Fleet Mass Calculator spreadsheet. Requires a Google account. Select File, then "Make a copy". You'll have your own version to modify as you see fit.
WARNING: this sheet doesn't magically know if a hole in your chain has been massed prior to your use of it. If your fleet got rolled out despite the sheet saying it would fit, I can only chuckle and advise more propitiations to Bob.
Pick ships and amounts and you're good. Change the hole mass reduction percentage if you want more or less leeway in the conservative transits column.
If you already have a fleet out, paste your fleet comp into a dscan tool which provides a mass readout (such as dscan.info) then paste said value over C18 for a quick check of whether you'll all fit through a hole. Note: the sheet uses tonnes i.e. Megagrams, as does dscan.info; not all tools do, so pay attention to your orders of magnitude!
Other EVE-related stuff.
An example of what the sheet does: