back–of–the–envelope adjective
always used before a noun informal : done to provide a rough idea of something
▪ I can't tell you the exact amount, but my back-of-the-envelope [=rough] calculation indicates a cost of about $300,000.
▪ I can't tell you the exact amount, but my back-of-the-envelope [=rough] calculation indicates a cost of about $300,000.
