by someone processing a queue, and length reduces the chance your specific request is identified and acted on. Anything beyond the argument itself — timelines, correspondence history, invoices — belongs in a numbered annex referenced from the letter.
Because each point has to be evidenced. A complaint asserting poor service achieves nothing; one stating the date, what was promised, what happened and what document proves it can be acted on. That structure needs roughly 100 words per point, which is why complaints run longer than requests.